The number of β-particles emitted in the transformation of radium

Abstract

The recent researches of Baeyer, Hahn, and Meitner have shown that the β -radiation from a substance can usually be resolved by a magnetic field into a number of distinct groups of approximately homogeneous rays. It has been shown by Danysz that more than 23 such groups are present in the radiation from radium B and radium C. From our knowledge of the approximate value of the charge carried by the β -particles from radium it is clear that each atom in disintegrating cannot emit one β -particle of each kind, and it is therefore safe to conclude either that different atoms of the same substance give rise to different radiations, the process of disintegrating being different in each case, or that the radiation which results from the disintegration is originally uniform but becomes modified by its passage through the atom. It is in this connection of importance to know the average number of particles emitted by an atom. If this proves to be an integer, the process of disintegration is probably simpler than it appears at first sight. At the suggestion of Prof. Rutherford, a careful investigation was undertaken to determine the number of β -particles emitted by radium C. This work has included a study of the best methods of measuring accurately the number of β -particles from an atom, with the intention of subsequently applying these methods to as many cases as possible. It has been found that, on an average, an atom of radium B or radium C emits slightly more than one β -particle, while radium E appears to emit on an average little more than one-half a β -particle. The only substance for which this number had hitherto been determined is radium C. The first determination, by Wien, gave only approximate results; the second, by Rutherford, needed correction by a somewhat uncertain factor, owing to the reflection of some of the rays from the metal cylinder on which the active material was deposited; the third, by Makower, eliminated the error due to reflection, and gave one, or possibly two, as the number of β -particles emitted. The chief sources of uncertainty in this case were, firstly, the correction made for the radiation from radium B; and, secondly, the correction for the absorption of the rays by the tube in which the active material was contained. In the present work a direct comparison of the radiations from radium B and radium C avoided the first source of uncertainty. The second was minimised by using as an active source radium emanation contained in a glass tube sufficiently thin to allow the passage of the α-particles, and then covering with just enough material to stop these particles; for the α-particles in their passage through matter produce δ-particles in numbers sufficient to render measurements of the charge carried by the β -rays impossible. A careful study has been made of the absorption of the β -radiation, and it is believed that in the case of radium C no serious error is introduced by the inevitable extrapolation.