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Gas gain in proportional counters

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White, KG (1989) Gas gain in proportional counters. PhD thesis, University of Tasmania.

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Abstract

Gas gain measurements were made for nitrogen, xenon and x~non with 2,3
dimethyl-2-butene (DMB) as an additive. Two different methods of measuring
gas gain were examined, pulse matching methods and current comparison
methods.
A preliminary investigation was made into a new approach to the pulse matching
method. Previous authors had shaped their test pulses to the same shape as their
detector pulses or used a calibration factor to correct for the difference between
the shape of the test pulses and the detector pulses. These approaches would be
valid only if the detector pulses were always the same shape. The pulses from
the detector vary in shape depending upon where the primary ion pairs are
formed in the detector sensitive volume. The new approach' I have investigated
shapes the detector pulses and the test pulses to the same shape so that the
variation in the detector pulse shape becomes irrelevant. This method of
measuring gas gain was tested using P-10 and 148Gd as an alpha particle source and
gave similar results to gas gains determined by current comparisons.
The measurements of gas gain finally used to obtain the data were all performed
using the current comparison technique. When the electrons were collected at the
central wire and 55Fe was used as the radiation source the ion saturation curves
for xenon showed no definite plateaus, only points of inflexion. I found that ion
saturation currents could be determined by fitting the observed currents and
associated operating voltages to an expression proposed by Johnson. This
expression also enabled me to determine the voltage region where gas gain
began.
The gas gain characteristics of nitrogen and xenon were measured for a range of
gas densities. From these measurements it was shown that the gas gain of a
cylindrical proportional counter operating in the proportional region is a function
only of the reduced field strength at the surface of the anode.
Aoyama has proposed an expression for the first Townsend coefficient and he
has been able to show that previously proposed expressions are only special
cases of his expression. I was able to obtain an excellent fit to Aoyama's
expression using my gas gain data for xenon and a value for one of the
parameters determined by Kowalski.
It is widely held that the mechanism of gas gain in proportional counters is
primarily by electron impact. Ionization may also occur in binary mixtures by the non-metastable Penning effect, and the metastable Penning effect. A binary
mixture of xenon and DMB was used to investigate these effects. It was found
that the gas gain in xenon could be greatly increased by the addition of a small
quantity of DMB. A broad peak in gas gain appeared when the concentration of
additive was between ~ 0.4% and ~ 0.75% and the molecular number density of
the gas filling was 2.7 x 10 [to the power] 25 m-3 (~ 1 atm). When the gas density was reduced
to 1.4 x 10 [to the power] 25 m-3 (~ 0.5 atm) the peak became sharper and better defined and
reached its maximum when the concentration of additive was ~ 0.43%.
Lowering the density to 7 x 10 [to the power] 24 m-3 (~ 0.26 atm) resulted in a very sharp well
defined peak centred on an additive concentration of ~ 0.23%. The gas gains
used to define these peaks were of the order of 10[ to the power] 5, whilst in pure xenon at
identical anode potentials the gas gain was of the order of 10. The mechanisms
proposed for these increased gas gains were the non-metastable Penning effect
and the metastable Penning effect in the case of high density mixtures, the
Pennning effect in the' case of the intermediate density mixtures and for the low
density mixtures it was suggested that doubly excited xenon molecules were
ionizing the additive.
The W-values of the low density xenon plus DMB mixtures were also measured
to 5% accuracy. The trend in the mean values of these measurements indicates
that the Penning effect is occuring and the minimum value in the W-value occurs
at ~ 0.4% which suppports the above proposed mechanisms for the increased
gas gains.
Detectors filled with mixtures of xenon plus large concentrations of DMB
developed double full energy peaks in a relatively short amount of time using the
55Fe source. Passing a large current through the anode removed these double
peaks demonstrating that the cause was uneven deposits on the anode. These
deposits were probably due to polymerization of free radicals formed in the
electron avalanche. Small percentages of DMB did not result in double peaks but
drops in gain were observed when the detector was subjected to prolonged
usage. The absence of double peaks is attributed to the diffuse nature of the
avalanches. If the Penning effect or ionization of the additive by UV photons is
taking place the avalanche activity would always completely surround the anode
giving rise 'to uniform coatings around the entire circumference of the anode.

Item Type: Thesis (PhD)
Keywords: Proportional counters
Copyright Information:

Copyright 1988 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1989. Bibliography: p. 152

Date Deposited: 04 Feb 2015 23:21
Last Modified: 28 Nov 2016 00:55
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