Construction features of a circuit for coincidence measurements and applications of the circuit to nuclear problems

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Material Information

Title:
Construction features of a circuit for coincidence measurements and applications of the circuit to nuclear problems
Series Title:
United States. Atomic Energy Commission. MDDC ;
Physical Description:
10 p. : ill. ; 27 cm.
Language:
English
Creator:
Moak, C. D
Argonne National Laboratory
U.S. Atomic Energy Commission
Publisher:
Atomic Energy Commission
Place of Publication:
Oak Ridge, Tenn
Publication Date:

Subjects

Subjects / Keywords:
Nuclear counters -- Design and construction   ( lcsh )
Coincidence circuits   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Bibliography:
Includes bibliographical references (p. 9).
Restriction:
Date Declassified: May 20, 1947
Statement of Responsibility:
C.D. Moak.
General Note:
Manhattan District Declassified Code
General Note:
Date of Manuscript: April 30, 1945

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005023430
oclc - 465256585
System ID:
AA00008515:00001

Full Text

7.I


A',


MDDC


- 1010


UNITED


TATE


ATOM


ENERGY


COMMI


ION


CONSTRUCTION FEATURES OF A CIRCUIT FOR COINCIDENCE MEASUREMENTS
AND APPLICATIONS OF THE CIRCUIT TO NUCLEAR PROBLEMS


by
C. D. Moak


Argonne National Laboratory


Date of Manuscript:
Date Declassified:


April 30, 1945.
May 20, 1947.


Issuance of this document does not constitute


authority for declassification of


classified


copies of the same or similar content and title


and by the


same author.


Technical Information Division, Oak Ridge Operations


U S OEPOSI~O


AEC, Oak Ridge, Termnn.,


12-20-48


--850-A4001


--


/6





































Digitized by the Internet Archive
Digitized by the Internet Archive


in 2011


with funding from


University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation





a) Conversion Coefficients of Aus98 and Xe'ss


b) Gamma-Ray Energy Measurements by Compton-Recoil Absorption


By C. D. Moak


INTRODUCTION

The inherent characteristics required of a coincidence circuit for nuclear research, as pointed
out by Norling1 are:


1. High resolving power.
2. The ability of the circuit to function properly at high single counting rates.
3. Low sensitivity to external electrical disturbances.
4. The circuit shall register a constant percentage, or preferably all of the coincidences.
E has been found here that each of the listed characteristics is more or less dependent upon the
e so that optimum operating conditions must be determined. For example, it is known that if the
resolving power of a circuit is made very high, the circuit will only count a fraction of the true
Lnces, an, effect which will be explained later. Such a condition might be improved to a small
extent by increasing the amplification of the circuit but this procedure usually increases the sensitivity
hjte circuit to external electrical disturbances. If the resolving power is decreased by a large
amount, the chance coincidence rate will obscure the real coincidence rate.
An outline of the uses of a coincidence circuit and an outline of the present basic design principles
are in order here. The coincidence circuit has been found valuable in the examination of the decay
cheese ontural and artificial radioactivities, i.e. the evaluation of internal conversion coefficients
o tbe determination of y/ ratios. It has been used to measure gamma ray energies by Compton
repco absorption. Some work has been done also in the measurement of very short half-lives. The
detection of certain radioactive isotopes in the presence of large radioactive contaminations is made
Possible through the use of the coincidence circuit.


DESCRIPTION OF THE CI UIT

The heart of the present day coincidence circuit is the Rossi stage.' Described in termsof o
triodes, the stage might be designed as shown in Figure 1. The circuit consists of any number triodes
Wth plates fed through a common resistor Rp but with grids driven by separate counter tubes, oqr
simplicity the two-channel case will be assumed throughout the paper, The vacuumntbe characterftiscs
of T1 and T, might appear as Line A in Figure 2 (a) with the load line of Rp as shown intersecting A at
Y. When the plates of T1 and T, are connected in parallel the characteristics of T, + T, would simply


CONSTRUCTION FEATURES OF A CIRCUIT FOR COINCIDENCE MEASUREMENTS
AND APPLICATIONS OF THE CIRCUIT TO NUCLEAR PROBLEMS










MDDC 1010


+HV


- OUTPUT


STN


Figure 1. Basic Rossi stage.



negative pulse strikes the grid of T1 cutting its plate current off, the operating point becomes that of
the single tube T1 operating at point Y on line A so that a small pulse equal to S appears at the output.
However, when two pulses each strike the grids of Ti and T2 simultaneously the current through R
is completely cut off so that the operating point moves from X up to Z or the high-voltage potential so
that a pulse of the size C, as shown, would appear at the output. These are coincidence pulses and
they may be caused by the simultaneous discharge of two counters. Using triodes in the Rossi stage,
these pulses may be three or four times as great as the small pulses produced by single pulses in
either channel. The assumption is made that the counter tube pulses have been sufficiently amplified
so that they will drive the grids of the Rossi stage to cut-off point. The situation is greatly complicated
by the fact that with very short, sharp pulses the interelectrode capacitances of the tubes limit the
action of the circuit.
Figure 2(b) shows the curves for a Rossi stage using pentodes. Theoretically the discrimination
between single counts and coincidence counts should be greater for the pentodes, as shown, but
interelectrode capacitances are greater so the advantages gained by using pentodes are small. One
thing is evident from this, that although the Rossi stage discriminates against single counts in favor
of coincidences, nevertheless single count pulses do appear in the output. The use of some sort of
stage with an excitation threshold above the voltage of these single count pulses following the Rossi
stage is clearly indicated. The writer has used a trigger stage following the Rossi stage for the reason
that its output pulses are of constant magnitude regardless of the magnitude of those excitation pulses
which are above the excitation threshold.
So far it has been assumed in the basic design that all counter pulses (or pulses striking the
Rossi stage grids) are of equal magnitude and of sufficient magnitude to extinguish a tube in the Rossi
stage. The ratio of pulse magnitudes of a standard glass Geiger tube and a mica-window 3 counting
tube may easily be an order of magnitude. Now it is desirable in the interest of high resolving power
that the dead time per pulse of a Rossi-tube be as small as possible. If simple amplifiers precede the
Rossi stage complications immediately arise; one can easily see that if the sensitivity of the amplifiers
is such that a pulse from a 0 counter will block a Rossi tube then if one replaces the 3 counter with a
standard glass counter, such a counter will block a Rossi tube upon rising to one-tenth its full pulse
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MDDC 1010


Figure 2(b).
pentodess.


Curves for Rossi stage using


Figure 2(a).
triodes.


[3










Curves for Rossi stage using


Another difficulty which arises is the fact that all counter tubes operated at high counting rates
give a certain percentage of so-called immature pulses. If one particle to be counted follows very
Closely upon another, it is likely that the counter will not be capable of returning to normal rapidly
enoug to, receive the following particle so that the counter may deliver a pulse of much less than
normal magnitude. If such a pulse were to come from a 0 counter in the previously mentioned circuit,
it would be incapable of blocking the Rossi tube to which it is applied, and, therefore, incapable of
producing a full-sized coincidence pulse if the other Rossi stage were blocked simultaneously, with the
result that a pulse would appear at the Rossi output which would fall somewhat short of a full -sized
coincidence pulse. From-these considerations, a trigger circuit preceding the Rossi stage whose ex-
citation threshold may be set at approximately one-third the pulse magnitude of a counter and whose
output ptulse shape and magnitude are independent of input pulse shape or margnitudeis clearly indicated
Such a trigger circuit has beein developed by Bradley- and is widely used in coincidence circuit wark
It is so designed that its output pulses are capable of turning a Rossi tube off and on in a very short
time interval.
*** <'^^**<* ** .- .*










MDDC


- 1010


experience is as follows: A two-stage amplifier followed by a trigger circuit in each channel, the
channels each driving a grid of a triode or pentode Rossi stage followed by a trigger circuit output


stage.


The diagram of such a circuit would be


as shown in Figure 3. Two circuits of this type have


been built here and both have nearly equal characteristics.


The layout here has involved the use


of three scalers. Bradley4 has designed a dual scaler for this work which consists of two Higginbotham
scales of 256 for counting the single counts directly. An Offner scale of 64 is used connected to the


output of the coincidence circuit.


With this arrangement single counts and coincidence counts may be


run simultaneously.


The three units are mounted together in a cabinet,


as shown in the photograph.


CHARACTERISTICS OF THE CIRCUIT

Having thus developed a basic coincidence circuit, it must now be ascertained whether or not the


properties desired,


as outlined in the beginning, have been attained,


The first of these properties is


high resolving power.


When counters are connected to the two channels of a coincidence circuit and


are made to count the radiations from two separate sources, there is a certain probability that a count


in one channel will occur within a small time interval bf a count in the other channel.


This probability


decreases


as the small time interval is decreased.


The time interval above which a circuit can


reject these chance coincidences is the resolving time of the circuit and a measure of its resolving


power, i.e., if pulses in each channel of a circuit must be 10"8


sec or less apart in time in order to be


registered


as coincidences, then one says that the resolving time of the circuit is 10-6


sec. This factor


may be determined in the following way:.


= 2N1 N2T for two channels


23 = 3N1 N N 2 Tfor three channels
..n = 3N1 1N... Nn 7n1' for n channels


where the N's


are the single counting rates and A the chance coincidence rate assuming r for any two


channels to be the same and assuming separate sources of counts for each channel. If T is different
for different pairs of channels:


A12 ***


=(NIT N2r2


1 1
i1


S.... +).


It is desirable that r be


as small


as possible so that the chance coincidence rate shall not obscure the


real coincidence rate. Here the advantage of the pentode Rossi stage and variable threshold output trigger


circuit is seen. Assume a Rossi stage whose resolving time is 2


time by, say, 1.5


x 10-


x 10-6 sec. If two pulses differ in


sec, the later pulse will strike its Rossi tube before the other Rossi tube is


completely conducting (i.e. back to normal). These "near misses" produce pulses of intermediate
size in the output of the Rossi stage. By setting the threshold of the output trigger stage so that real
coincidence pulses are barely great enough to trip the output circuit, those below that magnitude will
be rejected. The circuit described in this paper may be set to any resolving time within the range
10-'to 10-5 sec.
The second requirement of a coincidence circuit is that it be capable of functioning properly at
high single counting rates. It has been found that the effective resolving time of a counter tube (i.e.,


dead time) is about 3


x


sec so that one would expect the circuit to be better than the counter


tubes in this respect since the pulse duration in the circuit is everywhere less than 10-5


The third requirement is that the circuit have low sensitivity to external disturbances.
-... .1_ _s SI l ** **j* *


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MDDC 1010


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Electronic methods have been devised for testing the circuit. Periodic pulses resembling counter
pulses at a frequency of 2 x 104 cps were fed into first one channel, then the other, and then both.
When the pulses were fed through both channels simultaneously at 7 = 10-7 sec no loss of coincidences
could be detected. Also, the pulse size in one channel was varied relative to the pulse size in the
other. Again, no loss was observed. This proved that the coincidence circuit does not lose coincidences.
However, this did not prove that the combination of coincidence circuit and counter tubes would not
cause a loss of coincidences. It has been found6 that there is a variable time-lag from the instant a
particle traverses a counter until a pulse begins to appear on the positive wire of the counter. The
magnitude of this lag is of the order of several microseconds and varies slightly with counter voltage
and gas pressure.
It is generally believed that fluctuations in time-lag are the cause of real coincidences being lost
at low resolving times. Usually one can operate at a resolving time of 10- sec or higher without these
effects being appreciable. If one wishes to use a resolving time less than 106 sec, it is advised that
care be taken to determine if the system is losing coincidences, and, if so, to determine accurately
what the percentage loss is. It may be stated here that some counters using only a pure gas have been
shown to have time lags much less than several microseconds, some having lags less than 10-7 see.
However, if one wishes to use a conventional vapor-quenching argon-alcohol counter at low resolving
times, the loss of coincidences should be checked just before taking any coincidence data.
The procedure followed here in using two mica-window counters involves the use of a special type
counter. As shown in Figure 4, one of the counters has a .mall hole drilled in the closed end. A mica
window is waxed over the hole and a source of p particles placed directly against the small mica
window so that 3 particles may pass through the special counter into another conventional p counter.
With this geometry, any p particles entering counter 2 must have passed through counter 1 so that any
count produced in counter 2 must also be a coincidence count if the efficiency of counter 1 is unity and
if the circuit is losing no coincidences. Four readings are taken; S is the single count in 2 with the
source in place and SgB with the source removed. C and CB are the corresponding coincidence counts.
The following relation, then, should yield some information as to loss of coincidences:


C-CB


$2~S2B


= E, Ec


where E1 is the efficiency of counter 1. If the efficiency of counter 1 is unity, then a direct determination
of coincidence efficiency Ec is obtained. If EiEc is much less than unity, some effort should be made
to obtain separate evaluations of E 1 and Eg. Since Ec varies with T and E 1 does not, one could, by
varying 7, determine whether or not EE, remains constant. If EiEc remains constant as T is increased,
then one could conclude that E1 and not Ee is much less than unity. Next, after measuring Ec, one o
should use the same two counters with the same voltages in taking coincidence data. Any coincidence
rates taken may then be divided by Ec to get the true coincidence rate (after subtraction of the chance
coincidence background).
The writer would like to point out that in p-p coincidence work the geometry of the counters should
be so arranged that the counters may not "see" each other since scattering from one counter to the
other would cause spurious coincidences.

EXPERIMENTAL RESULTS


Work was carried out recently to determine the
eA^- ^-198 .:._=,a ... aj ^ a t-_ a.al* _1 -.. IN ..a a ., -


effective conversion coefficient for the y rays
a* ..^..1 A.--. n an a n ?.-T 0 ..nf'nf anfkAl T as nflWC Wkin flU





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Figure 4. Special type mica-window counter.



S5 = NAi + NaA1 NaA2 (3a)
S2 = NA2 + NaA2 NaA2 (3b)

The first terms represent counts due to 3 particles, the second those due to conversion electrons and
the third those due to 3 particles and conversion electrons entering the same counter simultaneously.
It is obvious that if N is to be determined from single counting rates as in the case of the Au flux
monitor, then a, the conversion coefficient, must be known. Another relation is used for determining
a. If one counts coincidences due to a conversion electron entering counter 1 or 2 at the same time a /3
particle enters counter 2 or 1, the coincidence rate should be:

C = 2NaAA2 (4)
Thus

C 2A2 a C 2A1a (5)
S4 1 + aA- Ada S2 1+a-Ai
so that if A1, A2 C, S,, and S are measured, a value for a may be determined and, consequently, a
value for N, the disintegration rate, may be determined. The value obtained for the effective con-
version coefficient is 7%. EEc of equation 2 was 0.94 during this run and r was 6 x 10-7 sec. No
attempt was made to separate Ec and E1. If E, were as low as 0.94, then there was notoincidence
loss during the run, whereas, if E1 were as high as unity, the coincidence loss would have been 6%.
Therefore, an error of + 3% must be recognized (after increasing the measured value by 3%) due to
the possibility of coincidence losses. Errors in measurement of A, and A2 carry the total estimated
error to approximately 10% so that the effective conversion coefficient for Aut8 would be quoted at
71%.
Previously some coincidence work was done in connection with the Xe5ss problem.7 It was felt that
if active contamination in the xenon samples used were present (due to Hg vapor, etc.) after irradiation,
erroneous conclusions might be drawn from single counting data as to the absorption cross section.
A rough measurement was made in which it was found that Xes35 has a converted y with a value for a of
about 10%. Since the Xern present in the samples has a y of very low conversion coefficient, and no
contamination activities which could be thought of would give rise to coincidences, it was decided that
the Xes35 problem would be run with both single counting and coincidence data. The ratio of the
activities of two Xes35 samples before and after the irradiation of one of the samples was taken by




M u


SOURCE


COMPTON ELECTRON
RADIATOR


Al SHIELD











-G-M TUBES


Figure 5.


Some work has been done here in the measurement of Compton-recoil energies by the absorption
method with the coincidence circuit. Considerable disagreement was found in the literature as to the
absorption curve for Al to be used for this work. Accordingly, it was decided to attempt to draw the


appropriate curve by measuring a few y sources of different energies which were fairly well known.
The geometry is as shown in Figure 5. Using two tubular thin-walled counters, the Compton electrons
from the radiator may give rise to coincidences by passing through both counters. Since the y efficiency
of the counter is less than 1%, the probability of both counters being discharged by one y ray is very
low. Absorbers are placed between the counters and a plot is made of coincidence rate against
img/cnm2 absorber. Since the e- energy is well defined, a sharp break occurs in the absorption curve.
ThU curves are dealt with as shown in Figure 6. The Al absorption curve which seems to fit the data
is shown in Figure 7.


CONCLUSION

The development of a fairly dependable coincidence circuit has been outlined, and its uses and
Jniitations have been pointed out. Values have been obtained for the conversion coefficients of Au"98
and Xe8s. Also, an absorption curve for Compton-recoil electrons has been obtained as an aid to y-ray
energy measurements.
The writer expresses his gratitude to Paul W. Levy for his construction of counters and his sug-
gested special counter, to L. A. Pardue for his help and advice, and to W. Bradley for his cooperation
in the development of the coincidence circuit and its dual scaler. E. O. Wollan's experience with
cosmic ray coincidence work has been especially helpful.


ZtFERENCES


Norling, Folke, Arkiv for
Rossi, B., Nature 125:636
Bradley and Epstein, Inst.
Bradley, CP2426.
Echart and Shonka, Phys.
Montromerv. C. G.. Rams


Matematik, Astronomi Fysik, BD 27A. N:O 27.
(1930).
Section Report No. 50299 Oct. 23, 1944.

Rev. 53:o 52 (1938).
ev. Cowie. D. D. Monteomerv. Phws. Rev. 56:635 (1939).


MDDC 1010


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0 50 100 150 200 250 300 350


Au1~ MG/CM2


Figure 6. Al
94.7 mg/cn


U
18




1.6




1.4




1.2 -


absorption of Compton recoils from the 0.42 Mev r
end-point at 94.7 + 57.5 = 152.2 mg/cm2.


's of Au" residual absorber was


Compton recoil absorption curve for aluminum.


























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