BETA-DECAY OF MG-20

The Beta-decay of Mg-20 was investigated. A secondary beam of Mg-20 io ns, produced in reactions between a 95 A.MeV Mg-24-beam and a Ni-nat-t arget, was isotopically separated by means of the LISE3 spectrometer a t GANIL. This secondary beam was implanted into a silicon detector arr ay surrounded by germanium gamma-detectors. The beta-delayed proton an d gamma-ray data, measured for this short-lived nucleus (T-1/2 = 95 +/ - 3 ms), were incorporated into an improved Mg-20-->Na-20 decay scheme . The 2645 keV level in Na-20 is of importance for the breakout from t he astrophysical hot CNO-cycle and the onset of the rapid proton captu re process via the reaction Ne-19(p,gamma)Na-20. An upper limit of 0.1 % for the beta-decay feeding of the 2645 keV level and a lower limit f or the corresponding log ft value of 6.24 were determined. The implica tions of this result for the spin and parity assignment of the 2645 ke V state are discussed. By comparing the Mg-20 beta-decay into the prot on-unbound 3001 keV state in Na-20 and the isospin-minored decay into the particle-bound 3488 keV level in F-20, an asymmetry ft(+)/ft(-) -1 = 1.69(-0.65)(+0.86) was observed. The comparison of the experimental ly determined B(GT) values for the Gamow-Teller beta-decay of Mg-20 wi th a calculation performed in a full sd-shell model space yields an ov erall quenching of the Gamow-Teller strength of 0.84, corresponding to an observed fraction of 71% of the calculated strength. The measured Gamow-Teller strength above excitation energies of 3 MeV is more fragm ented than predicted by the shell-model calculation.