New developments in instruments for counting and detecting nuclear particles

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Title:
New developments in instruments for counting and detecting nuclear particles
Series Title:
United States. Atomic Energy Commission. MDDC ;
Physical Description:
4 p. : ; 27 cm.
Language:
English
Creator:
Jordan, W. H
Oak Ridge National Laboratory
U.S. Atomic Energy Commission
Publisher:
Atomic Energy Commission
Place of Publication:
Oak Ridge, Tenn
Publication Date:

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Subjects / Keywords:
Nuclear counters   ( lcsh )
Nuclear physics -- Instruments   ( lcsh )
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federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Bibliography:
Bibliography: p. 4.
Restriction:
"Date Declassified: March 31, 1947"
Statement of Responsibility:
by W.H. Johnson.
General Note:
Manhattan District Declassified Code
General Note:
"Date of Manuscript: March 7, 1941"

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University of Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005024619
oclc - 288524552
System ID:
AA00009289:00001


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MDDC 811


SITED STATES ATOMIC ENERGY COMMISSION






NEW DEVELOPMENTS IN INSTRUMENTS FOR COUNTING

AND DETECTING NUCLEAR PARTICLES


W. H. Jordan




Oak Ridge National Laboratory


Date of Manuscript:
Date Declassified:


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D -. *
,;,-, ',
--. -.


March 7, 1941
March 31, 1947


This document is for official use.
Its issuance does not constitute authority
for declassification of classified copies
of the same or similar content and title
and by the same author.





Technical Information Branch, Oak Ridge Tennessee
AEC, Oak Ridge, Tenn., 3-9-49--750-A3374


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NEW DEVELOPMENTS IN INSTRUMENTS FOR COUNTING
AND DETECTING NUCLEAR PARTICLES*

By W. H. Jordan

Instruments for detecting and counting nuclear particles have
improved considerably in the past five years. There have been no revo-
lutionary discoveries--one still uses gelger counters, ion chambers,
linear amplifiers, scalers, and scopes. The tools are still the same
but the design and capabilities of them have improved. This is due to
several factors. The tubes are better, circuit techniques are better,
and our knowledge of the detailed operation has considerably increased,
thus enabling us to use the tubes more effectively. Components such as
condensers and resistors are more reliable and have longer life so that
one can build more complex circuits without having to worry about whether
the instruments will function long enough to get the data.

One of the first things that any nuclear physics laboratory needs
is a good oscilloscope. Chiefly as the result of radar, scopes are
vastly improved. Cathode ray tubes now give small, bright spots so that
high sweeps are possible. Sweep circuits have developed so that one can
see the details of pulses 0.1 p sec long. (One scope has been described
recently1 on which sine waves having a frequency of 10,000 Mc/sec can be
seen as individual waves. That's 10-10 sec I am talking about now.)
Video amplifiers with a bandwidth of 20 Mc are useful adjuncts to these
scopes. And for nuclear work the development of good delay lines has
been a great boon. One can thereby investigate the pulses that come from
geiger counters; for example, by first letting the pulse start the sweep,
then delaying it for a fraction of a miscrosecond before applying it to
the deflecting plates of the scope. Those of you interested in the de-
tails of these developments should procure appropriate volumes of the
Radiation Laboratory Technical Series2, soon to be published.

One does not detect individual nuclear particles directly but
rather by the ionization they produce. Thus we have ionization chambers,
geiger counters, proportional counters, cloud chambers, secondary elec-
tron multipliers, crystal counters, fluorescent screens. In many cases
it is necessary to follow these instruments with amplifiers, pulse height
selectors, and scalers. These are the primary tools of the nuclear phys-
icist and will be the subject of my talk today.

First consider the ionization chamber. High velocity electrons,
protons, as alpha particles ionize the gas. The ions are collected on
one plate, thus producing a current of electricity. Gamma rays are de-
tected with poor efficiency by the secondary electrons they produce.


*Presented at the April 1941 meeting of the Southeastern Section of
the American Physical Society, as an illustrated speech. The slides are
not being included in this article.


MDDC 811






2 MDDC 811

Neutrons are detected by several reactions. In hydrogenous materials
knock-on protons are produced. Alpha particles are produced in boron by
the B10 (n,a) LiT reaction so that the chamber may be filled with FS3 or
lined with boron. A significant development is the, lprovement in efficiency
of detection by the use of the separated isotope B10 which can now be
produced in significant quantity. Another very useful reaction for the
detection of neutrons is the (na) reaction. By coating the chamber with
fissionable material such as U235 one obtains very heavily ionizing fission
particles. Such fission chambers can be made very small and compact and
yet use a large area of plate by making the plates of thin strips wrapped
in a spiral.3

Let us next consider how one observes the current in the ion chamber.
When the current is large, a galvanometer is adequate. Very mall currents
are usually detected by measuring the voltage across a high resistance,
perhaps 1012 or 1015 ohse. The use of electrcoeter tubes such as the FP-5.
is well known, and recent improvements in its construction have been made.
Other small electrometer tubes have recently been produced by Victoreen.
Also, well known are the zero drifts that one encounters with DC amplifiers
so the vibrating condenser electrometer technique is fast coming into use ,
Here one converts the small DC voltage into an AC voltage by varying the
capacity of a condenser connected to the ion chamber. The amplification
of this voltage by an AC amplifier and its subsequent rectification is a
simple matter. The use of feedback makes a very stable system.

If one is interested in detecting individual particles the situation
is quite different and the use of electron collection techniques is one of
the most interesting developments in the past few years. Both the ion
chamber and the linear pulse amplifier following are involved and it will
be necessary to consider each in considerable detail.

First let us consider what goes an in the ion chamber following the
passage of a nuclear particle.7 Electran-ion pairs are created along the
path, about 30 ev of energy being substracted from the particle for each
pair created. The electron and positive ion move in opposite direction
under the influence of the collecting field. So long as the electron
does not become attached to a gas molecule, its velocity will be much
higher than that of the positive ion. The probability of attachment and
the velocity of the electron can be seen from Table 1 (Slide I)

If a gas with a small value for the probability of electron attach-
ment is used, the electrons are nearly all collected before they form
heavy negative ions. By using a mixture of gases, such as Argon and CO2
the time of collection can be made very small, of the order of 0.la sec.
When this is compared with the several hundred miscroseconds required to
collects heavy ions, one begins to see some of the advantage of electron
collection.

Let us now consider what happens to the potential of the collecting
electrode following an ionizing event. As the electron drifts toward the
positive electrode (assumed to be the collecting electrode) its potential
drops rapidly until the electrons are stopped. Then the potential changes
very slowly as the positive ions move out to the other plate. If one now
uses an amplifier that is insensitive to slow changes in potential, i.e.,
discards low frequencies by means of a short time constant coupling circuit,





MDDC 811


one gets out of it a sharp, high pulse. By making the differentiating
time constant only somewhat longer than the electron collection time one
can count ionizing events that are separated by only 1 Pesec. This per-
mits high counting rates (up to 100,000 per second) and accurate coin-
cidence measurements. It permits counting of alpha particles in the
presence of an extremely high $- orir- ray background, since there is
little time for "pile-up." Troubles with microphonics and hum are prac-
tically eliminated since the low frequencies have been discarded. These
items represent very real improvements in counting techniques.

There is one disadvantage with the scheme of using electron col-
leetion, namely, the pulse height depends on where the electrons started
in the chamber. This can often be circumvented by insuring that all
ionizing particles go through the same part of the chamber and in the
same direction. Where this is not feasible, the use of an extra grid
near the collecting electrode may be required.

Linear amplifiers for fast counting have been considerably improved
in the past few years0 I would like now to spend a few minutes describing
an amplifier which has recently been developed at Oak Ridge National Labo-
ratory and is typical of what can be done with present techniques.
(Slides 2 and 3). A part of the address presented at this point was read
directly from the paper, of "A General Purpose Linear Amplifier," by Jordan
and Bell.9

Following the amplifier and preceding the mechanical register
comes the scaler. A modification of the Ecces-Jordan circuit due to
Higginbotham represents a significant improvement in that the scaling is
much more reliable. Scalers '(Slide 4I10 using this circuit are available
commercially with resolving timer of 5 D.'se., Lawson has recently de-
veloped a decade scaling circuit which is being marketed as the YY2-1
scaler with a 0.1 J sec resolving time.

Our insight into the operation of geiger counters has been extended
in the past few years and described in publication by Montgomery, Korff,
Stevens, Rose, and Present,11 The use of quenching gades is now generally
adopted and the nature of the quenching mechanism fairly well understood.
Random delays in the time of firing are troublesome when very accurate
coincidence measurements are attempted. The delays amount to about 0.,
microsecond as has been shown recently by Sherwin.AL2 Two possibilities
for reducing these delays have been considered recently. One is the use .
of crystal counters3 and the other the use of electron multiplier tubes.Y

Whether the crystal counter is any better in this respect is now
being investigated. They are, however, interesting from another stand-
point in that the size of the pulse produced is proportional to the
energy of tha P-particle. Crystal counters are much like ionization
chambers except that the gas is replaced by a solid, in this case a
single silver chloride crystal at low temperature. Ions produced by the
passage of a beta particle are collected on one electrode, thus producing
a voltage pulse. Since the crystal is dense, a high energy beta particle
will be stopped in it, thus producing some 100,000 ion pairs, quite ade-
quate for detection when coupled to a linear amplifier. The higher the
energy of the beta particle, the more ion pairs produced. One serious
difficulty is that beta particles coming into the crystal may be scat-
tered back out before they have been stopped.





MDC 811


The secondary electron multiplier has proven valuable for counting
individual nuclear particles such as protons and alpha particles. It
needs considerable improvement to get it into a form convenient for every-
day use. But it does have a small delay, and a fast pulse rise time.
The fact that it operates in a vacuum is often a distinct advantage over
proportional counters end geiger counters.

Proportional counters have been used very extensively. When boron
coated or BF filled, they make excellent neutron detectors. Their char-
acteristics have been investigated recently by Bossi and Diven.15 They
showed that if care is taken, both in construction and filling, the volt-
age pulse obtained is proportional to the ionization produced. The leads
supporting the counter wire should be shaped to reduce the effects of
fringing electric fields. Also, the gas must have a low electron attach-
ment coefficient. Argon, hydrogen, and nitrogen work very well with gas
multiplications up to 1000. Multiplication up to 10,000 is possible with
the addition of a quenching gas such as methane.

RUEHEENCES

1. Proc. I.R.E. 1947.
2. Radiation Laboratory Technical Series, McGraw-Hill (In Press).
3. W. C. Bright, Spiral Fission Chambers, MDDC-91.
4. Lafferty and Kingdon, Jour. App. Phys., Nov. (1945).
5. Palevsky, H., R. K. Swank, and R. Grenchick, Design of Dynamic Con-
denser Electrometer. MDDC-38.
6. Scherbatskov and Feron, Phys. Rev. 70:96, (1946).
7. Snyder, T. M., Theory of Ion Chambers, MDDC-475.
8. Allen, J. S., and B. Rossi, Time of Collection of Electrons in
Ionization Chambers, MDDC-448.
9. Jordan, W. H., P. R. Bell, A General Purpose Linear Amplifier.
(Has been declassified and submitted to RSI for publication).
10. Higgenbotham, W., Scaler Unit Circuit, MDDC-114.
11. Van Nostrand, D., Electron and Nuclear Counters, Korff.
12. Sherwin, Paper at Jan. 29, (1947) Meeting of American Physical
Society.
13. Van Heerden, P. J., The Crystal Counter, A New Instrument On Nuclear
Physics. Published by N. V. Noord Hollandsche Uitgeders Maat-
schappij Amsterdam, 1945.
14. Allen, J. S., BSI, 12, 484, (1941). Bay, Z., RSI, 12, 127, (1941)
15. Diven, B. C., Operation of Proportional Counters at Pressures above
Atmospheric, MDDC-458.


END OF DOCUMENT




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