An electrical method of counting the number of α-particles from radio-active substances

Abstract

The total number of α-particles expelled per second from 1 gramme of radium has been estimated by Rutherford* by measuring the charge carried by the α-particles expelled from a known quantity of radium in the form of a thin film. On the assumption that each α-particle carries the ionic charge e = 3·4 x 10 -10 electrostatic unit, it was shown that 6·2 x 10 10 α-particles are expelled per second from 1 gramme of radium itself, and four times this number when in radio-active equilibrium with its three α-ray products, viz., the emanation,radium A and C. In order to reconcile the value of e / m found for the α-particle with that to be expected for the helium atom, it was later† pointed out that the α-particie should carry a charge equal to 2 e . On this assumption, the number of α-particles expelled per second per gramme of radium is reduced to one-half the first estimate. The need of a method of counting the α-particles directly without any assumption of the charge carried by each has long been felt, in order to determine the magnitude of the various radio-active quantities with a minimum amount of assumption. If the number of α-particles expelled from a definite quantity of radio-active matter could be determined by a direct method, the charge carried by each particle could be at once known by measuring the total positive charge carried by the α-particles. In this way, it should be possible to throw some light on the question whether the α-particle carries a charge e or 2 e , and thus settle the most pressing problem in radio-activity, viz., whether the α-particle is an atom of helium.