A series of measurements has been made of the relative currents produced in different gases by beta particles from Ni 63 and from tritium sources in an ionization chamber. In all cases only relative current measurements with argon as a standard gas have as yet been made. The value of W, the average energy to make an ion-pair computed relative to argon as a standard, is found to be the same for Ni 63 and tritium sources. If these relative Wp values are plotted as abscissas against previously determined W a values for polonium alpha particles as ordinates, a marked difference is observed in the gases investigated. For hydrogen and the noble gases the plotted points lie closely on a 45 degree straight line through the origin. Thus for these gases the ratio W a /Wp is constant. This constant may well be unity, but this is not proved as yet by these results. For all other gases so far investigated, the plotted points lie above the 45 degree line, indicating a higher efficiency of ionization (and a lower W) for the beta particles than for the polonium alpha particles. These results are in accord with the findings of Gray and Gurney. Gurney's results have been extended here to include a greater variety of gases for reduced alpha particles of approximately 1-Mev energy. Two postulates are advanced to explain the behavior of W a and Wp here found. E XPERIMENTS have been in progress for some time in which the relative ionization for different gases has been measured for beta particles emitted from tritium and Ni 63 sources. Although the results are as yet preliminary, a comparison of them with the corresponding alpha-particle values is of interest and would seem to warrant publication at this time. EXPERIMENTAL METHODS AND RESULTSThe ionization chamber used was a cylindrical one of brass of inside diameter 9.5 cm and height 7 cm (Fig. 1). The collecting electrode at the center of the cylinder was in the form of a wire ring 5 cm in diameter, supported by three wire stays rising from the central insulated shaft. In the plane of the collecting ring and filling the interior was a square gridwork of copper wires spaced 6 mm apart in each direction. In some experiments the individual wires were 2 mils in diameter and in others 6 mils. The wires in the central area of the collecting disk were coated with the beta-emitting substance under investigation. For Ni 63 this coating was accomplished by electroplating. The tritium, in the form of a solution of tritiated polystyrene, was applied with DETAIL OF ELECTRODE FACECopper wires with 6mm spacing in each direction-FIG. 1. Schematic diagram of ionization chamber for beta-particle current measurements. a brush to the gridwork and was made conducting when dry by a light application of graphite-soft lead pencil. The employment of the gridded electrode came as the result of a series of experiments where the source was applied to the extended surface of a plate electrode. The expected ionization seemed to be reduced by what we believe to be a process of backscattering of the very soft bet...

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...

The absolute average energy WB necessary to form an ion pair in nitrogen, ethylene, and ethane was determined for S 35 beta particles by the use of a sample assayed in a 4T counter. The total number of ions produced per second for each gas was determined by a measurement of total ionization in a chamber. The average energy per beta particle was calculated from the known shape of the distribution curve for S 35 . Corrected values of WB were found to be 35.3, 26.2, and 24.7 ev/ion pair for N2, C2H4, and C2H6, respectively. These values yield a corresponding value for air of 34.1 ev/ion pair. All these are in excellent agreement with values previously predicted from relative beta-and alpha-ray measurements in various gases. *

However, expressions for secondary yields and energy losses based upon this cross section actually vanish since, in this approximation, o-q is zero, a fact not recognized in previous theories. This is an immediate consequence of the orthogonality relation between the lattice eigenfunctions belonging to k and k+2-n-q/a, J tktk+2irqfa*dT = Q.Substitution of Eq. (3) into Eq. (7) yields 7FS m , n a n (k)d m *(k+27rq/a)J > exp[(27r^)(n-q-m)'r]rfr = 2 m a m+q (k)a m *(k+27rq/a)-0. (8) Substitution of Eq. (8) into Eq. (6) yields a zero cross section. In order to obtain a finite result, it is necessary to take into account the variation of a m (k') with K'. This can be accomplished by a Taylor series expansion of a m (W) about the point k , = k+27rq/a. This results in a cross section which for sufficiently high energies has the same form as that obtained by Be the 3 for the ionization of atoms, namely,

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