The distribution of velocity in the β-rays from a radioactive substance

Abstract
It is well known that the α -particles emitted from a thin film of radio­active matter of one kind are initially projected at an identical speed. No definite evidence, however, has yet been obtained whether a similar result holds for a substance which emits β -particles in its transformation. Hahn has found by the electrical method that β -rays from a simple radio­active substance are absorbed very nearly according to an exponential law, and has utilised this property to decide whether the source of β -rays consists of one or more products emitting β -particles. This method, in the hands of Hahn and Meitner, has proved very fruitful in bringing to light new and unsuspected β -ray products. In addition, it has been assumed by many writers that the exponential law of absorption is a proof that the rays are homogeneous, i. e ., that the β -particles are emitted initially at an identical speed, and, further, that the velocity of the β -particles does not change appreciably in traversing matter. W. Wilson has attacked this question from another direction. Using radium emanation, he has sorted out the rays from the active deposit by means of a magnetic field, and obtained rays which, if not homogeneous only cover a small range in velocities. Testing these rays by means of an electroscope, he found that when the ionisation was plotted against the thickness of absorbing matter, the curve obtained was not exponential, but very nearly a straight line. This indicated that the absorption of nearly homogeneous rays was not a constant, but increased with the thickness of matter traversed. Such a result can only be explained by a loss of velocity in β -rays as they pass through matter. It shows also that an exponential law of absorption does not signify homogeneous β -rays, but rather rays with a particular distribution of velocities.

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