Meson-exchange enhancement in first-forbiddenβtransitions: The case of50Kand

Abstract

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

Effective field theories, Landau-Migdal Fermi liquid theory, and effective chiral Lagrangians for nuclear matter

“…More recent measurements -and their analyses -in different nuclei [43,44] quantitatively confirm this result of Warburton. The theoretical prediction from (61) is…”

Shell evolution and theN=34“magic number”

“…Based on the energy of the lowest excited states in 48 Ca, the core-excited states are expected to contribute around 4 MeV. The assignment of a 3 − , 2 + 3 and 4 + states [2,17] is consistent with this expectation. The state at 4.9 MeV is identified with the p 3/2 f 5/2 excitation.…”

“…Such a method, to the best of our knowledge, has been employed for the first time. In 50 Ca the 1978.2(0.6) keV γ-ray was observed in coincidence with the 1026.2(0.3) keV transition and has been assigned to arise from the decay of a 2 + 2 and not from a 0 + state [17], based on the higher expected energy of the 0 + in 48 Ca. Other transitions seen in Ref.…”

A new effective interaction for $0\hbarω$ shell model calculations in the sdpf valence space

“…Besides that, in order to place the 3 2 − state of 35 Si at its experimental excitation energy, only the 0d 3/2 -1p 3/2 centroid was modified. As a consequence, SDPF-NR produces a 1 2 + ground state in 49 K, instead of the 3 2 + favored by the experiments [29], and does not reproduce the assignment of J=0 − to the ground state of 50 K proposed in [30].…”