Ionization by Penetrating Radiation as a Function of Pressure and Temperature

Abstract

To account for the experimental fact that the ionization of a gas exposed to $\gamma$-rays or cosmic rays is not proportional to the pressure, but approaches a limiting value for pressures of about 140 atmospheres, the hypothesis is suggested that at these high pressures the initially formed ions may remain so close together that they frequently reunite under their mutual electrostatic attractions. The probability is calculated for the ions to become separated by diffusion, and formulas are thus obtained for the saturation ionization current as a function of pressure. The most satisfactory formula is based upon an arbitrary but reasonable assumption regarding the ranges of the secondary electrons ejected by ionizing beta particles. Knowing the variation of ionization with pressure, this diffusion theory predicts a definite variation of ionization with temperature. Such a temperature variation is experimentally discovered and is in good accord with the theoretical prediction. The temperature coefficient is negligible for pressures less than 10 atmospheres, but at pressures over 100 atmospheres the ionization approaches proportionality to the absolute temperature.