Collection of Ions Produced by Alpha Particles in Air

Abstract

In the measurement of the ionization caused by alpha particles in air, recombination effects between slowly moving positive and negative ions (the latter formed by electron attachment to oxygen) have to be considered. The usual procedure in such measurements is to determine the saturation current (by extrapolation of reciprocal current versus reciprocal voltage curves to infinite field strength) according to the Jaff\'e theory. A paper by Wingate, Gross, and Failla has cast doubt on the validity of this extrapolation technique, in that the authors propose a field-independent part of the recombination amounting to 3.3% at atmospheric pressure in air. This proposal implied that all previous measurements of ${W}_{\ensuremath{\alpha}}$ for air were in error by this amount and that this error is a possible cause for the reported difference (3-4%) between $W$ values for alpha and beta particles in air. In view of our own $W$ measurements we felt compelled to re-examine this supposedly field-independent part of the recombination. Approximating the experimental conditions of Wingate, Gross, and Failla we have not been able to reproduce their effect and our experiments demonstrate the validity of the usual extrapolation techniques.