The shape of the continuous β -spectrum of thorium C. C'

Abstract

The theories of β -decay based on the neutrino hypothesis predict that a considerable proportion of the electrons emitted from a heavy nucleus will have low energies, owing to the Coulomb attraction between the electron and the nucleus. This prediction has been in apparent conflict with most experimental curves (Madgwick 1927; Scott 1935 ), which show the ordinate of the energy distribution falling to zero at the origin or even before it, thus even indicating a low energy end-point below which no β -rays are emitted. It is, however, probable that the experimental uncertainties in the methods which have been used are such that no definite conclusion can be drawn from them about the shape of the low energy end of the spectrum. In these methods the source is deposited on a solid mounting and the emitted β -particles pass through a window in entering the detecting apparatus, which may be a counter, a cloud chamber or an ionisation chamber. The window stops all β -particles below a certain energy, while those which pass through are reduced in energy and considerably scattered. These effects, which are well shown in curves given by Eddy (1928), produce a marked falling off in the observed number of β -particles of low energy. The use of a solid mounting for the source introduced opposite effects giving an increased number of slow electrons; for firstly, the fast electrons will eject slow secondary electrons from the solid mounting and, secondly, if the mounting is thick, a considerable reflexion of the primary electrons will occur with varying losses of energy inside the solid, so that the reflected spectrum will contain relatively more low energy rays.