Meson-exchange enhancement in first-forbidden β transitions: The case of 50K and 38Ca
Abstract
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 f7/2 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 Ex=2993 keV (Iβ<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.
- Publication:
-
Physical Review C
- Pub Date:
- October 1998
- DOI:
- 10.1103/PhysRevC.58.1970
- Bibcode:
- 1998PhRvC..58.1970B
- Keywords:
-
- 23.40.Hc;
- 27.40.+z;
- 23.20.Lv;
- 21.60.Cs;
- Relation with nuclear matrix elements and nuclear structure;
- 39<
- =A<
- =58;
- gamma transitions and level energies;
- Shell model