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A cs atomic vapor resonance ionization imaging detector

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A CS ATOMIC VAPOR
RESONANCE IONIZATION IMAGING DETECTOR














By

JAMSHID TEMIROV


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2003
































Dedicated to Diyora, and all the Temirovs.















ACKNOWLEDGMENTS

Now, when I am on the eve of a very important point of my career, looking back at

the last few years spent in the graduate school, I want to say that they have been certainly

very important period. They have been a period of learning science, discovering new

philosophy and cultures, appreciating virtues related not to the work environment, but

also valued in simple, everyday relationships among people and friends. For all of that I

owe the debt of gratitude to a number of people, first of all to my supervisor, Dr. James

Winefordner, great scientist, yet modest person, understanding and guiding advisor.

I would like to thank Dr. Nicolo Omenetto for his inspiring, contagious and

philosophical enthusiasm about science and life. I would also like to thank Dr. Benjamin

Smith, whose office door has always been wide open with him ready to help, support, and

advise on any occasion. I deeply express my gratitude to Dr. Oleg Matveev and his

family, a brilliant scientist and a person who is actually guilty for me being in this school.

He and his family have helped me a lot to settle down and adjust to both school and life

in Gainesville. I would like to say many thanks to Dr. Igor Gomrushkin and his family. He

has been a great friend since my first days in the University of Florida and has given

helpful advice and support many, many times.

I would like to thank the entire Winefordner group of students, past and present,

with whom I have had a chance to boil in the same big "graduate school pot."

Particularly, I would like to thank Dr. Michael Shepard for being a truly big friend. I

would especially thank Dr. Janina Gutierrez. The friendship with her has certainly been








and will always be treasured by me and my family. I would like to thank the Harrison

group students for contributing to the friendly and pleasant atmosphere around our joint

laboratory.

I want to thank Ms. Jeanne Karably for her kindness and help with paperwork. I

greatly appreciate the prompt and high quality job done by the Machine and Electronic

Shops of the Department of Chemistry.

I thank my parents, brothers and their families. Despite being physically far, I

believe they have always been with me in their hearts and thoughts. I also would like to

thank all my relatives and friends in Uzbekistan, who have wished me luck.

I want to thank my wife, Firuza, for her support and care. And finally, I would like

to thank the Creator for blessing me and my wife with the birth of our little angel

daughter, Diyora.















TABLE OF CONTENTS
Page

ACKNOW LED GM ENTS ................................................................................................. iii

ABSTRA CT...................................................................................................................... vii

1 INTRODUCTION TO SPECTRA L IM AGIN G ............................................................. 1

2 INTRODUCTION TO RESONANCE IONIZATION IMAGE DETECTORS .............9

Introduction................................................................................................................... 9
RIID Principle of Operation ......................................................................................... 9
Sealed-cell M ercury RIID ........................................................................................... 11
Active Elem ent for RIID ............................................................................................ 12
Conclusion .................................................................................................................. 16


3 CS PHOTOION IZATION AND LA SERS ................................................................... 17

Introduction................................................................................................................. 17
Cs Photoionization: Theoretical Considerations ........................................................ 18
Lasers: Availability..................................................................................................... 20
Cs Photoionization: Experim ental Studies ................................................................. 22
Two-step Ionization............................................................................................. 22
Three-step Ionization........................................................................................... 24
Lasers: Practicality...................................................................................................... 26
Conclusion .................................................................................................................. 27


4 CS RIID CHARACTERIZATION ................................................................................ 28

Introduction................................................................................................................. 28
Experim ental Setup..................................................................................................... 28
Experim ental Results and Discussion......................................................................... 31
Im age Acquisition and Processing ...................................................................... 31
Signal vs. H igh Voltage....................................................................................... 33
Signal vs. Num ber D ensity of Cs Atom s............................................................. 35
Signal and Resolution vs. Laser Pulse-energy .................................................... 37
Detector Efficiency and Spatial Resolution ........................................................ 39
Conclusion .................................................................................................................. 40








5 TIM E-RESOLVED EXPERIM ENTS IN RIID ............................................................. 41

Introduction................................................................................................................. 41
Tim e-Resolved M easurem ents in M ercury RIID ....................................................... 42
Experim ental Setup ............................................................................................. 42
Experim ental Results and Discussion......................................................................... 43
Theoretical Considerations.................................................................................. 43
Tim e-Resolved M easurem ent Results................................................................. 46
Low-m ass Ion Desorption ................................................................................... 49
Absolute Tim e-Resolution of Hg RIID ............................................................... 53
Tim e-Resolved M easurem ents in Cesium RIID ......................................................... 54
Dual-W avelength Tim e-Resolved Im aging with Cs and Hg...................................... 56
Conclusion .................................................................................................................. 59


6 FEASIBILITY OF SEALED-CELL CS RIID CONTRUCTION................................. 61

Introduction................................................................................................................. 61
Cs RIID : First Attempt ............................................................................................... 61
Cs Compatibility: Theoretical Considerations............................................................ 63
Cs Compatibility: Experim ental Verification............................................................. 66
Conclusion .................................................................................................................. 71


7 BUFFER GAS RESONANCE IONZATION IMAGING DETECTOR WITH
AVALANCHE AM PLIFICATION OF SIGNAL...................................................... 72

Introduction................................................................................................................. 72
Buffer Gas RIID : Design............................................................................................ 73
Buffer Gas RIID : Experim ental Results ..................................................................... 78
Avalanche Amplification of Cs Ionization Signal in Flam e....................................... 80
Experim ental Setup ............................................................................................. 81
Results and Discussion........................................................................................ 83
Conclusion .................................................................................................................. 90


8 CONCLUSIONS AND FUTURE W ORK .................................................................... 92

Conclusions................................................................................................................. 92
Future W ork................................................................................................................ 94


REFERENCES .................................................................................................................. 97

BIOGRAPHICAL SKETCH ...........................................................................................101















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

A CS ATOMIC VAPOR
RESONANCE IONIZATION IMAGING DETECTOR

By

Jamshid Temirov

August, 2003

Chair: James D. Winefordner
Major Department: Chemistry

Many modem analytical applications demand spectrally selective imaging devices

with high sensitivity and resolution. Spectral selectivity of an image detector would be

especially useful for imaging static or moving objects in highly scattering or high

background environments. For such applications, the detector must be able to detect

photons with a very small spectral or frequency distribution, typically less than 30 GHz.

The resonance ionization imaging Detector (RIID) proposed in this research is based on

an atomic vapor. The superior figures of merit of the RIID are determined mainly by

three factors: (1) extremely high spectral resolution, which comes from the narrowband

absorption of light in atomic vapor; (2) stepwise laser ionization of the excited atoms

with up to 100% efficiency; (3) and a large, almost hemispherical field-of-view.

To accomplish the goal of the project, a novel cesium RIID, the second existing

RIID (after the previously developed mercury RIID), has been developed and evaluated.

Cs is one of the few elements that are suitable for the construction of the RIID and can








offer certain advantages, such as high sensitivity due to strong atomic transitions of Cs

atoms and high spectral resolution, typically determined by the Doppler broadened

linewidth at room temperature (0.38 GHz). Also, the infrared detection wavelength of Cs

RIID at 852 nm allows important biological applications of the detector, because

radiation in this spectral range is safe for biological tissues and has good penetration

depth. The imaging capabilities of the Cs RIID have been evaluated, varying the RIID

parameters such as applied high voltage, the number density of Cs atoms and laser pulse

energies. The materials inert to Cs reactivity (sapphire, titanium, YAG) have been

experimentally evaluated; the results have proven the feasibility of the construction of a

compact, sealed-cell Cs RIID.

The flexibility of the resonance ionization imaging detectors has been demonstrated

with a non-imaging time-resolved mode of operation. Time-resolved ionization signal

measurements in addition to imaging information can enhance the performance of the

RIID by providing the basis for determination of the frequency shift from the resonance

for the photons to be detected. A dual-wavelength, time-resolved resonance ionization

detector with Cs and Hg vapors has been demonstrated. Such detector operating in time-

resolved mode could be very useful in imaging of two closely located targets.













CHAPTER 1
INTRODUCTION TO SPECTRAL IMAGING

The ultimate goal of this research was to develop a two-dimensional spectral

imaging detector of photons with superior figures of merit compared to available spectral

imaging techniques. Generally, two categories of photon detectors are used in analytical

instruments: single-channel and multi-channel. The detector, developed in this work, falls

into the second category, being two-dimensional. By a brief comparison of the detector

with conventional two-dimensional detectors used in analytical instruments and

applications, the motivation and basis for this research will be shown.

There are many modem applications demanding spectrally selective imaging

devices with high sensitivity and resolution (spectral and spatial). Spectral selectivity of

an image detector would be especially useful for imaging static or moving objects in

highly scattering or high background environments. Examples of applications that would

benefit from high spectral resolution detectors are chemical and biological Raman

imaging [1, 2], laser-Doppler velocimetry [3], optical communications and satellite

tracking [4]. For these applications, the detector must be able to distinguish very small

frequency differences in signal, typically less than 1 cm-1 (30 GHz) [5].

In most conventional imaging techniques, the multi-channel detector is a charge-

coupled device (CCD) with several thousand pixels in a rectangular arrangement.

Typically, CCDs have efficient spectral response over a broad range of wavelength,

which is favorable in a number of applications. However, in applications where high

spectral discrimination is needed, the CCDs must be coupled with some type of spectral








filter. Commonly available spectral filters are acousto-optic tunable filters (AOTFs) [6],

liquid-crystal tunable filters (LCTFs) [7], liquid-crystal Fabry-Perot interferometers

(LCFPIs) [8], dual-grating filters (DGFs) [9], scanning line imaging monochromators

(SLIMs) [10], fiber-optic bundle arrays (FOBAs) [11], and dielectric interference filters

(DIFs) [12]. All of these filters have some critical drawbacks. Major drawbacks are broad

bandwidth and/or limited transmission. AOTFs, for example, have relatively high

transmission efficiency (40 50%), but large spectral bandpass on the order of 50 cm-';

thus, they can not provide high spectral resolution. LCTFs and LCFPIs have smaller

bandpasses, on the order of 10 cm'1, and are commonly used as filters in commercial

Raman imaging instruments [1, 13, 14]. However, the transmission efficiency is low (15 -

25%). DGFs, which use two parallel, holographic transmission gratings to isolate images

according to wavelength, have a high transmission efficiency (75%) but large bandpasses

(200 cm'1). SLIMs have bandpasses limited by the spectrometer used (typically

1-10cm'), and generate line-scanned images which require a long analysis time to

produce a 2-D image of the sample. FOBAs have similar spectral characteristics;

however, the spatial resolution is limited by the diameter of the fiber, and current fiber

imaging quality is typically worse than other imaging filters. All these imaging filters

suffer from a trade-off between spectral resolution and throughput, as well as image

degradation. A narrowband atomic vapor resonance ionization imaging detector (RIID),

described in this work, is capable of true 2-D imaging with high sensitivity and spectral

resolution. These essential figures of merit of the system are determined by the physical

characteristics and spectroscopy of the active element of the system. The principle of

operation of the detector is based on absorption of radiation by atoms at the detection








wavelength, which typically is a resonance transition from the ground state, and

subsequent photoionization of the excited atoms. The narrowband atomic absorption

feature of the RIID, as will be shown in detail below, is the basis for high spectral

resolution of the detector. The laser photoionization method employed in the RIID makes

it a very sensitive photon detector.

As an example of RIID application, the principle of Raman imaging with the RIID

will be shown, because in the last few years, Raman spectroscopy has developed into a

2-D (wide-field) chemical and biological imaging technique and commercial instruments

are available for a variety of applications [15, 16]. The wide-field advantages are best

realized when high-fidelity images at a limited number of wavelengths (in the simplest

case, one wavelength) provide sufficient chemical and spatial information. Figure 1-1

shows a general schematic of RIID operation as a Raman imaging device. Let us assume

that the analyte has two spectral features with different Raman shifts, AARi and AIIR2,

indicated in Figure 1-1 as a red pentagon and a blue triangle. For example, these regions

could be healthy and diseased regions of a biological tissue in real-case applications. The

sample is globally illuminated with an expanded excitation laser detuned from the

resonance wavelength by exactly one of the Raman shifts (e.g., A2Ri). Thus, by detuning

the incident wavelength by an appropriate Raman shift, the Raman scatter of interest can

be made to coincide with the detection wavelength of the RIID. The atoms in the RIID,

excited with the radiation Raman scattered from the sample, will be photoionized with

the assistance of additional lasers. The ions formed will then be accelerated to the MCP

incorporated in the RIID, and electrons due to ions impinging on the MCP will be

accelerated to the phosphor screen of the RIID and form an image. The image produced








in the RIID will ultimately be captured with a CCD camera, as is done in conventional

Raman imaging.

Sample RIID

SRaman scattered CCD
Sradiation *




Excitation laser




Figure 1-1. Schematic diagram of Raman detection with the RIID. The red pentagon and
blue triangle represent two regions of sample with Raman shifts of AZI) and
A2R2, respectively. The excitation laser is tuned to 2A-A2R1 such that upon
interaction with the Raman active sample, the scattered light will be shifted
into resonance with the signal transition of the atomic vapor.

For this example of the detector application, the RIID and a conventional Raman

imager can be compared. In fact, in Figure 1-1, replacing the RIID with an LCTF tuned

to /1, would represent a schematic of a conventional Raman imaging device (with

focusing and collection optics omitted for simplicity). However, there remains a major

difference between the RIID and conventional Raman technology. Conventionally, the

collected Raman signal captured by the CCD camera is reduced in intensity due to the

limited transmission efficiency of the LCTF (typically 25%). In this way, only a portion

of the inherently weak Raman signal reaches the detector. In the case of the RIID, an

amplified Raman signal will be detected by the CCD, because Raman scatter from the

sample first will be converted into ion/electron pairs by stepwise photoionization after

absorption in the atomic vapor. This can be done with efficiency close to 100%. The ions








(or electrons) representing the Raman signal are then amplified in the MCP before

forming an image on the phosphor screen and detection by a CCD. As a result, the RIID

simultaneously acts not only as a filter but also as an efficient amplifier of the Raman

signal, eliminating the need for an expensive image-intensified CCD camera.

The narrow absorption linewidth of the atoms makes the system inherently

selective to the photons with specific wavelength. The system offers very high spectral

selectivity and sensitivity. The spectral resolution can potentially be in the sub-MHz

region. It has been shown that atomic vapor narrowband imaging detectors and filters can

have a spectral efficiency higher than any existing imaging system [17] (Figure 1-2).

Spectral efficiency or luminosity-resolving power (LR) product, the most significant and

universal figure of merit of any spectroscopic system, is the product of the resolving

power (R) and luminosity (L). Resolving power R is equal to 2/A2, where X is the

average wavelength of two spectral lines separated by the minimum resolvable interval

A2. Luminosity or throughput is defined as the product of the solid angle of acceptance,

.Q (sr) and the projected (acceptance) area, A (cm2). Figure 1-2 shows the calculations of

LR product for several selected spectroscopic techniques with typical parameters [17]. In

principle, all known types of spectrometers can provide a resolving power (R) as high as

105-106. However, for most conventional spectrometers there is an inherent problem of

decreasing throughput as resolving power increases. In other words, the product of these

two parameters is a constant. It has been shown that the LR product for atomic vapor

narrowband imaging detectors, in particular resonance ionization image detectors, can be

higher than 109 cm2 sr. Another important feature of atomic vapor detectors is that under

certain conditions (laser-cooled atoms), there is no trade-off between L and R; i.e., the LR








product is not constant, which means that an improvement in luminosity does not come at

the expense of decreased resolution (Figure 1-2). This trend is observed for heterodyne

detection technique as well. However, heterodyne systems have an inherently low field-

of-view, hence low luminosity in addition to complexity of the systems. Large

interference filters (Fabry-Perot or Michelson) can offer theoretically unlimited small

bandpass, but are extremely angle-sensitive and require additional pre-filters for efficient

background rejection. The RIIDs provide extremely narrow bandwidth (typically less

than a GHz) and simultaneously a hemispherical field-of-view, and therefore exhibit an

exceedingly high signal-to-noise ratio, which is influenced directly by the LR product.


102 Hg RIID (laser cooled)

10 Room temperature

1Z
C4 -2 -
10
"gi'- ----: u^s



S10-4
Diffraction grating (ideal)
1o Conventional 4 min
0-& -p*& 111l 1 Infrared lItm

Heterodyne detection 1W 0.25 lin

1 10 .*.... .. 1.. 1. ......
105 106 107 10s 10 100
Resolving power

Figure 1-2. Calculated LR product for selected spectroscopic systems [17].

Several imaging devices based on resonance absorption of radiation in an atomic

vapor have been reported. The first resonance ionization imaging detector based on








mercury atoms was developed by Matveev et al. [18]. A three-step ionization scheme

was used with the 253.7 nm resonance transition of Hg in the first step (the detection

wavelength). The electrons created in the ionization process were accelerated toward a

luminescent screen by a voltage applied between the screen and the quartz input window.

A sealed-cell compact version of the Hg RIID with microchannel plate was reported [19].

A resonance fluorescence imaging monochromator (RFIM) based on mercury was

described by Finkelstein et al., [20] and Matveev et al., [21] with one and two-step

excitations, correspondingly. In the RFIM, a fluorescence of the excited atoms is

detected. The spectral line profile of naturally occurring mercury atoms is 25 GHz. The

best reported spatial resolution has been 130 tm [19].

A cesium-based RFIM has been described and evaluated for space communication

satellite tracking by Korevaar et al. [4]. The most recent cesium RFIM was reported by

Pappas et al., with less than 200 tm spatial resolution. The spectral resolution of 400

MHz was limited by the Doppler broadened line profile of Cs atoms at room temperature

[22]. The spectral resolution of the Cs RFIMN was further improved (270 MHz) using a

Doppler-free excitation scheme [23]. A detector of far IR radiation using Rydberg atoms

has been reported by Drabbels and Noordam with 300 (Lm resolution [24]. They produced

a sheet of Cs Rydberg atoms in a vacuum chamber (10-6 mbar) by a UV light and directed

an IR radiation, carrying the image of the mask, perpendicular to the sheet. The detector

was sensitive to all IR photons with energies sufficient to ionize weakly bonded Rydberg

atoms, and therefore showed poor spectral selectivity.

A RFIM is similar in principle of operation to a RIID in that both detectors are

based on the narrowband absorption of radiation by an atomic vapor, and both detectors









have similar spectral resolving power. However, signal detection in RFIM is based on

fluorescence and in the RIID on photoionization. Typically, the cross section of

photoionization is 3-4 orders of magnitude larger than the fluorescence cross section. The

superiority of an RIID over RFIM in terms of sensitivity is obvious.

With this brief review of the modem spectrally selective 2-D imaging techniques,

we proceed to the next chapter, which will introduce the RIID design and principle of

operation in detail. Once again, the ultimate goal of this work was to develop a

methodology and device for a two-dimensional photon detection and imaging based on a

narrowband absorption in atomic vapors. To accomplish the goal, a novel cesium RIID

has been developed and extensive temporal studies of the mercury RIID have been

performed.













CHAPTER 2
INTRODUCTION TO RESONANCE IONIZATION IMAGE DETECTORS

Introduction

This chapter describes the general and detailed principle of operation of an RIID.

The performance of the sealed-cell Hg RIID, the only existing RIID until this research,

will be discussed along with some inherent and design problems discovered during the

extensive previous studies [18, 19, 21, 25-27]. Also, chemical elements, that could

potentially be used as an active element in RIIDs, will be listed with the comparison of

important parameters.

RIID Principle of Operation

Figure 2-1 shows a general detailed schematic of the sealed-cell RIID using a three-

step ionization scheme. The principle of operation of the RIID is based on the

narrowband absorption of radiation by an atomic vapor in a cell and subsequent laser

ionization of the atomic vapor filling the cell. The cell consists of an input window and a

phosphor screen, which are coated with a thin layer of metal in order to apply a high

accelerating voltage. Also incorporated in the cell is a microchannel plate (MCP) for

amplification of the photoionization signal. An "image carrier" beam at 2A (detection

wavelength) enters the cell through the input window and excites the atoms from the

ground state to the first excited state. This excitation occurs only in the region of the

atomic vapor which is a 2-D projection of the object being imaged. Depending on the

application, the image carrier beam, reflected or scattered from the object, can be either

of the same wavelength as a probe laser (from a static object) or its wavelength can be





10

Doppler- or Raman-shifted with respect to the probe laser beam. In the latter case, this

shift should be compensated by detuning the probe laser beam in order to bring the

scattered radiation to the detection wavelength at 2A (see for example Figure 1-1). Two

other laser beams at the wavelengths of 22 and 2A3, coming in through the side windows as

a thin sheet of light, form a planar region where atoms are excited to higher states and

photoionized. This ionization region should be as close to the input window as possible in

order to obtain the highest possible signal.


Ion k2 Ionization Region ja lm
W Atom /
Electron _"_" _

l to CCD




... .... .
0

Input Phosphor
Window 4 MCP Screen


Figure 2-1. Schematic diagram of a sealed-cell compact RIID.

By switching the polarity of the voltage applied between the input window and the

MCP, either formed ions or electrons can be detected. Figure 2-1 shows the case where

the ions are accelerated to the MCP. Electrons produced by charged particles striking the

MCP are accelerated by an electric field to a luminescent phosphorr) screen, where the

image of the object is captured by the CCD camera. In terms of design, the RIID is

similar to a standard image intensifier. Image intensifiers use a photocathode to transduce


Grun sat








an optical signal into an electrical signal; the electrons, amplified by the MCP

incorporated in the image intensifier, are then accelerated toward a luminescent screen. In

the RIID, the photocathode is replaced by a high-density atomic vapor. The optical image

is transduced into an electrical signal by the laser stepwise ionization of the atomic vapor.

Sealed-cell Mercury RIID

Previously a sealed-cell Hg RIID was designed and evaluated in our laboratory.

This compact cell was 5 cm in diameter and 4 cm in width, with a front input widow

diameter of 2.5 cm and side window widths of 0.7 cm [19, 27]. The cell was constructed

according to the diagram shown in Figure 2-1 (NPP Radian, Moscow).

In laser resonance ionization photon detection, which is the basic principle of the

RIID, it is desirable to have an efficient and simple photoionization scheme using a

minimal number of lasers and minimal energy of laser radiation. A stepwise

photoionization scheme of Hg has been the subject of many studies [18, 21, 25, 28, 29].

In all schemes, the first step was a resonance transition from the ground state at 2A =

253.65 nm. Figure 2-2 shows two successfully realized and studied schemes for the Hg

RIID routes of photoionization, one of which will was used in this work and will be

discussed in following chapters.

It has been shown that the sealed-cell Hg RIID was a very sensitive detector of

light; as low as 1000 photons have been detected by image summation [19]. The detector

showed a 130 jim spatial resolution when the ionic component of the imaging signal was

detected [19]. However, a few inherent and design problems have been encountered with

the detector. The detection wavelength of the detector (253.65 nm) was in the deep UV

region. First of all, this limits the application of the detector to important in vivo medical






12


applications such as imaging of blood circulation and tumors, because of the harmful

effects of UV radiation to human tissue. In addition, studies have shown an unwanted

photoelectric effect from the metal coated input window and the surface of the MCP due

to the UV transition of Hg [19, 30]. The energy of A,1 = 253.65 (4.89 eV) is higher than

the work-function of the metal coatings of the input window and MCP. These non-

selective electrons are a source of additional noise in the measurements. Recent studies

have shown that the detector suffers from temporal image distortions due to surface

charges accumulated on the input window [31].


'S' 0S, 'P -D
ns ns np nd
/rzizz//// LLL/J/////, 10.44 eV


C 'C
S 8.85eV -8

7.73 eV 7- 7
\% /


4.89 eV 6





-6 Hg


Figure 2-2. Partial energy level diagram and selected transitions of mercury.

Active Element for RIID

The performance and figures of the merit of an RID are determined by the physical

characteristics and spectroscopy of the active element of the system. As mentioned

earlier, the narrowband atomic vapor detectors with Hg and Cs have been reported in the








literature [18-24]. The choice of the active element is limited to a few atoms such as Rb,

Cs, Hg and inert gases which provide sufficient atomic vapor density at room temperature

to absorb a large number of resonance photons at the detection wavelength. Depending

on the application, one may be advantageous over another [5]. Table 2-1 lists potentially

suitable elements for the construction of the RIID, along with some important related

parameters.


Table 2-1. A list of elements suitable for RIID construction.

Element Hg Cs Rb He Ne Ar Kr

IP (eV) 10.4 3.9 4.2 24.6 21.6 15.8 14.0

MP(C) -39 29 39

,nsa(cm-3) 6x1013 4x10



Table 2-1 shows the first ionization potential, melting point and saturated vapor

density of potentially candidate active elements for RIID. The listed three metals, Hg, Cs,

and Rb, are preferred over noble gases, because the noble gases are difficult to ionize and

their electronic transitions lie in the deep UV spectral region, which may be difficult to

reach with commercially available lasers. Among the three metals, Rb may need

additional heating in order to obtain a high enough vapor density, due to its high melting

point. That leaves only Hg and Cs, two elements based on which resonance ionization

imaging detector's have been developed to date and those are the subject of this research.

It is pertinent here to compare closely essential physical and spectroscopic

characteristics of these two elements (Table 2-2), since the active element determines the

detector's operation and performance.








Table 2-2. Comparison of Hg and Cs properties.

Properties\Element Hg Cs

Detection wavelength (nm) 253.65 852.12

Oscillator strength of the detection transition,f 0.025 0.72

table isotopes 7 1

Atomic linewidth at room temperature, AX (GHz) 25 0.38

Chemical reactivity Low High

Photoelectric effect(PE) Yes No

Biological applications Limited Yes



The detection wavelength for Hg is 253.65 nm and for Cs is 852.12 nm. The

oscillator strength of the transition at the detection wavelength is much greater for Cs.

Mercury has 7 stable isotopes, whereas Cs has only one. The atomic line broadening at

room temperature is 25 GHz for natural Hg, which is a Doppler broadening with all

mercury isotopes taken into account. Doppler broadened atomic linewidth of

monoisotopic Hg atoms is approximately 1 GHz. Doppler broadened atomic linewidth is

only 0.38 GHz for Cs, making Cs more spectrally selective active element for the RIID.

The calculated cross sections of the transitions at the detection wavelength for Cs and

monoisitopic Hg atoms are 5.6x 10" cm2 and 5.7x 1013 cm2, respectively, showing the

better sensitivity of the detectors based on Cs atoms, provided that the number density of

atoms is sufficient to absorb the large number of signal photons. The saturated number

density of Hg atoms at room temperature is 3 orders of magnitude greater than that of Cs.

The UV detection wavelength of mercury may cause an unwanted photoelectric effect










(PE) from the metallic surfaces of the detector, which has been observed for sealed-cell

Hg RIID. For Cs, this effect is not expected and will be shown experimentally to not

occur. The infrared detection wavelength of Cs, which is safe for human tissue and can

penetrate deeply through a tissue, will allow biological applications of the detector. The

UV radiation at 253.65 nm may be harmful for human tissue. Despite the apparent

disadvantages, Hg RIID can provide better spatial resolution and signal-to-noise ratio

because it operates at a shorter wavelength. The LR products of Hg RIID and selected

high spectral resolution devices were compared in Figure 1-2. Figure 2-3 compares the

LR product for two Cs and Hg RIIDs with 100 cm2 acceptance area. It should be

mentioned that there is no practical limitation for construction of the detector with

considerable larger size.



10 3

-Hg, 1.00 GHz
.....Cs, 0.38 GHz

C" 102
NM .
E

C..)
0
.C
2
I 1 ".. ..." ...........





010-


105 106 107 108 109 101
Resolving power



Figure 2-3. Calculated LR product for Cs and Hg RIIDs for Doppler-broadened
absorption profiles at room temperature.








In addition to better spatial resolution, mercury is not particularly reactive with the

cell materials such as quartz, metal coatings on the windows, electrodes, and thus a

sealed-cell Hg RIID was easily constructed. However, some difficulties must be

overcome in order to construct a sealed-cell Cs RIID due to the reactivity of Cs. The

feasibility of sealed-cell Cs construction from compatible with Cs materials will be

discussed in Chapter 6.

Conclusion

In this chapter, the general principles of operation of a resonance ionization

imaging detector have been discussed. The Hg RIID developed prior to this work proved

that the RIID could provide efficient photon detection with comparable imaging

performance to conventional techniques. Also, it has been shown that Cs offers certain

advantages over Hg as an active element in RIID. In the following chapter, ionization

schemes of Cs and laser systems appropriate for the RIID are experimentally

investigated.













CHAPTER 3
CS PHOTOIONIZATION AND LASERS

Introduction

In order to reach the best performance of the Cs RIID, an ionization scheme had to

be carefully chosen, for it is the major factor determining the quantum efficiency

(sensitivity) of the detector. If high-energy pulsed radiation is used for photoionization,

theoretically there is no fundamental limitation to obtaining a near-unity ionization

efficiency. A single-photon detection capability of the RIID has already been

demonstrated [29]. A pulsed RIID based on the photoionization with powerful lasers is

therefore advantageous over the cw diode laser-based RFIM's mentioned in Chapter 1

with regard to quantum efficiency. Currently, commercially available diode lasers cannot

provide sufficient energy for effective photoionization.

Despite the advent and achievements in the field of lasers in general and especially

in the field of tunable lasers, still there is no single laser that would satisfy the end user in

terms of availability, practicality, cost, tunability, power characteristics. For example,

tunable dye lasers are a valuable research tool, covering a wide spectral range, from deep

UV to mid-IR. However, they are difficult to use and maintain, in addition to being

unable to provide sufficient power for some applications. Tunable and powerful sources

of radiation would broaden the range of applications of lasers and/or improve the

performance of existing applications.

This chapter describes the spectroscopy of Cs as well as the laser system used in

these studies. An efficient ionization scheme will be evaluated based on the spectroscopic








parameters of Cs from the viewpoint of the availability and practicality of the commercial

lasers. Results of the non-imaging Cs resonance ionization studies will be shown.

Cs Photoionization: Theoretical Considerations

Cs is an unique metal, an element with the lowest ionization potential of all

elements (3.89 eV). Due to its unique properties, Cs atoms have been a subject of many

studies [32-39]. Cs atoms have been used as narrowband atomic filters in a number of

studies [40, 41, 42]. When choosing an excitation scheme for resonance ionization

imaging detectors, two factors are most important, the cross section of the transitions and

the availability of radiation sources for chosen transition. The energy required to saturate

the transition is directly proportional to the cross-section of the transition, which can be

approximated with the following formula (in cm2):

S= 8.19x1020 fk "J2ik (3-1)
A2,

where, fk is the oscillator strength of the given transition dimensionlesss), Aik is the

detection transition and AAD is Doppler broadened atomic linewidth at the given

conditions (both in nm). For Cs, the natural linewidth is 5 MHz and the Doppler-

broadened linewidth at room temperature is 380 MHz. The width of the absorption

profile is determined by external factors, such as temperature and nature of the

environment, whereas the oscillator strength is an intrinsic property directly determining

the cross-section of the transition.

Figure 3-1 shows two ionization routes for Cs. The transitions of Cs

6s2S/2->6p2P3/2 (852.12 nm) 6p2P312->6d2D3/2 (917.23 nm), shown in Figure 3-1, have

an oscillator strength of 0.72 and 0.33, respectively, and are the two strongest transition

of Cs. Logically, an efficient ionization scheme should include these transitions.









2 11/ 2 -F.V 2D-512
ns np nd
/////////// /////////'//// 3.89 eV



6
I 7- 2.81eV





6 1.46eV








6 Cs


Figure 3-1. Partial energy level diagram and selected transition of Cs atoms.

The excited 6d2D32 state lies 1.08 eV below the ionization limit of Cs (3.89 eV). Ideally

the photoionizing laser energy h v should match the energy difference between the excited

state and the ionization limit. The photoionization cross-section is frequency-dependent

and drops drastically when photoionization energy considerable exceeds the ionization

limit [43]. The simplest laser available for photoionization of Cs from the excited 6dD312

state is the fundamental harmonic wavelength of Nd:YAG laser at 1064 nm with photon

energy of 1.16 eV. In order to obtain the highest possible efficiency (-100%), all steps

in the stepwise ionization must be saturated. Calculated cross-sections of the

6s2S12-->6pP32 (852.12 nm) 6p2P3/2->6d2D3/2 (917.23 nm) transitions according to








formula (3-1) are 5.6xl0"11 cm2 and 2.5xl011 cm2, respectively, for the Doppler-

broadened absorption profile of Cs at room temperature (380 MHz). Therefore, saturation

of these transitions in the experiments can be easily reached by commercially available

compact, low-cost diode lasers. The photon saturation irradiance of the transition is given

by (in photons.cm-.s -):

1=
I o- (3-2)
2crr

where, r is the lifetime of the excited state (s), assuming the laser beam profile matches

the atomic absorption profile at the given conditions and the laser pulse duration is

shorter than the lifetime of the excited state. For the Cs transition from the ground state

(852.12 nm), the saturation irradiance is calculated to be -3x1017 photons.*cm-2.*s-1, which

can be reached with typical diode lasers with a power of a few mW.

However, the situation is dramatically different for the final photoionization step

due to its non-resonance nature. The calculated or experimental value of the

photoionization cross-section from the excited 6d2D3/2 state have not been found in

literature, but can be expected to be on the order of 10-16 1017 cm2, which is 5 6 orders

of magnitude smaller than the strong transitions of Cs mentioned above. Therefore, it is

critical to have a very high-power pulsed laser in the third step for obtaining the highest

possible ionization efficiency.

Lasers: Availability

Once the question with the spectroscopy of the atoms clear and the transition are

chosen, the next important issue is the selection of light sources for excitation and

photoionization of the chosen transitions. One of the most often used and flexible light

sources in atomic spectroscopy is the tunable dye laser and it is useful for a wide








wavelength range of the electromagnetic spectrum. However, the dyes tunable in the IR

region are very inefficient and unstable, and require frequent change of the toxic dye

solution. Radiation at 852.12 nm can be obtained with a dye laser operating with a

solution of Styryl-9 in DMSO (Lambda Physik, Acton, MA). The maximum energy

conversion efficiency given by the manufacturer is only 9% when pumped by a XeCl

Excimer laser at 308 nm. For laboratory conditions, this efficiency is expected to be even

less. Despite this, the dye laser with Styryl-9 can operate with relatively stability and

acceptable efficiency at 852.12 nm for short period of time. The only available dye for

917 nm, HITCI, has 4% efficiency and is much less stable compared to Styryl-9. The

more efficient dye Styryl-14 has recently been discontinued, reflecting the interest in all

solid-state and semiconductor lasers.

A number of solid-state lasers have been developed as the result of the research in

the area of tunable solid-state lasers and non-linear crystals, providing an efficient

tunable source of radiation in the IR spectral region [44, 45]. Advantages of solid-state

lasers over dye lasers are compactness, efficiency and ease of operation. Lasers based on

LiF:.-F2+ color centers are very attractive as highly stable and powerful sources of tunable

radiation, exhibiting high gain and a low threshold of laser generation. A stable, efficient

LiF:F2+ laser, tunable in the 800-1200 nm region, has been evaluated and described in the

literature [46, 47]. A unique and practical source for excitation of Cs atoms would be a

LiF:F2+ laser capable of producing the 6s2S,/2--6p2P3/2 (852.12 nm) 6p2P3/2->6d2D3/2

(917.23 nm) transitions simultaneously since it would simplify the spatial alignment,

beam geometry, and the timing of the excitation pulses. This feature has already been

realized with tunable dye lasers and described in the literature [48].







Cs Photoionization: Experimental Studies
Both ionization schemes shown in the Figure 3-1 have been experimentally
investigated using the lasers available in the laboratory, including tunable dye lasers
pumped by an excimer laser at 308 nm, and LiF.F2+ color center pumped with various
sources. The ionization schemes were evaluated for a Cs resonance ionization detector
(RID), by measuring the ionization signal from a cell shown in the Figure 3-2. The cell is
an evacuated Pyrex cylinder (60 mm long, 25 mm diameter) containing solid Cs (Opthos
Instruments). The cell is furnished with a pair of nickel electrodes with feedthroughs for
application of voltage and detection of ionization signal. The design of the cell allows the
light can go in only through two side windows and can not be used for imaging.



)"




Figure 3-2. A schematic diagram of the cell used for Cs RID evaluation.

Two-step Ionization
Figure 3-3 shows the schematic diagram of the experimental setup of the two-step
ionization studies. Two dyes lasers (Molectron DL-II, Portland, OR) pumped by an
excimer laser (LPX-240i, Lambda Physik, Acton, MA) operated with XeCl (A = 308 nm)
were used in the experiments. The first dye laser at 852.12 nm entered the cell from one
end of the cell and excited the Cs atoms from the ground state into the first excited state
(62P3/2). The energy difference between the excited state and ionization limit is 2.44
eV. This corresponds to laser energy with a wavelength of- 508 nm.






23





Beam splitter Mirror

Excimer 3
o| 5308 nm




Dye Dye
laser laser
1 2



S. ...Cs cell
E) Oscilloscope __
c,,- ______ \,852 nm ~^-508 nm






Figure 3-3. A schematic diagram for two-step ionization setup.


The counter-propagating second laser was frequency scanned in the tunability

range of the dye from -490 nm to 530 nm and had a maximum in the vicinity of 508

nm. The ionization signal was observed with an oscilloscope. Figure 3-4 shows the

ionization signal as a function of ionization laser tunability.


150
508 nm
125-
E
S100- 490 nm
C
t0 75 -
C \
.o 530 nm
I 50-
o \\
S 25-

0
-25

-25 -- -- -- -- -- -- -- --
-15 -10 -5 0 5
Time (ps)



Figure 3-4. Two-step ionization signal of Cs as a function of wavelength.









Three-step Ionization

For three-step Cs ionization, a system based on Nd:YAG laser and LiF:.F2+ crystal

color center laser was used. The schematic diagram is shown in the Figure 3-5.


telescope
Nd:YAG
< M,
Glass Raman 1 'nm KTP 'nm
Plate shifter r



MV L
Plt e



Cs 1
% ~ M5
M,

Oscilloscope 9 nm
;.--, / 852 nmvSJ LiF:F \
G, rating 1- !? | '


Figure 3-5. A schematic diagram for three-step ionization setup.

The laser beam with the diameter of -6 mm at 1064 nm from Q-switched Nd:YAG

(Laser Photonics) laser was used to pump the Raman wavelength shifter with Ba(N03)2

crystal (Solar, Belarus). The Energy of the Nd:YAG laser was -80 mJ, with 10 ns pulse

duration in multi-mode regime with repetition rate of 1-10 Hz. The telescope was used to

reduce the diameter of the laser beam to -2 mm. The shifted radiation at 1198 nm (-12

mJ) was focused by the cylindrical lens L1 into the KTP crystal for the second harmonic

generation. The radiation at 599 nm (-2 mJ) was separated from the fundamental 1198

nm by M1 cold mirror (Edmund scientific). Radiations A, = 852.12 nm and A.2 = 917.23

nm were obtained simultaneously by pumping the LiF:-F2+ crystal with 599 nm radiation.

The size of the crystal was 40x20x6 mm3 and the input windows were cut at Brewster








angle to reduce the reflection. The 599 nm laser beam was directed by an aluminum

mirror Mz and focused by a lens L2 onto the crystal. A resonator was formed by a pair of

aluminum mirrors (M4, M5) and M3 hot mirror (Edmund Scinetific). The hot mirror had a

high reflectance for 750-1000 nm and high transmission for the pump radiation (599 nm).

The diffraction grating with 1200 gr/mm was used as the wavelength selection element in

the resonator. The distance between M3 and the diffraction grating was -10 cm. A blaze

angle of the diffraction grating was chosen to be small (5) in order to expand the

tunability of the laser. Each of the two resonator mirrors M4 and M5 reflected a portion of

the zero-order diffracted beam. The mirrors could be adjusted separately to provide two

independently tunable output radiations at 2A, = 852.12 nm and 22 = 917.23 nm. A portion

of the fundamental radiation of Nd:YAG laser at 1064 nm was directed and focused to

the cell by a glass plate, M6 mirror and L3 lens. The ionization signal of Cs was

monitored in an oscilloscope by applying a 12 V bias voltage to the electrodes as in the

case of two-step ionization. Energy of the lasers at A, = 852.12 nm and 22 = 917.23 nm

were -50 [J each. The energy of the photoionization laser at 1064 nm (the third step) was

-8 mJ.

The results of the experiment are shown in the Figure 3-6. The curve 1 represents a

resonance three-step (852-*917->-1064) ionization signal of Cs atoms. The signal,

although very small, was observed in the absence of 1064 nm radiation due to Cs

photoionization with the same 917 nm radiation in the third step (curve 3). In addition,

there could be some contribution of the collisional excitation/ionization in the signal

since the cell was under saturated vapor pressure. Also, a small signal was observed from

1064 nm radiation only (curve 2). In this case, the signal was believed to be due to






26


collisional excitation of Cs atoms and photoionization with 1064 nm radiation. The ratio

of the signal when all three beams were present to the signal when only 852 and 917 nm

radiations were present was more than 350. The ionization signal with three-step scheme

was approximately 200 times larger compared to the signal with 852->917 two-step

scheme (Figure 3-4). This means that with the powerful Nd:YAG laser for the third

photoionization step, an ionization efficiency improved more than 2 orders of magnitude

for the given experimental conditions.



4.0

3.5

3.0 1

2.5

2.0
1.5
U)
1.0

0.5


-0.0-

0 20 40 60
Time (ts)



Figure 3-6. Three-step ionization signal of Cs; curve 1 represents a resonance three-step
(852-*917--> 1064) ionization signal, curve 2 signal from 1064 nm radiation
only, curve 3 signal with two wavelength present (852->917). Note that
signal values in curves 2 and 3 are multiplied by 100 for illustration purposes.


Lasers: Practicality

Despite the dramatic increase in an ionization efficiency of the three-step

ionization, the configuration shown in Figure 3-5 was too complicated and far from

practical, especially for applications to imaging. Obtaining an appropriate wavelength for








pumping the LiF.F2+ crystal was done in multiple steps involving two non-linear

phenomena, first shifting the fundamental output of Nd:YAG laser and then second using

the harmonic generation from the wavelength shifted radiation. This could be done only

with very low efficiency (approximately 2% in this case).

As an alternative source of radiation to pump the LiF.F2+ crystal, a single-

wavelength (-610 nm) dye laser with Rhodamine B was constructed. A dye solution

circulating cuvette was placed between a pair of mirrors, one with 100% reflectance, the

other one with 20% reflectance, thus forming a resonator without any dispersive element.

The dye laser was pumped with an XeCl excimer laser (308 nm). The stronger laser beam

to pump the LiF:F2+ crystal, compared to the case depicted in Figure 3-5, was obtained

with a pulse energy of approximately 5 mJ. Despite this improvement, when pumped

with this radiation, the output of the LiF:.F2+ crystal did not considerably improve,

probably due to beam quality at 610 nm. In addition, coupling this system with an

Nd:YAG laser for efficient three-step ionization would require a complicated

synchronization circuit.

Conclusion

In this chapter the efficiency of three-step ionization scheme for Cs has been

theoretically discussed and experimentally validated. However, the laser system for the

three-step ionization scheme was not practical and was not applied to present imaging

studies. The only practical and reproducible laser system on hand suitable for Cs

photoionization was two-step ionization with dye lasers pumped by an excimer laser.

Using this two-step ionization scheme, the imaging capabilities of Cs RIID was evaluated

and will be presented in the next chapter.














CHAPTER 4
CS RIID CHARACTERIZATION

Introduction

A novel Cs RID reported in this work, inherited many design features from the

sealed-cell compact Hg RIID, developed and evaluated in our laboratory [19]. In this

chapter, the first images obtained with the Cs RIID will be shown. All experiments

reported here have been performed using the two-step photoionization scheme

(852->508) of Cs, described in the previous chapter. The dependence of the images on

the parameters of the system, such as high voltage applied to the system, Cs vapor

density, and exciting/ionizing laser pulse energies, will be evaluated.

Experimental Setup

Figure 4-1 illustrates the schematic of the experimental setup used for evaluation of

the detector in imaging mode of signal detection. The detector consisted of a metal-

coated input window, a 25 mm long linear Cs metal dispenser, and a microchannel plate

coupled with a phosphor screen in a vacuum chamber as shown in Figure 4-1. A six-way

cross chamber was constantly evacuated with a mechanical and turbo-molecular pumps.

Vapor of Cs was produced using the linear dispenser (SAES Getters USA Inc., Colorado

Springs, CO). The dispenser contained a mix of alkali metal chromate (Cs2CrO4) with a

reducing agent to provide a controlled amount of alkali metal. The Cesium chromate salt

and reducing agent mixture was held within a nichrome metal container having a

trapezoidal cross section with a slit to allow evaporation of the alkali metal (Figure 4-2).










When heated under vacuum by flowing a current through, the dispenser released atomic

Cs as the result of an oxidation-reduction reaction.


Figure 4-1. Schematic diagram of Cs RIID setup.


- FT type-


3!w
UtD 't
4 *.
0U*


a


Figure 4-2. Schematic diagram of the Cs metal dispenser [SAES Getters USA Inc.
http://www.saesgetter.com].








A two-step ionization scheme described in the previous chapter was used for the

imaging experiments. The first dye laser beam, at the detection wavelength of

A/= 852.12 nm, entered the cell through the input window carrying an image of the

cross-shaped transmission mask and excited the Cs atoms from the ground state into the

6 P3/2 state. This excitation occurred only in the region of the atomic vapor which was

illuminated by A1 laser beam passing through a mask and the perpendicular A2 laser beam.

The second photoionizing dye laser beam, at 22 508 nrim, was directed from the side,

perpendicular to A,, as a thin vertical sheet of radiation and ionized the Cs atoms. The

distance between the input window and MCP was 30 mm and could be adjusted. The

distance between the MCP and the phosphor screen was fixed at 10 mm. Figure 4-3

shows the photograph of the test image with dimensions, which is a four-wing cross-like

shape cut through thin metal sheet. Wings do not meet at the center, so the center of the

cross is opaque.


Figure 4-3. Photograph of the test mask with dimensions.








Figure 4-4 shows the voltage division between electrodes (input window, MCP,

phosphor screen) of the RIID. A negative voltage between -9 kV and -11 kV was applied

to the MCP and the input window was grounded to accelerate Cs ions to the MCP. The

phosphor screen was held positive relative to the MCP in order to accelerate the electrons

created by ions striking the MCP.


MCP



Input Phosphor
Window Screen





r / \ _80 Mn
-(9 11)kV

120 MQ 30 M


Figure 4-4. Voltage division between electrodes of the Cs RIID.

Experimental Results and Discussion

Image Acquisition and Processing

The images of the test mask formed on the phosphor screen were captured by a

monochrome CCD camera (PC23, Supercircuits, USA). The CCD camera was interfaced

with a computer via an image acquisition board (PCI 141, National Instruments, USA).

The image frames were synchronized by pulses from the excimer laser. The images were

acquired in 8-bit gray-scale mode (256 shades of gray, 0 for black and 255 for white)

with the IMAQ Vision Builder 6.0 software package (National Instruments, USA).

Figure 4-5 shows examples of images that could be obtained and processed with the





















































c)M
Figure 4-5. Two and three dimensional representations of: a) a single image; b) a filtered
single image (see text for filter description); c) average of 20 images.








given hardware and software for images acquisition. Three dimensional views of the

images were produced in IMAQ Vision Builder and presented for illustration purposes.

In Figure 4-5, a single image, a single filtered image and the average of 20 images are

compared (a, b and c). The IMAQ Vision Builder convolution filter highlights regions in

the image where there are sharp changes in pixel values. These regions correspond to the

boundaries of the mask and other noisy pixels that may be present in the image. Table 4-1

shows the improvements in the image signal-to-ratio. In this work, a signal-to-noise ratio

of the images was defined as the ratio of a maximum pixel value of the image to the

standard deviation of the pixel values along the vertical dashed line as shown in Figure

4-5a. All the images reported in this chapter are averages of 20 individual images.


Table 4-1. Signal-to-noise improvement with image averaging and filtering.

Signal Noise Signal-to-noise
ratio
a) 143 7.8 18

b) 252 10.7 24

c) 164 6.4 26



Signal vs. High Voltage

Figure 4-6 shows the dependence of signal on the high-voltage applied to the

system. In order to correlate the image intensity with the photoionization of Cs, signal

was calculated as the total intensity of the images, i.e. sum of all the pixel intensities of

the images of the test mask. Calculated in such a way from the images, signal or image

intensity would be proportional to the resonance photoionization signal of the Cs atoms.

An almost exponential signal increase with high voltage is supposed to be determined by






34


the non-linear gain of the MCP with the applied voltage. The S/N ratio as defined in the

previous section was monitored and reached a maximum at about -10.1 kV; and any

further increase of the negative high voltage did not improve the signal-to-noise ratio. At



240-

220-

200-

180-

160-

S140-

120-
01) Ii
01
S100-
E

80-

60

40-, I I -,
9.6 9.8 10.0 10.2 10.4 10.6
HV (kV)



Figure 4-6. Signal vs. high voltage applied to the system.


-10.5 kV, the pixel intensity saturated, reaching the maximum gray level of 255,

indicating that the images obtained at potential more negative than -10.5 kV would not

truly represent the photoionization of the atoms. Further correlation of the image intensity

and ionization signal would require attenuation of the luminescence of the phosphor

screen. A conclusion from these measurements was that the optimal operating voltage

was -10.1 kV for the given configuration of the detector and experimental conditions.

Other system parameters were studied at the optimal voltage.








Signal vs. Number Density of Cs Atoms

Absorption of resonance photons is directly proportional to the number of

absorbers n, i.e. number density of Cs atoms which was varied by changing the current

flowing through the linear dispenser. The studied range of number densities between 107

and 108 cm-3 was significantly small compared to saturated number densities of Cs atoms

at room temperature (n,, 4.1010 cm3). The number density of Cs atoms was estimated

by measuring the absorbance with a hollow cathode Cs lamp. Figure 4-7 shows the series

of images taken at different number densities of Cs atoms. Even at such low number

density of Cs atoms, images with high contrast ratio were obtained, indicating large

photoionization signal. Such high signals were due to the strong cesium transition at the

detection wavelength, hence large cross-section of the absorption.


















Figure 4-7. Series of images at different Cs number densities (cm3). From left to right,
densities are: 1.3x 107, 2.4x107, 6.1x107; bottom row: 1.2x 108, 1.9x 108.

Figure 4-8 is a log-log plot of signal (total intensity of images) vs. Cs number

density. Theoretically, the signal should depend on the number density of atoms via the

expression of the absorption factor given by [5]:










a = exp[-cn/] (4-1)


where I is absorption path length (in cm), i.e. the penetration depth of 2A into Cs vapors, a

is the absorption cross-section (in cm2) defined in equation (3-1) and n is the Cs number

density (in cm3). Generally, both 1 and a depend on the detection wavelength [49, 50]. In

order to achieve a high signal level the following condition must be fulfilled:


onl t 1 (4-2)


For optically thin conditions (nl -* 0), which was valid for our case, the absorption factor

is linearly related to the number density of atoms n for small values of n. At high number


density of atoms, a reaches its limiting or saturation value. It is seen from the Figure 4-8

that at the given experimental conditions, the signal vs. number density of Cs atoms

shows similar to typical atomic absorption experiments behavior. Saturation level of the

signal was not reached in the studied range of Cs number densities.



1000







cn
C
C
0)
loo
o 100
E
0)
o
_j




0.1 1 10
Log Cs number density (x108 cm3)


Figure 4-8. Signal vs. number density of Cs atoms.








The experimental values of the absorption coefficient, number density of Cs atoms and

absorption path length were 5.6x10-" cm2, 1.9x108 cm3 and 5 cm respectively. The

product of these quantities was equal to 0.05. In the experiments the number density of

Cs atoms were estimated by measuring the absorbance with a hollow cathode Cs lamp.

At n = 1.9x 108 cm3, saturation of the pixel intensity of the CCD camera was observed.

When dealing with atomic population of atoms with strong resonance transition,

such as Cs atoms, one must consider the effect of radiation trapping (diffusion). The

excited Cs atoms may be de-excited by emitting resonance radiation at 852.12 nm before

being ionized. This process may occur repeatedly and result in ionization at points far

away from the point where the original incident photon was absorbed. This phenomenon

is more pronounced for dense atomic populations. Despite the low Cs number densities

studied in this work, with an increase in the number density of atoms, a small degradation

(<10%) of spatial resolution of the images was observed (Figure 4-7), which was

attributed to the resonance radiation trapping (diffusion) effect. The effect of resonance

radiation trapping on the spatial resolution of the detector is expected to be minimal or

even negligible if one uses an efficient ionization scheme and lasers. Because of the

broad linewidth of the laser used in our experiments, the efficiency of ionization was low.

Therefore, for the best detector performance, it will be critical to have a laser system

capable of providing 100% efficient ionization, which is theoretically possible for a

method based on laser photoionization of atoms.

Signal and Resolution vs. Laser Pulse-energy

Figure 4-9 shows dependence of the signal on the irradiance of the excitation and

ionization lasers. The linewidths of the dye lasers used in the experiments were estimated












to be 1-1.5 cm1 (30 45 GHz), very large compared to the Doppler broadened profile of


Cs atoms at room temperature (380 MHz). Again, because of to this broad linewidth of


the lasers, the pulse energies of the lasers were not enough to saturate the transitions,


even the strong transition from the ground state at 852.12 nm, having oscillator strength


of 0.72. This can be seen from the plots of signal vs. laser irradiances in Figure 4-9. The


laser irradiances were varied using a set of calibrated filters.


280-
260-

240-
220-

5 200-
'r 180-
160-
4)
M 140-
E
120-
100-
80 .. .


S 10 20 30 40 50 60
Irradiance of X, (,JIcm -2)


70 80


160-


" 120-
4)

t 80-
E


40-


0 2 4 6 8
Irradiance of 42 (mJ/cm')


Figure 4-9. Signal vs. laser pulse energy: left 2A = 852.12 nm; right A2 = 508 nm.


22
Figure 4-10. Spatial resolution vs. laser pulse energy at X2. Irradiance: left 0.7 mJ/cm;
right 40 mJ/cm2.


10 12








Both signal and spatial resolution depend on the power characteristics of the lasers

used in the excitation scheme. It was especially important for the final ionizing step

because of the low cross-section of photoionization due to non-resonance nature of the

transition. Figure 4-10 illustrates the improvement in the spatial resolution with increase

in the irradiance of the second photoionizing laser beam at 508 nm. Two images were

taken with laser beams unfocused and tightly focused with a cylindrical lens. With

focusing, the irradiance of the laser beam increased from 0.7 to 40 mJ/cm2 (see

improvement in Figure 4-10).

Detector Efficiency and Spatial Resolution

The quantum efficiency (QE) and the ionization efficiency (IE) of the detector were

estimated as the following:

QE = # of laser induced charges (43)
QE : ------ (4-3)
# photons absorbed

JE # of laser induced charges (-
/E = -----------(4-4)
# of Cs atoms in the illumination volume

These two quantities are related by:

QE = axlE (4-5)

where, a is a fraction dimensionlesss) of photons absorbed or absorption factor (see

equation 4-1). The efficiencies were calculated from the measurement of the signal as a

current from the phosphor screen by 50 Q termination. Figure 4-11 shows an oscilloscope

trace of the measurement (average of 128 shots). The number of electrons calculated

from Figure 4-11 was 5.5x108. Assuming the MCP gain as 104, it can be concluded that,

on average, 5.5x104 ions were created per laser pulse. The number of Cs atoms in the

ionization volume was estimated to be -107. The ionization efficiency according to the






40


equation 4-4 is 0.005. Equation 4-5 then gives the value of the quantum efficiency of the

detector as QE = 0.05x0.005 = 2.5.10-4. Using the equation (4-3) the number of absorbed


photons was calculated to be 2.2-108.



0-

-20


E -40
C
05, -60-
m -so-

-80

-100

0.5 1.0 15 2.0 2.5
Time (ps)



Figure 4-11. Signal measured as a current from phosphor screen of the detector.


Absolute spatial resolution of the system with the current configuration and parameters

was determined by using a 1951 USAF test chart and was found to be 280 JIm.

Ultimately, an optimized RIID should have a spatial resolution limited by the resolution

of the MCP and/or image capturing device (CCD) on the order of 1 .m.


Conclusion

In this chapter, the Cs RIID was evaluated in the imaging mode of signal detection.

The conclusion of the chapter is that, for comprehensive evaluation of the detector it is

important to have an efficient ionization scheme with powerful lasers to perform the

ionization. In the next chapter, important and interesting fundamental temporal studies

performed in the non-imaging signal detection mode with both the Cs and Hg RIIDs

separately and in combination will be shown.













CHAPTER 5
TIME-RESOLVED EXPERIMENTS IN RIID

Introduction

As predicted by Matveev [51], there are four main noise sources that degrade the

figures of merit of a resonance ionization imaging detector: (1) thermally or collisionally

excited atoms in the state, which is to be excited only by the radiation to be detected; (2)

multiphoton ionization (MPI) of atoms by radiation in the second and/or third steps of

stepwise ionization; (3) thermal ionization (TI) of atoms; and (4) photoelectric effect (PE)

due to background radiation and radiations of the second and/or third steps. The

photoelectric effect can occur from the thin metallic film of the input window, side

windows and from the MCP. This chapter is dedicated to studies of some of these noises

by monitoring the time-resolved ionization signal in the RIID. Of these noises, PE in

particular, was observed in previously evaluated compact, sealed-cell Hg RIID [19, 27].

Understanding the sources of these noises is essential for both the Cs and Hg RIIDs. The

time-resolved signal detection mode of the RIID operation is based on a current

measurement from the phosphor screen using an oscilloscope. Time-resolved ionization

measurements in addition to imaging information can enhance the performance of an

RIID by providing the basis for determination of the frequency shift from the resonance

for the photons to be detected as was done in [49]. Such possibilities will be discussed in

this chapter, along with a unique combination of Cs and Hg in a single RIID with dual

imaging capabilities.









Time-Resolved Measurements in Mercury RIID

Experimental Setup

Three-step ionization of Hg (see Figure 2-2) in the experiments was achieved by

using two Molectron DL-II dye lasers pumped by a XeCl Excimer laser at 308 nm.

Radiation at 2/ = 253.65 nm (6'S0 -->'63pO) was generated by frequency doubling the

507.3 nm radiation from a dye laser with Coumarin 500. Radiation at 22 = 23 = 435.8 nm

for the transitions 63J0 -->7SI and 73S, -> ionization continuum were generated from the

second laser with Coumarin 120 dye solution. Pulse energies of 10 pJ and 450 gJ were

measured for radiation k, and X2, correspondingly, operating at 10 Hz. The linewidths of

the lasers were between 1 cm1 to 1.5 cm1.


a.... ... _


I I '
//
SII to CCD
...~~ ~..... ............... ......., ...o,..........


n I I
Ring MCP phosphor screen
S1 ^ I----


Figure 5-1. Schematic diagram of the vacuum chamber Hg RIID.


_1___

-^SS




Quartz tube
_J-


|








Two versions of the mercury RIID were used in these experiments. The first one

evaluated in references [19, 27] and mentioned in Chapter 2 was the sealed-cell Hg RIID.

This compact cell was 5 cm in diameter and 4 cm in width, with a front input widow

diameter of 2.5 cm and side window widths of 0.7 cm. The second one was a semi-sealed

cell vacuum chamber RID as shown in Figure 5-1. The distance between the input

window and the MCP in this case was 40 mm. The end of the sealed quartz tube served

as an input window in this case. Mercury vapor pressure in the cell could be varied from

10-5 to 2 10-3 Torr. In this study, depending on the task, radiations at X2 and X3 were

focused with a cylindrical lens into a rectangular sheet approximately 0.1 0.7 mm in

width or with a spherical lens into a spot with predetermined diameters. The laser beam

with a wavelength of 253.7 nm, which carried the imaging information in these

experiments, could be expanded by a telescope to a 1.5 cm2 spot size.

Experimental Results and Discussion

Theoretical Considerations

In our case, the spectral line width of Al dye laser (A V/,ser 30 GHz) was much

broader than the width of the Doppler-broadened Hg atomic lines at room temperature

(A to = 1 GHz) and much of the incident radiation was not capable of being absorbed.

Therefore, the Al beam impinged on the channels of the MCP and created an unwanted

photoelectric effect (PE). In the imaging signal detection mode, the response of the

detector to PE electrons was identical to that of the mercury ions. If a collimated beam

was directed towards the RIID, the image formed on the phosphor screen due to PE

electrons would be identical with the image formed due to selectively ionized mercury

atoms. This PE image was easily observed experimentally when laser beams A2 and 23









were blocked from the RIID. Under normal operating conditions, the image formed

contained both PE and ionization components. Generally speaking, these two components

of the signal could be discerned in the imaging mode only if a rather short pulsed signal

was detected and time-resolved information could be acquired from every pixel of the

RID. The mercury RIID was not designed to acquire time-resolved imaging information

from every pixel in the nanosecond range. In this experimental study, the time-resolved

signal was measured with digital oscilloscope from the whole phosphor screen. Shown in

Figure 5-2 is a time-resolved measurement, typical for non-imaging signal detection

mode of the sealed-cell Hg RIID, where a second peak between the photoelectric and the

resonance ionization peaks was observed [27]. The exact reason for the appearance of the

second peak is still not clear; however, after the experiments it is believed that the most

probable reason is desorption of low-mass ions (Na, Ca, or K) from the surface of a side

window [52, 53]. The flight time of the Hg ion can be varied by changing the distance

between the ionization region and the MCP, as well as by changing the high voltage

applied to the system.



0-



Unidentified
I20-
Photoelectric signal
-30-
Resonance Ionization signal

-40-
-50o


Time (ns)


Figure 5-2. Typical time-resolved measurement in a sealed-cell Hg RIID [27].








The duration and temporal behavior of the ionization signal depended on the

penetration depth I of resonance radiation into the volume of RID. As described in

reference [49], 1 depended on the Al wavelength. The width of the ionization area is

narrowest (or the penetration depth is shortest) when the radiation is tuned exactly into

resonance. The absorption factor (a) of the atomic vapor is described by equation (4-1).

The time-resolved signal in the RIID cell can be described by the first derivative da/dl,

when 1 (distance) is substituted in terms of time t. For mercury vapor at room temperature

(T = 300 K), the number density of atoms, n, is approximately equal to 4.3x 10"3 cm-3, the

cross section a is -6x 10'13 cm2 for the resonance transition, and the Doppler-broadened

linewidth is 1.05 GHz. Approximately 60% of resonance radiation tuned to the center of

the absorption line will be absorbed for path length of 0.05 cm under this condition. If

there is some detuning from resonance, then this path length will be longer. The time

response of the ionization signal will be longer as well in the case of detuning far from

the center of the absorption line. In Figure 5-3, the results of time-resolved signal

calculations based on equation (4-1) are shown [50]. As can be seen, the shape of the

signal depends on the frequency shift and the spectral composition of the detected light:

narrowband or broadband. The signal for broadband radiation (quasi-continuum source)

was normalized with the signal when very narrowband radiation was tuned to the center

of the line.

Here, narrowband radiation is considered as the laser beam with a spectral

linewidth, much smaller than the atomic linewidth at the given experimental condition

(Avlaser << Avatom). And a broadband radiation means that the spectral linewidth of the

laser beam is much broader than the atomic linewidth (Aviaser>> Avatom). Narrowband






46


radiation 200 MHz shifted from the center of the line can not be resolved as well as for

the case when it is shifted by 500 600 MHz. This is understandable since the first

derivative versus frequency has zero at the center of the line and has a maximum between

500 600 MHz. In Figure 5-3, the ionization signal was plotted vs. distance (in arbitrary

units) from the input window, which represented the absorption pathlength. As was stated

earlier, temporal behavior of the signal would be the same.


Distance from input window (a.u.)


Figure 5-3. Signal profile vs. absorption path length for Hg atoms at room temperature;
1 for narrowband laser (Aviaser s AVatom), 2 for broadband laser (Avia.ser >>
Ava.tom) and four cases when the narrowband laser is frequency-shifted from
resonance.

Time-Resolved Measurement Results

The linewidth of the dye laser (-30 GHz) was approximately 30 times greater than

the Doppler-broadened mercury line. For such a line, the shape of the time-resolved

signal should not be very dependent on laser frequency shift. To observe and simulate the









frequency-dependent ionization signal by manipulating the width of the ionization region,

an experiment with different thicknesses of the 22 beam in the sealed-cell RIID was

performed (Figure 5-4). In the first case, the thickness of the beam sheet was -0.1 mm

(solid line) and, as predicted, the time response of the ionization signal was determined

by the duration and the shape of the dye laser pulse, which has two partially overlapping

peaks with time of ~20 ns between them. In the second case, the thickness of the beam

sheet was -5 mm. The duration of the ionization signal was longer than in the first

example and supposedly could be controlled by the width of the ionization region

(dashed line).


- P--~-~.


I 1


0 100 200
Time (ns)


300 400 500


Figure 5-4. Signal dynamics in case of narrow 0.1 mm (solid line) and 5 mm (dashed
line) 22 beam.


In Figure 5-5, the results of another experiment are shown. The 22 laser beam was

focused into a sheet and directed to three locations of the RIID, close to the MCP (1), to


-0.01 -


-0.02-


-0.03-


-0.04-


-0.05-


J V






48


the center of the cell (2) and close to the input window (3). The ion peak close to the

MCP has smallest amplitude, apparently due to absorption of radiation within the cell.

Consistent with theoretical calculation, shown in Figure 5-3, for broadband radiation,

peak 3 must have larger amplitude than peak 2 in the middle, and peak 1 must have the

smallest amplitude. In a first approximation, these data show that the time-resolved

signal can carry useful information about spectral composition of the detected radiation.

It must be emphasized that if the detected radiation has very narrow linewidth, it is not

necessary to have the whole time-resolved signal from the RIID cell. From Figure 5-3, it

can be derived that the information about the frequency of detected radiation can be

obtained by taking only two measurements. For example, from a point close to the input


I


S-0.005-
(0 ,

C
.2)
-0.010-
C
0
-,.
C
"N
0
- -0.015-




-0.020 ,
0 100 200 300 400
Time (ns)


3


5I I I 80
500 600 700 800


Figure 5-5. Spatial scan of 22 beam showing three location of the beam inside the RIID
cell; 1- close to the MCP, 2 in the center of the cell, 3 close to the input
window.








window and from a point further than the region where all curves are crossed. If the

RIID is calibrated in terms of frequency shift, then the ratios of signal amplitude at these

two points will unambiguously determine the frequency shift from resonance for the

detected radiation. The reason for taking two measurement points on opposite sides of the

region where all curves cross is seen from Figure 5-3.

Low-mass Ion Desorption

As we hypothesized above, the second peak shown in Figure 5-2 was due to

desorption of low-mass ions from the surface of side window. To verify this hypothesis

and to study possible ways to eliminate noises related to the low-mass ions, another set of

experiments was carried out in a semi-sealed version of the RIID as depicted in Figure 5-

1. The chamber was under constant evacuation by a turbo-molecular vacuum pump and a

constant supply of mercury atoms was produced from a drop of mercury within the

chamber. The mercury vapor pressure could be varied from 10-6 Torr to the level of

saturated vapor 2x 10-3 Torr by changing the active aperture of a throttle-valve between

the chamber and the turbo pump. To eliminate the effect of absorption of A, radiation by

mercury atoms, a sealed 12 cm long quartz tube with a diameter of 25 mm was placed in

the chamber as shown in Figure 5-1. A Chevron type MCP with P-20 phosphor screen

was used to detect ions. The distance between input window (i.e., the end of the quartz

tube) and the MCP was set to 40 mm.

It was noticed (Figure 5-6) that when A2 was aligned precisely in parallel with input

window, no second peak was observed. The distance from the side windows of the

sealed-cell RIID to the MCP was approximately 1 cm. For the vacuum chamber, this

distance was 16 cm, which was sufficient to eliminate any noise-producing desorbed






50


ions. Focused and unfocused A2 beams showed clearly distinguishable difference in pulse

duration. As can be seen from Figure 5-6 when the A2 beam was tightly focused with a 60

cm focal length spherical lens, the ionization signal duration was 20 ns. When the 2

beam was directed into the chamber unfocused with a diameter of 6 mm, the pulse

duration was 130 ns.


0.000

-0.004

-0.008

-0.012

-0.016

-0.020

-0.024


1500
Time (ns)


2000


2500


Figure 5-6. Effect of focusing of 22 beam on the ionization signal.

When the A2 beam was directed to the surface of the input window at small (-5)

angle, a signal from low-mass ions was seen on the oscilloscope. In order to form clearly

distinguishable and repeatable desorbed ions from the surface of the input window, the 22

beam was focused; the results are shown in Figure 5-7. As can be seen, the second peak

in this case is split, showing three discrete peaks compared to the sealed-cell RIID case

(Figure 5-2). By blocking the 2A laser beam, the photoelectric (PE) and the resonance









ionization (RI) peaks were eliminated, which corresponds to the solid-line trace on the

graph.



0.005

0.000 -

S-0.005
oi* *' .':
S -0.010 .
R)
U) -0.015 RI
c
0
-0.020 :
N
C
.o -0.025 -

-0.030
Desoprtion
-0.035- PE

-0.040 n-i
0 1000 2000 3000
Time (ns)



Figure 5-7. Low-mass ion desorption from the surface of input window with the 22 beam.
Dashed line shows peaks due to photoelectric effect from MCP (PE),
resonance ionization of Hg atoms (RI) and low-mass ion desorption. Solid line
is the case when 2A beam was blocked, showing only peaks due to low-mass
ion desorption.


By appropriate alignment of the lasers, it was possible to image all three peaks. In

Figure 5-8a, spatially separated images of three peaks related to three different physical

processes are shown. The positions of all the peaks were precisely where they were

expected to be geometrically. When A, was blocked, the image of only one spot

(Figure 5-8b) due to low-mass ion desorption was seen on the phosphor screen. In order

to further investigate the origin of the peaks that, according to our conclusion, were due









to low-mass ion desorption from the surface of the input window an experiment with a

XeCl excimer laser (308 nm) was performed (Figure 5-9).


a)

Figure 5-8. Imaging the processes in the RIID.


1000


I
2000
Time (ns)


3000
3000


Figure 5-9. Low-mass ion desorption from the surface of input window excimer laser
beam.








In this experiment, 6 mm diameter excimer laser beam with 1 mJ a pulse energy was

hitting the surface of the input window at -10 angle. This experiment proved the non-

selective nature of the peaks, indicating that the peaks were not related to resonance

excitation and/or ionization of Hg atoms.

It should be emphasized that we use the term ion desorption. However, the exact

physical mechanism of ion creation [52, 53], when a surface of a solid material is

illuminated by comparatively soft laser radiation is not known. In fact, we never noticed

any traces of a physical damage on the surface of the input window in our case, unlike the

techniques of MALDI and/or LIBS, where the surface is ablated by the laser.

Absolute Time-Resolution of Hg RIID

The next experiment was performed to evaluate the absolute time resolving ability

of the RIID, i.e., to verify if a small disturbance in the distribution of the ion cloud,

created as a result of resonance ionization of mercury atoms, could be time-resolved. The

laser beam of the second step (A2) was expanded into 1 cm wide horizontal sheet by

focusing with a cylindrical lens and directed to the Al laser beam path. Paper strips with

different widths were inserted into the 22 beam partially blocking its radiation. A gap in

the pulse, proportional to the width of the strip was observed. In Figure 5-9, the shape of

the pulse is shown when instead of strip a 0.54 mm diameter pin was inserted into the

beam. The half-width of the gap was 30 ns. This experiment demonstrated not only the

potential time resolution of RIID but also one of the possible applications to detect two

closely placed targets when they are not distinguishable by the imaging device. Ideally, if

time resolution was very high even the shape of the target could be determined. For such

tasks, the Resonance Ionization Imaging Detectors have tremendous advantage since it









practically does not detect any ambient light, responding only to the target illuminated by

laser radiation.


0.000-


-0.002-


-0.004-


-0.006-


-0.008-


-0.010-


1000 2000 3000
Time (ns)


Figure 5-9. Absolute time resolution of the RIID; 30 ns gap in the resonance ionization
peak of Hg atoms corresponds to a pin with 0.54 mm diameter.


Time-Resolved Measurements in Cesium RIID

A time-resolved ionization signal in Cs RIID was shown in Figure 4-10. As

expected no photoelectric effect was observed due to radiation at the detection

wavelength of the Cs RIID (852.12 nm). Figure 4-10 was taken when a laser beam

852.12 nm laser beam entered the RIID through the input window, with the 508 nm laser

beam coming into the RIID from the side as a thin sheet of light (so called standard

imaging geometry). In order to further investigate the photoelectric effect, the geometry

of the beams was reversed. The ionization laser beam at 508 nm impinged on the MCP

carrying the image of the test mask and the 852.12 nm laser beam entered into the cell








from the side as a thin sheet of light. Spatially separated photoelectric and resonance

ionization images of the test mask from the phosphor screen are shown in Figure 5-10.

This was achieved by directing the 508 laser beam into the RIID at a small angle.

















Figure 5-10. Photoelectric effect from the MCP in the Cs RIID when the laser
wavelengths are 2A = 508 nm and A2 = 852.12 nm.

It should be noted that the spatial resolution of the mage due to PE electrons was

better than the image die to Cs ions. This is because the PE image was formed by

electrons traveling the distance only between the MCP and the phosphor screen. As for

the RI image, in this case the image was due to Cs ions that had to travel to the MCP first.

For better spatial resolution the ionization region should be closer to the MCP. Spatially

separated photoelectric and resonance ionization peaks were time-resolved and the result

is shown in Figure 5-11.

The low-mass ion desorption process discussed for the Hg RIID was not observed

for 508 nm radiation in the Cs RIID, at least at the available laser pulse energies.

However, the presence of PE due to 508 nm radiation indicated that the work function of

the electrons from the MCP corresponded to the laser energy with a wavelength greater









than 508 nm but definitely smaller than 852.12 nm. Therefore, PE noise will not be

present in the Cs RIID with the three-step ionization scheme described in Chapter 3.


500 1000 1500
Time (ns)


2000


Figure 5-11. Time-resolved measurement in Cs RIID. PE is a peak due to photoelectric
electrons from the MCP and RI is a peak due to Cs ions.

Dual-Wavelength Time-Resolved Imaging with Cs and Hg

Coincidentally, the same dye laser with Coumarin 500 solution was used for both

Cs and Hg ionization. The first harmonic of the laser at -508 nm was used for ionization

of Cs and the second harmonic of the laser at 253.65 nm was used for Hg excitation. This

provided a ground for a very interesting and intriguing idea of combining the two vapors

in a single RIID. Simultaneous ionization of Cs and Hg atoms was investigated using a

two-step ionization scheme for Cs (852 nm -> 508 nm) and a three-step ionization

scheme for Hg (254 nm -+ 436 nm -+ 436 nm); the wavelengths are rounded off for

simplicity. This unique simultaneous ionization of Cs and Hg atoms was accomplished








using only two dye lasers and a LiF.-F2+ color center laser. The Cs atoms were produced

using the described above alkali metal dispenser. Hg atoms were produced from a small

drop of a mercury metal placed in the vacuum chamber. The experimental setup is shown

in Figure 5-12.

/^---?- DL2-----^-----


308
Excimer
'__ LiF:F2I
---- ~610 y
DL 3


Oscilloscope

CT / 852
<--' 0 c Mask 254
00 --- o 508 '254

50 f 436 Rl 508
= RIID

Sto CCD

Figure 5-12. Setup for dual-wavelength time-resolved imaging with Cs and Hg vapors.

Two Molectron DL II dye lasers were pumped by the excimer laser to generate

radiations with a wavelength of 254, 508 and 436 nm. A third, homemade dye laser,

based on a amplified spontaneous emission at -610 nm was used to pump the LiF.'F2+

color center crystal in order to obtain the detection wavelength of Cs at 852 nm, which

illuminated the cell from the front side, carrying the image of the test mask. The UV

beams for Hg ionization (254 and 436 nm) were counter propagating, parallel to the MCP

to avoid the formation of the photoelectric effect from the MCP. The time-resolved

ionization signal could be measured simultaneously with image acquisition. The images








due to both Cs and Hg atoms are shown in Figure 5-13, along with the diagrams of the

beams forming the images.


Figure 5-13. Dual-wavelength imaging with Cs and Hg vapors.

The image on the top right was taken with all four beams present. The image of the

test mask was due to Cs ionization, and the stripe was formed by resonantly ionized Hg

atoms. By blocking the appropriate laser beams, separate images due to only cesium or

only mercury could be taken (middle and bottom images). The separate images

confirmed that the Cs and Hg vapors were ionized simultaneously, but independently.

This was confirmed by turning off the heating current of the Cs metal dispenser. With no

Cs atoms present, the only image observed was a stripe. The simultaneous ionization of

Cs Hg was time-resolved by monitoring the signal in an oscilloscope and the result is


852 nm

436 nm 8 rm

a i 254 rnm


852 nm


508 nm
.. .... ..............
.I .I....; .


436 nm 2

254 nm









shown in Figure 5-14. Despite the precautions, a small PE peak due to scattered from the

parts of the RIID UV radiations was observed at t = 0. The ratio of the times of flight of

the Hg and Cs ions could be calculated as follows:


tg M F200.59 1.22
tcs VM V132.905


where, M is a molar mass of the atoms. The experimental value of this ratio was -1.21,

showing a good agreement with the calculations.




0 -F

t-0
t=O
-10


E -20

0)
) -30-


-40
Cs
Hg
-50

0.5 1.0 1.5 2.0 2.5
Time (ts)



Figure 5-14. Time-resolved ionization of Cs and Hg atoms.


Conclusion

The flexibility of the resonance ionization imaging detectors has been demonstrated

with non-imaging, time-resolved mode of operation of Hg RIID. This mode of signal

detection has provided improved figures of merit and could be performed simultaneously

with the imaging mode of signal detection. Furthermore, the non-imaging mode of RIID

operation provided a correction method for photoelectric noise during the imaging mode








of operation. This was achieved by temporal resolution of the ionization signal from the

photoelectric signal. A new source of noise, most likely due to low-mass ion desorption

from the walls of the RIID cell, has been discovered. A method to eliminate this noise has

been demonstrated. The feasibility of using time-resolved information to determine the

frequency shift of detected radiation has been discussed.

Also, an enhanced resonance ionization imaging detector with dual-wavelength

imaging capabilities has been demonstrated using Cs and Hg vapors. This version of

RIID with simultaneous imaging and time-resolved modes of signal detection could

broaden and improve the applications of the RIID. For example, it adds valuable imaging

information to the detection of two closely located targets.













CHAPTER 6
FEASIBILITY OF SEALED-CELL CS RIID CONSTRUCTION

Introduction

In this chapter, the issues related to Cs reactivity are discussed. Cesium is a strong

reducer that generates an explosive reaction with water and is not compatible with many

materials such as aluminum, copper and zinc. Even the alloys of noble metals such as

gold and silver are not compatible with Cs and other alkali metals as will be shown.

However, through a literature search and experimental work, materials that are inert to Cs

have been found. The possibility of sealed-cell Cs RIID construction will be shown in

this chapter.

Cs RIID: First Attempt

Our very preliminary experiments with Cs was performed in a semi-sealed cell

which consisted of a sapphire cylinder confined between Al coated input window and

phosphor screen. The schematic of the experimental setup is shown in Figure 6-1. The 20

mm long sapphire cylinder had a diameter of 50 mm. The sapphire cylinder was

sandwiched between 3 mm thick sapphire input window and phosphor screen. The

phosphor screen was a 6 mm thick lead glass substrate coated with P22G phosphor with

7.5 mg/cm2 surface density. The inside surface of the input window was coated with a 20

nm layer of Al. A 5 nm thick Al layer was deposited on the phosphor surface. A 25 mm

long linear dispenser of Cs, described in Chapter 4 (Figure 4-2), was installed on the

bottom part of the cell; through two 3x0.3 mm2 slits, dispenser terminals were connected

to a current supply. The cell was evacuated through the same slits. The dispenser was








heated by flowing current through it to release vapor of atomic Cs. The sapphire cylinder

between the input window and phosphor screen was heated by a wire coil wrapped

around it to minimize any Cs atom adsorption on the inner surface of the cylinder. The

cell was installed inside the six-way cross chamber depicted in Chapter 5 (Figure 5-1),

which was evacuated down to 105-10.6 torr pressure. A negative high voltage in the

range of 5-8 kV was applied to the input window and phosphor screen was grounded.





CCD

1 Screen UR



5 6
10 -10 Torr vacuum



Figure 6-1. The schematic of the semi-sealed Cs RIID.

The two-step ionization scheme (2A = 852.12 nm; 22 = 508 nm) mentioned in

Chapter 3 was used for Cs ionization. The laser beams were counter-propagating. The

reason for counter-propagating geometry of the beams was the illumination of the

phosphor screen by both lasers when the beams were directed to the cell from the front

side. The very thin Al coating on the phosphor screen along with the phosphor layer was

not capable of blocking light from reaching the CCD. With lasers counter-propagating

parallel to the phosphor screen, the resonance ionization image was observed. Electrons

created by selective photoionization of Cs were accelerated to the phosphor screen and a

bright stripe was observed. However, the detector was operable for only a few hours. The








aluminum layer on the surface of phosphor screen and input window reacted with Cs

forming some sort of amalgam. As a result, an electric field between the screen and

window could no longer be maintained. Figure 6-2 shows a photograph of the cell taken

from the input window side of the cell. The first thing to note was that the input window

was completely transparent, almost nothing was left from the 20 nm layer of aluminum

coating of the window. The phosphor layer of the screen was also completely exposed.

Furthermore, the phosphor layer of the bottom half part of the screen, which was close to

the dispenser, was destroyed too, indicating the incompatibility of the P20 phosphor with

Cs at high temperatures.





















Figure 6-2. Photograph, showing the damage of the phosphor screen due Cs reactivity.

Cs Compatibility: Theoretical Considerations

For the initial construction of a sealed-cell Cs RIID, it was experimentally observed

that there was incompatibility of Al with cesium metal due to the solubility of the metals

in each other. Al was chosen because of its availability and ease to work with. Search in









literature for materials inert to Cs reactivity resulted in Table 6-1, which lists the

compatibility of materials with Cs [54]. Even though the cell temperature in the

experiments was much lower than 450C (AL alloys), Al still reacted with Cs. Therefore,

the temperature, given in the table and up to which the metals are compatible, was

considered as an indication of the degree of reactivity of the alloys with cesium. As can

be seen, noble metals such as gold and silver, which would first come to mind in terms of

inertness, were not compatible at all with alkali metals.


Table 6-1. Compatibility of materials with alkali metals [54].


Compatible with alkali metal up to ('C)
Factors influencing
Material Li Na K Rb and Cs compatibility

Mg alloys n.c." n.c,. 300 300 Metal solubility, oxygen exchange
Al alloys n.c, .350 400 450 Metal solubility
Cu alloys 300 400 400 400 Metal solubility
Ag and its alloys n.c. n.c. n.c. n.c. High metal solubility
Au and its alloys n.c. n.c. n.c. nc. High metal solubility
Zn coatings n.c. n.c. n.c, n.c, High metal solubility
Pb and its alloys n.c. nc. n.c. n.c. Very high metal solubility
Sn and its alloys n.c. n.0. n.c. n.c. Very high metal solubility
Fe 500 700 700 700 Nonmetallic impurities
Low-alloy steels 500 700 700 700 Nonmetallic impurities
Ferritic steels 500 700 700 700 Nonmetallic impurities
High-Cr steels 500 700 700 700 Nonmetallic impurities
Austcnitic steels 450 750 750 750 Nonmetallic impurities
Ni alloys 400 600 600 600 Flow velocity
Mo alloys 1000 1000 1000 1000 Nonmetallic impurities
W alloys 1000 1000 1000 1000 Nonmetallic impurities
Ti alloys 700 700 700 700 Nonmetallic impurities
Zr alloys 700 700 700 700 Nonmetallic impurities
Y alloys 700 700 700 700 Nonmetallic impurities
Nb alloys 700 700 700 700 Nonmetallic impurities
Ta alloys 700 700 700 700 Nonmetallic impurities
Sintered AlO3 350 500 500 500 Thermomechanical action
stab. ZrO2/CaO 350 350 350 350 Intergranular corrosion
stab. ThO2/Y20, 400 550 550 550 Intergranular corrosion
Glass n.c. 250 250 250 Chemical reaction
UO2 750 Excess of oxygen
UC 750 Nonmetallic impurities
Sn.c. = not compatible.






65


According to Table 1-1, the most compatible metals would be W and Mo, followed

by Ti, Zr, etc. The more readily materials Ni and Cr also seemed to be good candidates.

Based on this table, a few elements were selected as a potential metals for coating the

input window and the phosphor screen. Penetration of these materials by high energy

electrons was simulated using Casino V2.42 (monte Carlo Simulation of electron

Trajectory IN sOlids) software package (http://www.gel.usherb.ca/casino/). Calculations

for different materials and different electron energies were performed. Figure 6-3 shows

the simulation results for different materials bombarded with 10 keV electrons.



100 100- *
5 Ti
J o \:J80- U Ni0 5keV
C Ti i i 10 keV
0o- 60. .* 15keV
tB 0 \ 60 *
w 40- 40 -
a a |
S20- 20 .
E E
Z 0- z 0 *

0 200 400 600 800 1000 0 200 400 6o00 800oo 1000 1200
Penetration depth (nm) Penetration depth (nm)
a) b)

Figure 6-3. Penetration depth of electrons for different materials and electron energies.


Penetration depth decreased with the increase of the atomic number of elements.

From Figure 6-3a, it can be seen that the Ti is easily penetrated by electrons. Therefore Ti

can be used to coat the phosphor screen with a thick layer to make it optically opaque, but

transparent to electrons which will reach the phosphor layer and cause a luminescence.

The coating of the input window, on the contrary, should be as thin as possible to

transmit the most of the photons to be detected. Again, Ti is well-suited for the purpose;

the bombardment of Ti metal by electrons with different energies was simulated (Figure

6-3b).








Cs Compatibility: Experimental Verification

Two custom-made (Del Mar Venture, CA) luminescent screens coated with a layer

of Ti metal were experimentally evaluated. One of the screens was a conventional P20

phosphor coated with 200 nm Ti. The second screen was a novel luminescent YAG

crystal which was coated with 80 nm of Ti. YAG crystals have proven to be highly

resistant to alkali metals at high temperatures [55]. Figure 6-4 show the experimental

setup used for evaluation of the luminescent screens.


Polished E Luminescent
Al disk Screen Oscilloscope
electrode

*CCD





-6-18 kV
00C
Excimer S
308 nm _


Figure 6-4. Schematic diagram of the setup for luminescent screen test.

A cell was formed with 3 mm thick well polished Al disk electrode and one of the

luminescent screens. Both screens were tested in exactly same way. Negative voltage up

to -18 kV was applied to the Al electrode. The luminescent screen was grounded through

a 50 Q resistor and the current, i.e. the number of electrons per second striking the screen

was monitored by an oscilloscope. Simultaneously, the image from the screen was

captured with a CCD camera. Photoelectric electrons were produced by illuminating the

Al electrode with the excimer laser beam (A = 308 nm). The excimer laser beam with a







67



pulse energy of 300 .tJ and a diameter of 3 mm impinged the center of the Al electrode at


30 angle. Measurements were performed at 7. 10-6 Torr pressure. Figure 6-5 shows signal


measured from the screens with the oscilloscope at -12 kV applied to the Al electrode.


0.5. YAG screen tested with excimer P20 screen tested with excimer
0.0- 00
-0.5 -
-1.0
-i 0
E5 -15 E -20- FWHM =25-30 ns
2 FWHM = 25-30 ns
c -2.0- *SI
U i -30-
-2.5- Ma m Max =4.71 mV
Max = 3.75 mV
-3.0- -40
-3.5-
-5.0
-40-
-0.5 0,0 05 1.0 1.5 20 -0.5 0.0 05 10 1 5 20
Time (ps) Time (1ps)



Figure 6-5. Current due to electrons striking the luminescent screens.


The calculated number of electrons for both screens was approximately 1.5x 107. At


approximately -1 kV applied to the Al electrode, the signal amplitude reached the level


shown in Figure 6-5 and further increase of the voltage did not noticeably change either


the time-resolved signal intensity, or the time of flight of the electrons. More informative


results were obtained from the measurements of the image intensities. Figure 6-6 shows


series of images obtained with the P20 and YAG screens at different voltages. Each


image is an average of twelve individual image acquisitions. With increase in the voltage


the images became brighter, indicating an increase in the number of electrons penetrating


the metal coating. The experimental results were in good correlation with the simulation


results. At 6 kV a very faint image was obtained; according to Figure 6-3b, a very small


percent of electrons can penetrate 200 nm Ti layer with 5 keV energy. At energies greater


than 10 keV, almost all of the electrons can be expected to penetrate the 200 nm metal






68


layer according to Figure 6-3, which could be concluded from the plot of image

intensities vs. accelerating voltage in Figure 6-7. In general, P20 phosphor was a more


efficient luminescent substance.


6kV 10 kV


14 kV 18 kV


Figure 6-6. Series of images at different accelerating voltages. Above row P20 phosphor,
bottom row monocrystalline YAG.


* P20 coated with 200 nm Ti
* YAG coated with 80 nm Ti


* 0


6 8


S 0 *
0


14 16 18


10 12
HV (kV)


Figure 6-7. Image intensity vs. accelerating voltage.


As for the optical parts of the cell, sapphire was chosen due to its known resistance

to alkali metals. In the work by Bouchiat et al., electrical conductivity of glass and


*
U








sapphire cells exposed to cesium vapors are compared [56]. The walls of cells made of

different glasses became electrically conductive when exposed to Cs vapors (typical

resistances approximately 103 Q) whereas monocrystalline sapphire cells maintained a

resistance on the order of 109 Q in similar conditions. The measurements were done by

capacitively coupling the outer surface of the cells to the inner surface [56]. They

concluded that in the case of glass cells, the conductance was due to Cs atoms adsorbed

on the inner walls of the cells. For the sapphire cell, they could see no indication of wall

surface conductivity, attributing the conductance to collisional processes occurring

between the Cs atoms, especially at number densities of more than 1014 cm-3. The

conclusion was made based on the cubic temperature dependence of surface conductance

of the inner cell walls. The adsorption energy for alkali atom glass wall was estimated

to be 0.66 eV [56]. In the work by De Freitas et al., the adsorption-desorption of cesium

in a fused quartz cell was estimated 0.2 eV and was found to weakly depend on

temperature [57].





o -


Figure 6-8. A photograph of the sapphire Cs cell with Nb electrodes.

An experiment with a sapphire cell with niobium electrodes inside was performed.

A photograph of the cell is shown in Figure 6-8. The cell was approximately 8 cm long

cylinder with a diameter of 15 mm. The distance between electrodes was -15 mm. The

cell was evacuated and sealed with solid Cs inside. Niobium is a metal, compatible with

Cs according to Table 6-1. A high voltage up to 20 kV was applied to the electrodes to










the electrodes and VA characteristics of the cell was obtained. Figure 6-9 shows the


experimental results, the dependence of cell conductivity and current flowing through the

cell vs. applied high voltage.


0.9- 34-
08 32-
07- 3.0-
0,6- 28-
05. 26-
I04- % 24-
0.3- Ill 2.2-.
032- 22
0.2 B 2,
0 18.
021 20 *. S 5

2 4 6 8 10 12 14 16 18 20 22 2 4 6 18 0 12 14 16 18 20 22
HV (kV) HV (kV)
a) b)

Figure 6-9. VA characteristics of the sapphire Cs cell with Nb electrodes; a) conductance,
b) current vs. high voltage.


At room temperature, metallic Cs adsorbed on the inner walls of the cell which can


be seen by eye (inner walls were coated with a thin, golden-color film). The cell was

heated as a result of the applied voltage and the current flowing through the cell. If the

applied voltage is considered as an indication of the temperature of the cell, then Figure

6-9a shows a dependence very similar to the temperature-dependence of the conductance


discussed in reference [56], where the conductance in the sapphire was attributed to

collisions between Cs atoms in gas phase. However, as shown in Figure 6-9b, the drastic

decrease of the current with the voltage could be attributed to desorption of Cs atoms

from the walls, which could lead to the conclusion that the atoms adsorbed on the inner

surface of the cell was responsible for the conductance of the cell. As the results reported

in references [56, 57] were most likely interpretation of the results, the decrease of the

current with the voltage in our experiments was not clearly understood.








The current flowing through system dropped and stayed constant at a level of about

2 mA (see Figure 9-6b). The resistance of the cell increased from a few tens of kQ up to

-9 MQ due to the heat from the current flowing through the cell. At 5 kV applied

voltage, the temperature of the cell was measured to be -200 C. Two important

conclusions from the experiment are: (1) Cs metal adsorbed on the walls Cs metal could

not be completely evaporated even from sapphire; (2) however, intense electric field (>13

kV/cm) was realized in a sapphire cell filled with Cs atoms without producing a

discharge. The reasons that the wall conductivity could not be eliminated from sapphire

even at high temperatures could be due to surface and composition quality of the sapphire

used for the construction of the cell.

Conclusion

In this chapter, the possibility of constructing a sealed-cell Cs RIID construction

using sapphire and metals that are compatible with Cs has been shown. The chemical

inertness of the sapphire to Cs was confirmed in experiments with the sapphire cell and

Cs metal inside the cell. Sapphire, heated up to 200 C, did now show any signs of

reaction with Cs. In glasses heated up to such high temperatures, an irreversible reaction

between silicon oxide and alkali metal will occur and glasses become discolored and

brittle. The high electric field up to 13 kV/cm, produced in the sapphire cell with Cs

metal, did not cause an electric discharge and breakdown. For the metallic parts and

coatings of the windows of the potential sealed-cell Cs RIID, Ti was chosen among a few

other suitable metals based on a literature search. The electron penetration of Ti was

theoretically and experimentally studied by evaluating luminescent screens with P20

phosphor and YAG crystals.













CHAPTER 7
BUFFER GAS RESONANCE IONZATION IMAGING DETECTOR WITH
AVALANCHE AMPLIFICATION OF SIGNAL

Introduction

It has been shown that the high spectral resolution, which comes from the

narrowband absorption in atomic vapor combined with the efficiency of laser ionization,

furnishes the detector with superior figures of merit compared to conventional spectral

imaging techniques. However, there are certain applications where detectors with broad

bandpass are required. For example, Raman bands of molecules are typically -30 GHz or

broader. Narrowband RIID responds to only a small fraction of the Raman signal.

Despite this intrinsic disadvantage of the RIID for the detection of Raman scatter, based

on the sensitivity and high luminosity, the RIID still can be more efficient compared to

conventional Raman imagers. In addition there are ways of broadening the spectral

response of the RIID, while keeping the flexibility and efficiency. One way involves

collisional broadening of the atomic lines by addition of a buffer gas.

In reference [55], sub-Doppler absorption spectroscopy of Cs atoms was studied in

a sub-micron thin cell. Thin cells are very attractive idea for the design of RIID, because

they not only allow sub-Doppler high spectral resolution, but also provide improved

spatial resolution. In fact, the ultimate imaging with atomic vapors, both in terms of

spectral and spatial resolution, would be in atomic sheets.

In this chapter is discussed a completely different concept of a detector based on

atomic vapors. The novel detector is based on avalanche amplification and electrodeless








optical detection of resonance ionization signal and could address the question of line

broadening in a buffer gas and a very thin cell could be designed. The cell contains no

MCP or phosphor screen. The image is formed by optical emission of the buffer gas

excited by collisions with high energetic electrons accelerated in high frequency electric

field. Electrons are created as a result of resonance ionization of the atoms and form an

avalanche effect when accelerated in high electric field. This detector takes advantage of

the selective nature of the resonance ionization method and the sensitivity of particle

detectors, which are well developed and applied in the field of elementary particle

physics. This chapter discusses the design and the fundamental idea behind the buffer gas

RIID, along with the preliminary work done in this direction.

Buffer Gas RIID: Design

The photoionization detection of atoms in a buffer gases has been studied in many

experiments. The first successful experiment on the detection of single atoms of Cs by

means of two-step photoionization in buffer gases was shown by Hurst et al. [58]. Cs

atoms formed by the molecular fission of 252Cf were photoionized and the resulting

photoelectrons drifted into a proportional counter and were detected. The first optical

emission detection of resonance ionization signal of Hg atoms in a buffer gas (Ne) was

reported by Matveev et al. [59].

As atoms are detected in a buffer gas, the time of interaction of atoms with the laser

beam, i.e. the residence time of atoms in an exposed region is defined by their diffusion

in the gas. The time of diffusion of atoms from a spherical region with a diameter a can

be expressed [43]:

a2 *6 (7-1)
r"=16D 760








where D (cm 2/s) is the diffusion coefficient at a pressure of 760 Torr and P is the buffer

gas pressure (Torr). In order that all the atoms in this region are probed, the laser pulse

repetition rate must satisfy the condition:


f =-- (7-2)
Tdiff

for atmospheric pressures time of diffusion changes approximately from 2.5 ms to

250 ms as the diameter increases from 0.1 to 1 cm.

Electrons and ions produced by the ionization of Cs atoms diffuse by multiple

collisions with the buffer gas atoms. In the absence of an electric field, the diffusion

distribution is Gaussian and the mean free path of diffusion, I is [60]:

-= 1
-(7 ~ (7-3)


where, n is the number of atoms per unit volume (cm3) and oc() is the energy dependent

collision cross section (cm2). It is desirable to have rather high pressures in the cell (of

the order of 1 atm or higher), which leads to more efficient collisional excitation of buffer

gas atoms and improvement in the spatial resolution of the cell by limiting the free escape

of ions from the ionization region.

If the charge carriers (ions and electrons) are exposed to an electric field, an

ordered drift along the field will be superimposed over chaotic diffusion. The drift

velocity is directly proportional to the applied electric field and inversely proportional to

the pressure. Therefore, in terms of charge acceleration, high pressure may seem to have

a negative influence. Obviously, there is an optimal electric field strength for a given

pressure.










Front view Pon
Point gas
discharge

0/O OS 0 00 0 \
"'_'_''"__ Power
0 0 OSI @Supply





k3 I Pin electrodes


Figure 7-1. The schematic diagram of the buffer gas RIID.

The design of the buffer gas RIID and the principle of operation with three-step

ionization are shown in the Figure 7-1. Again, as described before, the image carrier

beam at the detection wavelength (A,) enters through the input window and excites the

atoms in the cell. Laser beams at A12 and A3? further excite and ionize the atoms. The

electrons formed are accelerated towards the very thin wire (pin) electrodes, which are

installed outside of the cell. There is no direct contact between the atoms and the

electrodes. However, the thickness of the cell wall separating the electrodes should be as

small as possible. Upon acceleration, the electrons form an avalanche effect, as shown in

Figure 7-2. The avalanche electrons will collisionally excite the buffer gas atoms and as a

result, a gas discharge localized to the point of ionization in the vicinity of the electrode

is formed (Figure 7-1).

In order to keep the signal proportional to the primary ionization, the avalanche

amplification factor should match the linear dynamic range of the detector. The lower

limit of this range corresponds to the field strength at which electron multiplication starts.









electron


\ gas ions


\electron avalanche

secondary electrons




Figure 7-2. Formation of an avalanche effect [60].

The upper limit is the starting point of new secondary and tertiary avalanches, which will

merge with the primary avalanche and no longer depend on the primary ionization. The

secondary and tertiary avalanches are due to photoelectrons produced by energetic UV

photons. At very high electric field strengths, the accelerated electrons also liberate

electrons from inner shells of gas atoms. This leads to de-excitation of excited gas atoms

by UV photon emission. These energetic UV photons will produce further electrons by

ionizing the gas atoms. This problem can be solved by addition of a quenching gas which

will absorb energetic UV photons, usually an organic gas. Most organic compounds in

the hydrocarbon and alcohol families are efficient in absorbing photons in the relevant

ranges. The molecules dissipate the excess energy by elastic collisions.

One should keep in mind, that not only the strength of the applied electric field is

important, but also the form of the field is critical. When the applied electric field is

continuous in time (Figure 7-3a), the signal will have a threshold character. In other

words, as the number of produced charged particles increase, they begin to have an effect

on the external field and saturation occurs. Signal amplitude is then no longer







proportional on the primary ionization (corresponds to the Isa, Figure 7-4). However, if

the field is applied in a form of short pulses (Figure 7-3b) with a duration of r, the signal

amplitude will then be different for different numbers of primary ionization products
(Figure 7-4: I,2,3).

E E

ANWMkkAA PAMI MJVi


t r t
Figure 7-3. Continuous and pulsed high frequency electric field.





31
12





Figure 7-4. Signal behavior vs pulse duration.

Other advantages of a pulsed electric field are that it allows one to apply a stronger
electric field without entering the saturation stage, and it gives time for ions and electrons
to recombine. In a continuous electric field, the ion/electron pair does not have much of a
chance to recombine. This is exaggerated at high field strengths, which can lead to
accumulation of charges on the cell walls. Thus, electric field investigations include








optimization of the pulse duration to produce a discharge whose emission intensity is

proportional to the initial number of electrons created by resonance ionization process of

the atoms. The repetition rate of the electric field must match with the repetition rate of

the lasers.

Figure 7-5 illustrates the principle of image acquisition is shown in the buffer gas

RIID. The dichroic mirror, transparent for the image carrier beam at the detection

wavelength, is placed at 45. The gas discharge emission in is reflected from the mirror

and collected by a CCD camera.

X Dichroic mirror








|Mask
:' '




Gas discharge
CCD


Figure 7-5. The schematic of the image acquisition in buffer gas cell.

Buffer Gas RIID: Experimental Results

The experiments were performed in a cell, the cross-sectional diagram of which is

shown in Figure 7-6. The cell was constructed with a stainless steel body and a quartz

front window. Electrodes were 0.3 mm copper wires inserted into small capillary tubes.

Approximately 200 capillaries were bundled and tightly packed into a glass tubing with a

diameter of 15 mm. The glass tubing with electrodes then was inserted into the opening








in the stainless steel body of the cell. To prevent the direct contact of the electrodes with

the gas molecules and atoms, a thin layer of transparent vacuum UV glue was spread onto

the surface of the bundle of the electrodes and cured with the excimer laser. The cell

contained Ne gas and Hg atoms from a drop of mercury inside the cell.




Quartz ........... Stainless
window steel


12 kV

30 kHz
... .. ...., ,

Separating
layer .
0.3 mm wire
/ electrodes


Figure 7-6. Buffer gas RIID cell and the Ne gas discharge.

A continuous oscillating electric field with a frequency of 30 kHz and an amplitude

of 12 kV was applied to the electrodes. The pressure inside the chamber was varied and

images of the Ne gas discharge were manually taken with a digital camera. Figure 7-7

shows a series of images taken at different pressures of the buffer gas.


Figure 7-7. Ne gas discharge at pressures (left to right, in Torr): 15, 30, 120, 240.








The Ne gas discharge was initiated randomly and did not respond to the resonance

ionization of Hg atoms. As stated above, the applied field parameters are critical and a

pulsed field with certain frequency and amplitude must be applied to produce a controlled

discharge proportional to the selective ionization of atoms. Unfortunately, it was not

possible to construct such a power supply due to technical difficultness. However, the

formation of a gas discharge, localized to the tip of the electrode and the response of the

detector to the pressure change observed in the experiments are promising. The concept

of the buffer gas RIID with avalanche amplification and optical detection of resonance

photoionization of the atoms could be very interesting continuation of RIID development.

Avalanche Amplification of Cs Ionization Signal in Flame

The avalanche effect was studied in flame with two major goals: (i) to better

understand the conditions for avalanche with possible application of the results to the

design of the RIID; (ii) to apply the effect of avalanche amplification of signal to the

fundamental laser-enhanced ionization (LEI) technique. LEI is well known as one of the

most sensitive elemental analysis techniques, which takes advantage of the spectrally

selective nature of laser excitation of analyte atoms and of thermal environment at which

atoms are produced, leading to their effective collisional ionization [61]. Ionization of Cs

atoms, enhanced by two-step excitation, was detected in hydrogen and propane flames.

This work not only proved the feasibility of the idea of avalanche amplification in flame

environment, but also by employing the effect of avalanche amplification of electrons, a

limit of detection (30 fg/ml) for laser-enhanced ionization (LEI) technique was achieved

[62]. The most often used atomic reservoir for LEI is a flame, and the best reported limits

of detections (LOD) are in the range of tens of pg/ml [61]. There are three main factors

limiting the LOD of atoms in a flame when using the LEI method, namely: (i)








fluctuations of the background current between the electrodes which detect the signal [63,

64], (ii) non-selective ionization of atoms and molecules in the flame and, (iii) Johnson

noise of the input resistance of the signal preamplifier [64]. As shown by Matveev and

Omenetto [63, 64], there is a several orders magnitude gap between experimental limits

of detection and the theoretical values which can be calculated by taking into account

these main sources of noise. As suggested by Matveev [63], one of the most promising

ways to bridge this gap is to take advantage of the avalanche amplification of the signal

as is done in the field of nuclear physics when high-energy nuclear particles are detected

with gas-filled counters [60]. Previously, the avalanche effect has been obtained for

mercury atoms and an inert gas contained in a quartz cell at atmospheric pressure with

argon and P-10 (10% methane in argon) gases as a buffer [65]. The resulting detection

limit was approximately 15 Hg atoms in the volume illuminated by the laser radiation. In

this work the possibility of the effect of proportional avalanche amplification of an LEI

signal in a flame reservoir has been experimentally demonstrated.

Experimental Setup

The experimental setup (Figure 7-8) consists of two Molectron DL-II dye lasers,

pumped by the XeCl excimer laser (LPX-1 lOi, Lambda Physik, Acton, MA), a laboratory

constructed burner supporting propane and hydrogen fueled flames, and electronics for

signal measurement. Cesium atoms were excited into the 8d 2Ds/2 state by tuning the dye

lasers to the 6s2S/2 -- 6p2P3/2 (// = 852.12 nm), and 6p2p32 -> 8d2Ds/2 =2 = 621.3 nm)

transitions, respectively. Excited Cs atoms were collisionally ionized in the flame, and

the ionization signal was detected with a 0.4 mm diameter Pt wire (12 mm long)

electrode set 4-5 cm above the burner head. A negative voltage of up to -5 kV was









applied to a planar (screen) electrode consisting of a stainless steel wire mesh (1 x3 cm2).

The planar electrode was positioned just outside the flame (barely touching the flame) at

the same height as the probe. Introduction of this planar electrode was necessary in order

to prevent electrons originating in the primary combustion zone of the flame from

reaching the probe and contributing to the background noise. The counter-propagating

laser beams were coaxial with the Pt probe and had a diameter of 3 mm. The dimension

of the probe volume, defined by the dimensions of the exciting laser beams was 3x3x12

mm3. The distance between the 0.4 mm Pt probe and the mesh electrode was -10 mm.


Dye lasers
/ --] ,/ ~]





SScreen i
electrode Probe



7.5 MQ 15OpF -

0-5 kV Preamplifier Oscilloscope
(+ or-)
\ JiOk

Burner head

Figure 7-8. Schematic diagram of the experimental setup

A high-voltage power supply (Series 205, Bertan Associates, Inc.) was connected

to the planar electrode and the Pt probe was connected to the signal detection system,

consisting of a laboratory-built, charge-sensitive, low-noise preamplifier and a digital

oscilloscope (Tektronix, TDS 620A). By changing the polarity of the applied voltage,








either the ions or electrons could be collected with the Pt probe. Radio frequency

interference noise was minimized by placing the burner and probe inside a shielded metal

box. The internal 20 MHz lowpass filter of the oscilloscope was used to further reduce

high-frequency noise.

Results and Discussion

A preliminary set of experiments was performed in a hydrogen/air flame with a

detection scheme using an electric field formed between the Pt wire probe and the burner

head. A positive high voltage was applied to the probe while the burner head was

grounded. The distance between the probe and burner head was 4 cm. The two-electrode

scheme used was characterized by poor S/N due to the presence of high background

noise. The source of this noise was identified as the electrons produced in the primary

combustion zone of the flame, which is located close to the burner head. In the strong

electric field between the probe and the burner head, these non-selective electrons can

easily reach the probe, causing a high level of background noise. Due to this high

background, it was not possible to produce a stable avalanche effect because all electrons,

no matter how they originated, experienced the same avalanche effect under these

conditions. At rather moderate voltages (-3 kV), electrical breakdown between the probe

and burner head occurred. As mentioned above, the third planar electrode was

introduced to overcome this problem. Both the probe and the burner head were kept

positive with respect to the planar electrode, which was at a negative potential. As a

result, selective electrons (produced by laser-enhanced ionization of Cs atoms) were

accelerated toward the probe and detected. All electrons from the primary combustion

zone of the flame were removed, via the burner head, to ground. Thus, the three-

electrode scheme formed or produced a "silent" (low thermal electron) zone in the upper








part of the flame providing a favorable environment for avalanche amplification of the

selective, laser-induced electrons.

In a weakly ionized plasma such as a flame, very often the electric field between

the electrodes is not uniform and is modified by a positive ion space charge or "sheath",

adjacent to the cathode [61, 66]. When the concentration of ions is high enough and

sufficiently low voltages are applied between electrodes, the electric field strength has a

maximum at the cathode, decreasing linearly with the distance from the cathode to zero at

the edge of the sheath. However, when higher voltages are applied, a condition of

"electrical saturation" with a non-zero field and a positive space charge at all positions in

the flame, is observed [66]. At higher voltages, the sheath can extend to the primary

combustion zone with its non-uniform temperature and composition. As discussed above,

the electrons produced in the primary combustion zone of the flame were the main source

of noise and the cause of breakdown in our experiments and was eliminated by using the

three-electrode scheme. In the work of Travis, the sheath length was evaluated

experimentally for Na atom LEI in an air/acetylene flame using a 1 ptg/ml solution [67].

The field was formed between a 5 cm single slot burner head and a 0.635 cm diameter

stainless steel tube. At applied voltages of up to the 1200 V, the sheath length was 11-13

mm. In the experiments reported here, up to 5 kV was applied to electrodes separated by

only 10 mm. Moreover, the very dilute Cs solutions (100 pg/ml) aspirated into the low

background hydrogen/air flame assured the presence of high electric field at all points

between the electrodes. The small diameter (0.4 mm) of the Pt wire probe also should be

emphasized because that is the key to having a very strong electric field, which is

essential for avalanche amplification. The field map in the three-electrode scheme is









complex, but a field between the Pt wire probe and the planar electrode could be

approximated as an idealized field between a cylinder and a plane [68]. The field

intensity is inversely proportional to the wire probe diameter.


350-
1,96-A
96-a) 3 b)

1.92- A
A 250-
A/
1.88 / *
A 200- 2/
1.84- 'Ai E 1 "....50. CS(100pg/ml)
._ 150-
/ r A Li (1 g/ml) /
LM 100-
A_ 8o Cs (100 pg/ml)
S' 7 Li (1 plg/ml) 500o.
1-76 *
0-
1.72- *
-50-1
0.0 04 08 12 1.6 20 20 2.4 2.8 3.2 3.6 4.0
HV, kV HV, kV



Figure 7-9. LEI signal vs probe voltage: a) ion, and (b) electron signals for Cs and Li
atoms


Evidence of avalanche multiplication is seen from the comparison of the electron

and ion signals obtained with 100 pg/ml Cs and 1 Vtg/ml Li solutions under the same

experimental conditions (Figure 7-9 a, b). Ions were detected by switching the polarity of

the planar electrode to positive so that both the probe and the burner head were negative

relative to the planar electrode. When ions were detected, the signal increased linearly

with the electrode voltage until it reached a plateau showing no avalanche behavior. In

contrast, when electrons were detected, the exponential increase in signal as a function of

voltage shows the typical shape of an avalanche process, continuing until breakdown. In

the strong electric field between the probe and the burner head, electrons can gain enough

kinetic energy to liberate electrons from inner shells of gas atoms and molecules leading

to de-excitation by UV-photon emission. These energetic UV-photons will produce








further electrons by ionizing the gas atoms. This results in a transition from proportional

avalanche to a streamer followed by electrical breakdown. This transition process is well

known and has been extensively studied and described in the nuclear particle detector

literature. The breakdown time is very small, of the order of a few tens of nanoseconds

[69]. In this particular experiment, the breakdown occurred at voltages close to 4 kV. The

breakdown voltage varied with the distance between the electrodes and the solution

concentration aspirated into the flame. The planar electrode should be kept just outside

the flame, but as close as possible in order to provide the maximum possible electric

field. If the planar electrode is immersed in the flame, then electrons which are thermally

emitted from the surface of the electrode will reach the Pt wire probe, increasing the

noise and the risk of breakdown. A maximum avalanche amplification factor of

approximately 105 was observed. The avalanche amplification factor was estimated as the

ratio of the signal obtained just before breakdown (at 3.8 kV) to the smallest detected

signal (at 2.4 kV) detectable when the avalanche process was absent. The experiments

were repeated with a 1 gg/ml Li solution in order to prove that the avalanche effect is not

due to any unique properties of Cs. The ionization rate of Li atoms in the flame was

enhanced by tuning the dye laser to the 2d2D -> 3/F resonance transition of Li (610 nm).

The behavior of the ion and electron signals was exactly the same as in the case of Cs.

Three types of flames were studied: hydrogen/air, hydrogen/20% oxygen/argon and

propane/air. The hydrogen/air flame gave the highest proportional ionization signal

compared to the other two flames (Figure 7-10). The signal in hydrogen/air flame was

approximately two orders of magnitude larger than that in the propane/air flame, and one

order of magnitude larger than that in the hydrogen/oxygen/argon flame. As known from









the literature, all three flames are expected to provide similar temperatures, and

consequently similar thermal ionization efficiencies. As can be seen in Figure 7-10, the

three flames exhibited largely different signal amplitudes at the critical voltage just below

the breakdown point. This different behavior reflects the different chemical composition

of the flame mixtures and the resulting different species responsible for the avalanche


3.0
*
2.5 /
-m- Hydrogen + Air
2.0 -*- Hydrogen + (Ar/O2)
A Propane + Air
> 1.5 /
C
1.0-

0.5 U- /

A
0.0 *-- -- *- A A

0.5 1.0 1.5 2.0 2.5 3.0
HV, kV



Figure 7-10. Signal vs. voltage behavior in flames showing the occurrence of the
avalanche effect. Cs: 1 [tg/ml.

effect. Since the signal decreased substantially when argon was substituted for nitrogen,

i.e., when a H2 /02 /Ar flame was substituted for the hydrogen/air flame, it can be

concluded that nitrogen is the most probable source of avalanche electrons. This

conclusion is supported by the fact that (i) its ionization potential (15.5 eV) is slightly

less than that of Ar (15.7 eV); (ii) nitrogen molecules in Hz flames are usually more

efficient collisional partners, compared to argon atoms, and the ionization rates are






88


therefore higher in N2 than in Ar; and, (iii) Ar-diluted flames usually exhibit lower


temperatures than the corresponding N2-diluted flames [70]. The poorest results were


obtained with the use of a propane-air flame, which can be attributed to the avalanche

quenching properties of the organic molecules in the flame. Additionally, hydrocarbon-


fueled flames are known to have a much higher background current compared to


hydrogen flames [61]. This background current has to be kept as low as possible for both


conventional LEI and avalanche-LEI, since it will ultimately determine the detection


limit and the selectivity of the technique.



S1600
14W I
/S
F_ -36kW] 1200-
01 4kV
/ 41000-
> Boo/ BO- .
0" 001 U)
600-
400-
1E-3- 4 0
.200- ,

1E-4 -, ., ., -, .. .. .. ,.,
i.3 001 0o i 10 100 1000 10000 10oo6ooo 3.4 3.6 3.8 40 42 44 46 48 50 52
Concentration, pg/ml HV, kV
a) b)

Figure 7-11. a) Calibration curves for Cs at two different electrode voltages in the H2 Air
flame b) Signal-to-background vs applied voltage. Cs: 100 pg/ml


Once the effect of avalanche amplification of the LEI signal was confirmed, the

limit of detection of the system was estimated in the hydrogen/air flame. The practical

LOD achieved in this work was 100 pg/ml (Figure 7-1 la), limited by memory effects of

the burner/nebulizer system. As mentioned above, the breakdown voltage varied with the

distance between electrodes. At the optimal distance between electrodes, with a voltage

of up to 5 kV, the signal-to-noise characteristics of the system were studied and the

highest S/B was observed at 4.4 kV (Figure 7-1 lb). The drop of the signal-to-background









ratio at higher voltages indicates that when the avalanche amplification is very high, the

system is getting close to the transition from proportional avalanche to a streamer. The

system was further optimized in terms of flow rate of the flame gases in order to obtain

the maximum possible signal at 4.4 kV. Five measurements of the signal and

background were carefully taken at the optimal voltage. Background measurements were

taken with no lasers and aspirating only de-ionized water into the flame, which was used

as a solvent for all solutions. Each measurement was an average of 1200 pulses at 20 Hz.

Sample measurements of averaged signal and noise are shown in Figure 7-12.


700
600- a)01 b)
50 0 -0 .2
400- 03


-100- E
-0.7
100- CO) -0.5-
0 kY 06-

-0.05 0.00 0.05 0.10 0.15 0.20 -50 0 50 100 150 200
Time (pis) Time (ps)


Figure 7-12. Temporal variation of (a) signal and (b) noise; Cs: 100 pg/ml.


For each measurement, the signal was calculated as the area under the curve, calculated

as the sum of intensities of 8 points (from -1 to 2.5 pts). For the noise calculation, the

entire time scale (500 points) was divided into 62 sections, each with 8 points. The area

under each 8 point segment was then calculated and the standard deviation of those 62

areas was taken as the noise. The signal-to-noise ratio was calculated based on 3y

criteria and found to be more than 104. Based on this S/N, an extrapolated, limit of

detection of 30 fg/ml was estimated (average of 5 measurements). For comparison, the








LOD was 40 fg/ml when calculated using the peak height. Table 7-1 compares the LODs

for Cs obtained by selected analytical techniques as well as previously obtained LODs

using LEI.


Table 7-1. LOD comparison.

Element Method LOD (ppt, pg/ml)

Cs AAS* 1000

Cs ICP-MS 1

Cs LEI in Acetylene/Air [1] 2

Li LEI in Acetylene/Air [1] 0.3

Cs Experiment 0.03

* The data from the LOD table of VHG LABS (www.vhglabs.com)

Conclusion

In this chapter, the concept of electrodeless optical detection of laser resonance

ionization signal with avalanche amplification has been discussed. Based on such

concept, the design of the resonance ionization imaging detector with a buffer gas has

been illustrated and the experimental results proving the concept has been shown. The

buffer gas RIID could be especially useful in applications requiring a broadband but

sensitive imaging detector, such as chemical and biological Raman imaging

spectroscopy. Also possible is construction of very thin imaging cells, with high spectral

and spatial resolution.

The avalanche effect was applied to the fundamental laser-enhanced ionization

technique. The LEI signal of Cs atoms in a hydrogen/air flame was detected using

avalanche amplification. Three types of flames were studied: hydrogen/air,





91


hydrogen/20% oxygen/argon and propane/air. The hydrogen/air flame gave the highest

signal-to-noise ratio, showing that nitrogen molecules might be the best environment for

avalanche amplification of the LEI signals in flames. The LOD of 30 fg/ml obtained in

the experiments was approximately 70 times lower than the best previously obtained

LOD for Cs in flame LEI (2 pg/ml) and 10 times lower than the lowest LEI detection

limit ever reported (Li, 0.3 pg/ml).













CHAPTER 8
CONCLUSIONS AND FUTURE WORK

Conclusions

The resonance ionization imaging detector with Cs vapors has been constructed and

evaluated. The detector, based on narrowband absorption of signal radiation in an atomic

medium and subsequent laser stepwise ionization of the excited atoms, offers

unsurpassed performance as a two-dimensional imaging detector compared to existing

ones. The superiority of the detector is determined by its high Luminosity Resolving

power (LR) product, which is orders of magnitude greater than any other spectroscopic

imaging technique. The cesium RIID is the second existing RIID, in addition to the

mercury RIID developed previously in our laboratory. Cs is one of the few elements that

is suitable for the construction of the RIID and can offer certain advantages, such as high

sensitivity due to strong signal transition of Cs atoms. Also, the infrared detection

wavelength of Cs RIID (852 nm) allows important biological applications of the detector,

because radiation in this spectral range is safe for biological tissues and has good

penetration depth. Two-step (2/ = 852, 22 = 508 nm) and three-step (2A = 852, A2 = 917,

23 = 1064 nm) ionization schemes of Cs have been investigated using various lasers,

including XeCl excimer, Nd:YAG, dye and color-center lasers. The efficiency of the

three-step ionization scheme has been shown in Cs the atoms resonance ionization

detection (RID). The imaging capabilities of the Cs RIID have been evaluated using two-

step ionization of Cs. Images were obtained with a conventional CCD camera, varying

the RIID parameters such as applied high voltage, the number density of Cs atoms and




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