Beta-decay of 2°Mg

The fl-decay of 2°Mg was investigated. A secondary beam of 2°Mg ions, produced in reactions between a 95 A.MeV 24Mg-beam and a "atNi-target, was isotopically separated by means of the LISE3 spectrometer at GANIL. This secondary beam was implanted into a silicon detector array surrounded by germanium y-detectors. The fl-delayed proton and y-ray data, measured for this short-lived nucleus (Tl/2 = 95 43 ms), were incorporated into an improved mMg-+2°Na decay scheme. The 2645 keV level in 2°Na is of importance for the breakout from the astrophysical hot CNO-cycle and the onset of the rapid proton capture process via the reaction 19Ne(p,y)2°Na. An upper limit of 0.1% for the fl-decay feeding of the 2645 keV level and a lower limit for the corresponding log f t value of 6.24 were determined. The implications of this result for the spin and parity assignment of the 2645 keV state are discussed. By comparing the 2°Mg fl-decay into the proton-unbound 3001 keV state in 2°Na and the isospin-mirrored decay into the particle-bound 3488 keV level in 2°E an asymmetry f t + / f t 1 = 1 ~o+0.86 . . . . 0.65 was observed. The comparison of the experimentally determined B(GT) values for the Gamow-Teller fl-decay of 2°Mg with a calculation performed in a full sd-shell model space yields an overall quenching of the GamowTeller 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 fragmented than predicted by the shell-model calculation. 0375-9474/95/$09.50 Q 1995 Elsevier Science B.V. All rights reserved SSDI 0375-9474(94)00791-8 510 A. Piechaczek et aL /Nuclear Physics A 584 (1995) 509-531