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The ␤ decay of 50 K and 38 Ca has been investigated with the main motive of determining more accurately the first-forbidden ␤ branches, in particular the rank-0, ⌬Jϭ0, ␤ transitions. 50 K and 38 Ca have been produced by fragmentation of U and Ti targets, respectively, with a 1 GeV proton beam and subsequent on-line mass separation. For 50 K, ␥-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 unbound͒ levels in 50 Ca. The level structure of 50 Ca can be compared to recent calculations which incorporate one-particle-one-hole excitations from the f 7/2 shell. The first-forbidden ␤ Ϫ transition 50 K(0 Ϫ)→ 50 Ca(0 ϩ) ground state has been evaluated for the first time by a direct measurement of ␤ and ␥ activities. Its strength (61.0Ϯ7.4%) is interpreted as an effect of the meson-exchange current ͑MEC͒ leading to an enhancement factor of 62͑5͒% in comparison with the value predicted by shell-model calculations using the impulse approximation. For the 38 Ca→ 38 K decay, chemical selective production was obtained through separation of the molecular ion CaF ϩ without contamination by isobars. In these conditions, the measurement of very weak ␤ branches, at a level of 10 Ϫ3 per 100 decays, could be made and a limit, at the 2 confidence level, has been obtained for the 0 ϩ →0 Ϫ branch to the level at E x ϭ2993 keV (I ␤ Ͻ0.0046%). 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. ͓S0556-2813͑98͒01310-7͔

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