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UBC Theses and Dissertations

The primary specific ionization of gases of positrons and electrons Silver, Lorna Margaret


Theories of P.S.I.* produced by electrons passing through gases show that one would expect the number of primary ion pairs formed to vary with the electron density in the gas and with the mean ionization potential of the gas. to vary inversely as the square of the velocity of the electron at low energies reaching a minimum value approximately at a kinetic energy equal to the rest energy and to be independent of the sign of the beta particle producing the ionization. It is consequently of interest to obtain measurements of the P.S.I, of beta particles at energies at which it is a minimum in various gases which are used for filling counters and ionization chambers including hydrogen, helium, neon, xenon, methane, chlorine and other quenching vapors such as methyl chloride which may become of practical concern for counters designed to operate over a wide temperature range. Present data on P.S.I. of electrons and mesons is limited to but few gases and the results are not in good accord. As mentioned, the P.S.I, is independent of the sign of the charge of the ionizing particle and, since all the indirect evidence and the principle of conservation of charge indicates the equality of the charge on the positron and negatron, it would be expected that the values of the P.S.I, would be identical for the two particles. However while it is known that the value of e/m for both particles is identical to within 2%, there is some possible theoretical indication that their masses might be slightly different. *The primary specific ionization will hereafter be denoted by P.S.I. Moreover the only direct estimation of the value of e⁺ is the measurement of P.S.I. from the original cloud chamber observation of a cosmic ray positron by Anderson¹ which established the charge equality of e⁺ and e⁻ to within 20%. Thus it appeared of some fundamental importance to compare with precision the P.S.I, of both positive and negative electrons of similar velocities in a gas of high atomic number such as neon and a gas of low atomic number such as helium. Finally it was hoped that the apparatus set up would be suitable for an investigation of the relation between the P.S.I. and velocity of the electron especially in the relativistic region in which the data is very meagre. This will need high energy beta sources (e.g. B¹²) which will "become available when the U.B.C. electrostatic generator is in operation. Apparatus has been set up to determine the P.S.I. of electrons in various gases by determining the inefficiency of a Geiger counter filled with gas. To eliminate the difficulty, of a variable path length of the particle through the ordinary cylindrical counter, rectangular and square envelope counters with thin windows have been constructed. The presence of a large thin window, even when a conducting surface, has been found to affect the spread of the discharge and results in the appearance of two size pulses analogous to the effect of an insulating bead oh the centre of the wire. Thus, contrary to the usual theory of Geiger operation, it seems that the spreading of the discharge does involve a cathode mechanism in this design. In order to select beta particles of homogeneous energy from the radioactive source, a small wedge shaped magnetic spectrograph has been built. The counter inefficiency is determined by passing the beta particles through the inefficient counter into a 100% efficient counter operating at normal pressures, and recording the coincidence rate as a fraction of the "efficient counter" rate. Co⁵⁶ was chosen as the most suitable positron emitter, and has been prepared in the Berkeley cyclotron by the Fe⁵⁶(d,2n)Co⁵⁶ reaction. RaE has been used as a negatron emitter.

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