MASON-EXCHANGE ENHANCEMENT IN FIRST-FORBIDDEN BETA-TRANSITIONS - THE CASE OF K-50 AND CA-38

The beta decay of K-50 and Ca-38 has been investigated with the main m otive of determining more accurately the first-forbidden beta branches , in particular the rank-0, Delta J=0, beta 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, gamma-ray spectroscopy, as well as delayed neutron spectroscopy b y time of flight, was carried out to obtain a detailed decay scheme to 20 (bound and unbound) levels in Ca-50. The level structure of 50Ca c an be compared to recent calculations which incorporate one-particle-o ne-hole excitations from the f(7/2) shell. The first-forbidden beta(-) transition K-50(0(-)), Ca-50(0(+)) ground state has been evaluated fo r the first time by a direct measurement of beta and gamma activities. Its strength (61.0+/-7.4%) is interpreted as an effect of the meson-e xchange current (MEC) leading to an enhancement factor of 62(5)% in co mparison with the value predicted by shell-model calculations using th e impulse approximation. For the Ca-38-->K-38 decay, chemical selectiv e production was obtained through separation of the molecular ion CaF without contamination by isobars. In these conditions, the measuremen t of very weak beta branches, at a level of 10(-3) per 100 decays, cou ld be made and a limit, at the 2 sigma confidence level, has been obta ined for the 0(+)-->0(-) branch to the level at E-x=2993 keV (I-beta<0 .0046%). Implications of these results on the general trend of meson-e xchange enhancements of first-forbidden transitions within the framewo rk of the spherical shell model are discussed. [S0556-2813(98)01310-7] .