Variation with Pressure of the Residual Ionization due to the Penetrating Radiation

Abstract

Residual Ionization in Closed Vessels: Experimental Method.---In order to throw light upon the type of radiation responsible for the residual ionization in closed vessels, the author has measured the ionization in a sphere one foot in diameter for pressures ranging up to 21.5 atomospheres. Any corpuscular radiation not absorbed in the ionization sphere has a penetration at one atmosphere equal to or greater than the product of the dimension of the sphere and the number of atmospheres pressure. A special device was used to compensate for any fluctuations of the battery furnishing the high potential required for saturation. This consisted of two ionization spheres of the same electrical capacity which were connected to the opposite ends of a ten megohm resistance, the mid-point of which was earthed. Connections from the battery were made to the ends of the ten megohms. The final experiments were performed over water to eliminate any effect due to radio-active substance in the soil.Nature of Ionization Curve.---The shape of the ionization-pressure curves gives information as to the process of ionization. A discussion of such curves obtained by other observers has been given in connection with those of the author. The curves obtained in the present experiment give a linear relation. This indicates that the ionization is not due to a soft radiation from the walls of the sphere, but that it is due either to the direct action of a penetrating radiation without any effect produced by secondary $\ensuremath{\beta}$ rays from the gas, or to a secondary, corpuscular radiation from the walls of the vessel. If the latter is responsible for the main portion of the ionization, the experiments indicate that it has a penetration of at least six and one half meters in air at atmospheric pressure.Diurnal Variation.---A few curves obtained during the preliminary observations have been given. Provided that the sky is not cloudy, these indicate the existence of a diurnal variation.Ionization in Air Due to Gamma Rays of Radium.---The ionization in a sphere one foot in diameter has been measured for pressures ranging up to 40 atmospheres. From four to twenty atmospheres a linear relation existed. The change in slope at twenty atmospheres to a value about six tenths of that at lower pressures may be attributed to some of the $\ensuremath{\beta}$ rays having completed their paths. At the high pressures there is a suggestion of an effect due to the secondary radiation from the air.