The production of positive electrons by beta-particles

Abstract

Abstract P art I. Cloud chamber experiments concerning the intensity and energy distribution of the positrons emitted by aRa( B + C) source are described. The ratio of the number of positive to that of the negative electrons was found to be 0.0103 in the first series of measurements (359 exposures) and 0.0110 in the second series (397 exposures). These results confirm the results of Chadwick, Blackett and Occhialini (24 positrons against 2000 negative electrons). From the strength of the source, the known composition of theγ-radiation ofRa( B + C), and the theory of Jaeger and Hulme, the number of positrons excited by the internal pair production byγ-rays was calculated (1.7 × 10 −4 per disintegration). The observed number was about 90 times as large. If the observed positrons are ascribed to the internal pair production by the β-particles, the probability of this process turns out to be about 0.085, which is of the order 10 4a`10 5 times the theoretical value calculated by Arley and Moller. The energy distribution of the positrons shows that by far the greater part of them have energies below 300 KeV. It is suggested that because of this low energy the counter measurements have hitherto given much too small a value for the intensity of the positron spectrum. P art II. The positrons excited in lead by the β andγ-radiation ofRa( B + C) are investigated with the Wilson chamber. In full agreement with the measurements of Skobelzyn and Stepanowa, it is found that the positrons observed can only for a small fraction be ascribed to the process of external pair production by theγ-rays. If the rest is ascribed to the process of pair production by β-rays, the effective cross-section for this process turns out to be of the order 10 −22 cm 2, which is about 10 4 times the value predicted by the theory. Just as was the case for the positrons emitted by the source itself, the energy distribution of these externally excited positrons, shows that by far the greater part have very low energies (< 200 KeV), and the intensity rapidly decreases with increasing energy.