The Longitudinal Polarisation of β-Particles

Abstract

The aim of my experiment was to try to detect a longitudinal polarisation of the electrons of β-decay, testing the space and charge symmetry rules. I hoped also to measure the energy dependence of the polarisation and obtain information on time reversal invariance. That a longitudinal polarisation would indicate a special symmetry can readily be seen. β-decay is the process by which a neutron changes to a proton within certain nuclei accompanied by the emission of two particles, an electron and an antineutrino. From statistics on the occurence of the decay it is found to be of random nature. With randomly oriented nuclei in the source we expect and find an isotropic distribution of the electrons about the source. Each electron has associated with it an intrinsic angular momentum called its spin, which can roughly be represented as a vector. On the average we would expect these spins too to be randomly oriented with respect to each other. If we find a nett alignment in any direction, (a polarisation), a space asymmetry must be present and parity not conserved. Longitudinal polarisation is a nett alignment along the direction of motion. I designed the apparatus to make measurements on polarisation and tested to make certain that it was working satisfactorily. I measured the polarisation of the decay electrons of thallium 204 at the average energy of .25 mev and had intended to make measurements at several energies on thulium 170 electrons. Unfortunately this source did not arrive from England. As a result of this work I was able to confirm the violation of parity and charge conjugation in the weak decay of thallium but had to leave untested the question of time reversal invariance. In Part I of this report I go over the experimental work in some detail describing the design and testing of the apparatus and the measurement of the thallium polarisation. In Part II I outline the theory of β-decay and show the way in which the symmetries affect the interaction and give rise to polarisation. I survey the published material on β-decay and discuss the results in conjunction with my own. Then I review the results on these and other symmetries among the elementary particles. The hyperons and mesons show interesting patterns of behavior, interacting as they do in both a strong and a weak manner. Our knowledge of why such modes occur is very limited; it is through the systematisation of symmetries among these particles that we hope form a consistent explanation for their properties.