Unique first forbidden β-transitions

Abstract

Theoretical expressions which include higher-order terms in the multipole expansion of the weak Hamiltonian and terms induced by strong interaction together with the variation of the lepton wave functions over the nuclei are presented for different β-decay observables. A detailed investigation of the experiments concerning 142Pr and 166Ho has been performed. Shape factor measurements for 42K, 86Rb, 90Y and 90Sr are also re-analysed. No useful information about pseudoscalar and pseudotensor interactions can be extracted from the available experimental data. Precise experiments determining shape factors, together with longitudinal polarization and/or βγ directional correlation, combined with better knowledge of the nuclear structure are required in order to draw definite conclusions.