Meson-exchange enhancement in first-forbidden β transitions: The case of ⁵⁰K and ³⁸Ca

The β decay of ⁵⁰K and ³⁸Ca has been investigated with the main motive of determining more accurately the first-forbidden β branches, in particular the rank-0, ΔJ=0, β transitions. ⁵⁰K and ³⁸Ca have been produced by fragmentation of U and Ti targets, respectively, with a 1 GeV proton beam and subsequent on-line mass separation. For ⁵⁰K, γ-ray spectroscopy, as well as delayed neutron spectroscopy by time of flight, was carried out to obtain a detailed decay scheme to 20 (bound and unbound) levels in ⁵⁰Ca. The level structure of ⁵⁰Ca can be compared to recent calculations which incorporate one-particle–one-hole excitations from the f_7/2 shell. The first-forbidden β^- transition ⁵⁰K(0^-)→ ⁵⁰Ca(0⁺) ground state has been evaluated for the first time by a direct measurement of β and γ activities. Its strength (61.0±7.4%) is interpreted as an effect of the meson-exchange current (MEC) leading to an enhancement factor of 62(5)% in comparison with the value predicted by shell-model calculations using the impulse approximation. For the ³⁸Ca→³⁸K decay, chemical selective production was obtained through separation of the molecular ion CaF⁺ without contamination by isobars. In these conditions, the measurement of very weak β branches, at a level of 10^-3 per 100 decays, could be made and a limit, at the 2σ confidence level, has been obtained for the 0⁺→0^- branch to the level at E_x=2993 keV (I_β<0.0046%). Implications of these results on the general trend of meson-exchange enhancements of first-forbidden transitions within the framework of the spherical shell model are discussed.