Comparison of the beta-alpha angular correlations in the Li8 and B8 beta decays, and the conserved vector current theory

Abstract

This paper describes a comparison of the beta-alpha angular correlations of the beta decays of Li8 and B8 to the 2.90-Mev alpha-unstable level of Be8. The data were fitted to an angular correlation in the laboratory system of the form W(θβα)=1+Acosθβα+Bcos2θβα, where A and B were independently determined for the Li8 and B8 beta decays from measurements at θβα=0°, 90°, and 180°. The coefficients A are due to the recoil of Be8* and have the same sign for both beta decays. The coefficients B arise from the interference of forbidden vector matrix elements with the allowed axial vector matrix element and have different signs in the two decays. The correlation for the Li8 decay was measured with average total beta energies, Wβ~5, 8, and 11 Mev. Both A and B were found to be approximately linear in Wβ. The correlation for the B8 decay was measured only with average Wβ~11 Mev. From the data with Wβ~11 Mev, the difference of the coefficients, B(Li8)-B(B8), equals (0.0070±0.0012)Wβ. This result is compared with theoretical predictions based on the older Fermi and the newer conserved vector current theories of beta decay. The experiment agrees with the prediction of the latter theory.