FACTORS INFLUENCING THE PLATEAU CHARACTERISTICS OF SELF QUENCHING G-M COUNTERS

38 Pages·01.715 MB·English

by  SPATZ, WILBER DE VILLA BERNHART

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.G7 Spatz, Wilber deVilla Bernhart, 1908-
1943 Factors influencing the plateau char-
.S7 acteristics of self quenching G-M coun­
ters... eNew York# 1943.
lp.l.,25,da typewritten leaves,
tables,diagrs. 29cm.
Thesis (Ph.D.) - Hew York university,
Graduate school, 1943.
Bibliography: p.25.
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FACTORS INFLUENCING THE PLATEAU CHARACTERISTICS
OF SELF QUENCHING G-M COUNTERS
Wilber deVilla Bernhart Spatz
k dissertation in the department of physics
submitted to the faculty of the Graduate
School of Arts and Science in partial ful­
fillment of the requirements for the degree
of Doctor of Philosophy.
June, 1945
TABLE OF CONTENTS
Page
Introduction 1
Voltage regions of G-M Counter
Problem of Investigation
Factors Previously Investigated 6
Apparatus 8
Experimental Arrangement and Observations 9
Dependence on Argon Purity 9
Ageing Tests 10
Pressure Variation 12
Photosensitivity 15
Tests on Methane Counters 14
Summary of Results 15
Discussion and Interpretation of Results 16
Increase of Pressure and of Starting Potential 19
Ageing Effects on Plateau Slope and Recovery 20
Effect of Percentage of Alcohol on Plateau Slope 22
Conclusions 24
Bibliography 25
1 6 0 6 3 2 .
INTRODUCTION
Numerous attempts have been made to explain the mechanism
whereby the avalanche discharge in a Geiger-Mueller counter (having
a center wire anode and co-axial cylindrical cathode) is initiated,
maintained, and quenched. It is well known that the physical di­
mensions of the counter and the nature and pressure of the gas^ de­
termine the minimum counter voltage at which discharges can be ini­
tiated. On the basis of the Townsend criterion, S. C. Brown^ showed
that the discharge was maintained through the agency of photoelectrons
ejected at the cathode as a result of photons being produced in the
process of neutralization of the positive ions formed in the discharge,
it is believed that the avalanche is initiated close to the center
wire where the field conditions are suitable. C. G. and D. D. Mont­
gomery^ and W. E. Ramsey^ proposed the quenching mechanism as due to
the lowering of the field by the positive ion sheath around the wire
to a point where it is incapable of supporting a discharge. The length
of time during which the field is held below the critical value de­
pends on several factors. External Factors are the size of the leakage
resistance connecting the anode to the high potential, the RC time of
external circuit, special electron switches of the Neher-Harper0 type,
etc. In addition, there is a formation about the center wire of heavy
positive ions which keep the field below the critical value. Due to
low mobility of these heavy ions the field is kept suppressed for a
length of time which is long compared to the traversal time of the elec­
trons to the anode. H. G. Stever® has estimated that the positive ion
sheath produces sufficient lowering t>f ~the field to prevent further
2
counting until it has moved approximately one third the distance to
the cathode.
With the use of pure rare gases such as neon, argon, etc. the
Q IQ
leakage resistance must be of the order of 10 to 10 ohms to quench
the discharge. This results in a long time period during which the
counter is insensitive and such counters are referred to as "slow"
7
counters. Self quenching counters, first described by A. Trost , use
argon and other gases mixed with organic "quenching" vapors such as
alcohol or ether. The role played by the organic constituent in pro­
ducing the self quenching is not at present fully understood. Counters
6
of this type may be operated on about 10 ohms, have a relatively short
resolving time (the time during which the counter is insensitive) of
the order of 10~^ seconds, and are referred to as "fast" counters.
When a G-M counter of the self quenching type is connected to an
external counting circuit such as a scale-of-eight scaler and mechan­
ical recorder, the relationship between the number of counts resulting
from a given source as a function of voltage across the counter is giv­
en by the familiar curve in Fig. A. The entire voltage range may be
VOLTAGE
Fig. A.
3
divided into three regions A, B, and C. Region A is the region of the
proportional counter. Due to the physical characteristics of the ex­
ternal apparatus attached to the Geiger counter this region appears to
be one where no counts or discharges occur. This is due to the fact
that the recording apparatus is sensitive only to voltage changes of
the center wire greater than a definite minimum size (usually several
volts) and in the low voltage region A, the voltage pulses are below
the size necessary to actuate the apparatus.
Region B is usually referred to as the plateau of the counter.
In this region the size of the voltage pulse will be the same regard­
less of the number of ions produced in the ionizing event which in­
itiates the discharge®. The lowest voltage at which the pulse is of
such size as to be able to actuate the external apparatus is known as
starting potential of the counter. The value of the starting poten­
tial for a counter of given physical dimensions depends on the composi­
tion of the gases and vapors and on their pressure as shown in Fig. 1.
A good counter is one for which the slope of the plateau is zero, or
in other words the plateau is flat. Physically, a flat plateau means
that the number of counts recorded is dependent onjy on the magnitude
and geometry of the particular radioactive source used and is inde­
pendent of the voltage across the counter. If an oscilloscope is used
instead of a counting circuit, each discharge will show as a pulse on
the oscilloscope screen and for a counter having a flat plateau the
number of pulses per minute will be independent of the counter volt­
age but the size (height) of the pulse will increase with the voltage.
Pulse height is thus a means of measuring the overvoltage on the counter;
4
the overvoltage being defined as the difference between the operating
voltage and the starting potential.
Counts may be produced by agents other than the particular source
used; cosmic radiation and radioactive background due to natural con­
tamination in the vicinity of the counter can produce counts. The
counting rate produced from such sources is comparatively low and is
termed the background of the counter. In experimental work it is desir­
able to hold the background to a minimum which can be done by lead
shielding. Along with the background counts occur spurious counts, which
are produced by events originating inside the counter itself, often as
a result of the discharge. The action of (l) negative ions, (2) positive
ions, (5) irregularities on the wire and cathode producing strong local­
ized fields, and (4) saturation of the absorption ability of the quench­
ing vapor are believed to be the main causes for spurious counts. The
actual mechanism of origin of spurious counts is not at present completely
understood.
With actual, counters the plateau has a slight slope. The slope of
the plateau is defined as the percentage increase of counts per volt of
counter potential. It is known experimentally that the slope and extent
of the plateau depends on the gas composition in the counter.
The transition from regions B to C is a gradual one; arbitrarily it
can be considered to take place at that voltage where the plateau departs
from a 11 straight" line. In this region the spurious counts increase
rapidly with the voltage, soon reaching a point where the counter goes
into continuous discharge. It is advisable never to operate a cotin ter in
this region since heavy currents may damage the wire.
5
The starting potential and the width and slope of the plateau are
called the plateau characteristics and are known experimentally to de­
pend on the type of organic vapor and other gases used in the counter.
The problem of this investigation is to determine what factors in­
fluence these characteristics. This is of importance in the counting
of electrons. Two conditions are desirable: (a) the starting po­
tential should be as low as possible without affecting the efficiency
of the counter and (b) the plateau should be as nearly flat as possible
over an appreciable voltage range so that the number of counts be in­
dependent of voltage fluctuations present in experimental apparatus.
6
FACTORS PREVIOUSLY INVESTIGATED
9—14
Preparation of Counters* Many articles have been written on
the problem of the preparation of alcohol-argon counters and on the
effect of the preparation on the characteristics of the plateau. Some
authors claim that elaborate care must be taken to achieve good re­
sults while others are of the opinion that such precautions are not
necessary. It depends on the manner in which the counter is to be
used as to whether such precautions are necessary or not. .If used in
multiple as in coincidence or anti-coincidence circuits rigorous re­
quirements on plateau characteristics are as a rule not imperative, for
if the spurious counts are not too numerous, the chance that one will
occur simultaneously in several counters is small. If, however, a
counter is to be used singly as in the case of electron counting, it
is necessary that all precautions be taken in the preparation and fill­
ing of the counter. All authorities agree that the center wire and
cathode must be free of foreign particles and rough spots to avoid in-
homogeneous fields. Flashing of the center wire while the counter is
evacuated will insure elimination of these undesirable conditions on
the anode. If the cathode is enclosed in glass, baking of the counter
to drive out adsorbed gases should be done wherever possible; this un­
fortunately is not possible in the case of silvered thin walled counters
due to the injurious effect on the silvering.
Temperature. In experiments of S. A. Korff, W. D. B. Spatz, and
N. Hilberry^ it was verified that alcohol-argon counters have a consider­
able temperature coefficient; i.e. the plateau characteristics varied
with the temperature. Above room temperature (26°C) the change was not