BETA-DECAY OF NA-31, NA-32 AND MG-31 - STUDY OF THE N = 20 SHELL CLOSURE

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 measurements. 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 calculation including two-particle-two-hole configurations in the model space, as far as only low-lying levels of 31Mg are concerned. In the 31Mg-->Al-31 decay, a new decay scheme involves ten beta branches and three states are reported for the first time. While satisfactory agreement with theoretical calculations is observed for excitation energies in Al-31, a strong discrepancy is observed for the intensity of the ground-state beta branch, the experimental one being highly quenched as compared to theoretical expectations. Finally, new spectroscopic results have been obtained in the 32Na beta decay. A previously noninterpreted 1436 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 latter 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 cascade in the descendant 32Al is also obtained.