G. Klotz et al., BETA-DECAY OF NA-31, NA-32 AND MG-31 - STUDY OF THE N = 20 SHELL CLOSURE, Physical review. C. Nuclear physics, 47(6), 1993, pp. 2502-2516
The Na-31, Na-32 and 31Mg beta decays were studied at the CERN on-line
mass separator ISOLDE by gamma, gamma-gamma, and neutron-gamma measur
ements. In the 31Na decay, the assignment of previously reported gamma
transitions and the observation of a new level at 3760 keV lead to a
revised decay scheme which is found in good agreement with a calculati
on including two-particle-two-hole configurations in the model space,
as far as only low-lying levels of 31Mg are concerned. In the 31Mg-->A
l-31 decay, a new decay scheme involves ten beta branches and three st
ates are reported for the first time. While satisfactory agreement wit
h theoretical calculations is observed for excitation energies in Al-3
1, a strong discrepancy is observed for the intensity of the ground-st
ate beta branch, the experimental one being highly quenched as compare
d to theoretical expectations. Finally, new spectroscopic results have
been obtained in the 32Na beta decay. A previously noninterpreted 143
6 keV gamma ray is now assigned in the 31Mg scheme. The 240 keV ray is
shown to arise from 31Mg produced in the one-neutron channel, and to
be related to the decay of an intruder state at E(x) =461 keV. The lat
ter is partially fed from the 1390 keV level. Both nicely compare with
theoretical predictions locating 1homegaBAR states at 0.40 MeV (7/2-)
and 1.57 MeV (11/2-). The first experimental evidence for a gamma cas
cade in the descendant 32Al is also obtained.