Ionization by Alpha Particles in Mixtures of Gases

Abstract

The presence of minute impurities greatly increase the ionization produced by Po alpha particles in helium. Systematic studies of this effect show an increase in ionization up to 40 percent for approximately 0.1 percent of argon. Similar results have been obtained with CO2, Kr, Xe, H 2 , N 2 , and C 2 H 4 as the contaminant. Preliminary experiments with mercury vapor in helium confirm the large effect already reported by others. A similar increase in ionization in pure argon is obtained by the addition of C 2 H 2 or C 2 H 4 . These increases in ionization seem to be caused by the production of ions, when metastable atoms in the parent gas undergo collisions with molecules of the impurity. In mixtures of H 2 , N 2 , or A with helium, the excess in ionization is observed to decrease by a few percent as the pressure of the gas mixture is increased from 48 to 110 cm of mercury. For no other contaminant gases tested thus far in helium is such a pressure change in ionization observed. F OR some time experiments have been in progress in this laboratory to study the rather striking changes in alpha-particle ionization in the noble gases produced by the introduction of minute quantities of other gases. Although a complete explanation of all the effects observed cannot be given at the moment, it seems worthwhile at this time to give a more extended description of the phenomena observed than has as yet been published. 1,2 APPARATUS AND METHOD Two methods were employed to measure the ionization produced by alpha particles in gaseous mixtures. In the first method 1,3 the ionization produced by single polonium alpha particles was measured by collimating these alpha particles along the axis of a long brass cylindrical ionization chamber (Fig. 1). The effective path was about 20 cm. The ions produced by each alpha particle were collected and fed into a vibrating-reed electrometer connected to a Brown strip chart recorder. The length of jump on the chart produced by each alpha particle was measured and the total of a large number of these averaged. With a knowledge of the voltage sensitivity and electrical capacity of the system one can determine the average number of ion pairs produced per polonium alpha particle. A very small correction for the ions lost within the collimating system was made.In later determinations of the effect of the pressure of the gas mixture upon the ionization, a more robust chamber was used, in which the energy of alpha particles from Am 241 was reduced to about 1 Mev by passage through a thin sheet of mica as they entered the chamber. A diagram of the arrangement used has 1755 this arrangement the total integrated current was measured.Both types of measurement could be compared for any given gas mixture by computing the ratio of the ionization observed for any given impurity concentration to that observed in pure helium.Elaborate precautions were taken to insure the purity of the gases used. The chamber shown in Fig. 1 was constructed with quartz insulators, to permit baking and pumping for a period of...

Investigation of the proportional discharge mechanism in nonelectronegative gases

“…In conclusion we compare our results with the results obtained in work [10], where the output of ionization electrons from α-tracks was measured in current conditions for the mixtures of He with Ar, Kr, CO 2 , Xe, N 2 , C 2 H 4 , H 2 , of Ne with H 2 , Ar, Xe and of Ar with C 2 H 2 and C 2 H 4 and it was shown that for concentration of admixtures ≥ 2·10 -4 rel. vol.…”

“…parts [9]. The gases He, Ne were purified of all impurities by fine filtration at a temperature of 78 K [10]. The Ar and Xe were purified of all chemical active impurities up to a level of ~ 10 -9 rel.…”

X-Radiolysis Ion Yields and Electron Ranges in Liquid Xenon, Krypton, and Argon: Effect of Electric Field Strength

“…One finds that the total ionization yield in liquid argon is nearly double the gas phase yield (29), whereas in krypton and xenon the ionization yields in the liquid are triple those in the gas phase (Table 3). Ionization in these liquids is evidently greatly facilitated by polarization energy, as it is in many solids (30,31).…”

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“…Jesse and Sadauskis (10) have shown that about 25 per I cent of the energy ot an alpha particle stopped in helium is expended in excitation of the metastable states. This percentage may be highor for fission fragments than for alpha particles at least over part of the range.…”