The upper limits of the continuous β-ray spectra of thorium C and C″

Abstract

It is well known that the disintegration electrons from a radioactive body are distributed over a wide range of velocities, and that a characteristic feature of this distribution is that there is an upper energy limit above which no electrons are emitted. The accurate determination of these upper limits and the corresponding maximum energy of the β-rays has become of much importance in connection with special theories which have been advanced to explain the nature of the β-ray disintegration. Numerous experiments have been done to find these upper limits or end points. Most of the existing data are based on range measurements in which the energy of the fastest β-rays from a source is deduced from their range. The chief advantage of the method is that it can be carried out with weak or rapidly decaying sources; the great disadvantage is that owing to the scattering suffered by the β-particles, the range found by experiment is an indefinite quantity and has no simple relation to the maximum energy of the β-particles. Because of this fact range methods, while frequently giving results in general agreement with magnetic analysis, are not capable of leading to accurate results for the upper limits. There remain two other methods of analysis, the magnetic spectrograph and the expansion chamber. The latter as used by Terroux and Alexander gives a higher upper limit than other methods. For radium E Terroux reported a tail extending to 3,000,000 volts, while other methods give an end point at about 1,070,000 volts. Champion, in repeating Terroux’s experiments, emphasized the precautions that must be taken in interpreting the experimental material. To eliminate the effect of scattering he was forced to adopt a criterion that a particle would be counted only if it had an undisturbed track greater than a certain length. The cloud-chamber method, while applicable to very weak sources, must be used with great care, and cannot give an accurate value of the end point without taking a very large number of photographs.