Rare beta decays and the spectrum-shape method

Abstract

This is a thesis consisting of seven publications and an introductory part on theoretical studies on rare single beta decays. Firstly, the theoretical framework is applied to the computation of partial half-lives for few selected cases of rare single beta-decay transitions. This includes the study on a possible ultra-low-Q-value decay branch of 115Cd as well as the highly forbidden beta decays of 48Ca and 50V. The double magic 48Ca is one of the few experimentally verified nuclei that decay via the two-neutrino mode of double beta decay. A theoretical study on the single beta-decay branches was used to inspect the competition between the single and double beta-decay channels. In the case of 50V the theoretical framework is used to examine the detectability of the beta-minus decay branch that leads to the first excited 2+ state of 50Cr. To access the finer details of the theory the usual analysis of beta-decay transitions is extended by the introduction of the next-to-leading-order terms of the beta-decay shape factor. A comparison between the leading-order and the next-to-leading-order contributions is performed in the case of the fourth- forbidden decay branches of 113Cd and 115In. Finally, an overview on the recently introduced spectrum-shape method is given. The spectrum-shape method (SSM) was developed for the extraction of the effective values of the weak coupling constants as a complementary approach to the usual partial half-life considerations. Highly encouraging results are obtained when SSM is applied to the beta decay of 113Cd. In this initial application of the method consistent values of the two coupling constant were found when the calculations were performed using three different nuclear models.