Study of the neutron-rich nuclei with N=21, 35Si and 33Mg, by beta decay of 35Al and 33Na Original Research Article

The first information on the level structure of the N=21 nuclei, 35Si and 33Mg, has been obtained by the beta decay study of 35Al and 33Na, produced by fragmentation of an UC target with 1.4 GeV protons at CERN/ISOLDE. The experimental technique involved β–γ, β–γ–γ, and β–n–γ coincidences, neutron spectra being obtained by time of flight measurements. Gamma detection was made either using large Ge counters or small BaF2 scintillators (for lifetime measurements). In the case of the 35Al decay, (T1/2=41.6(2.2) ms), a simple structure has been found for the level scheme of 35Si (Z=14, N=21) which has been interpreted with the level sequence : 7/2−, 3/2− and 3/2+ corresponding respectively to the ground state and the states at 910 and 974 keV. The life-time of the 974 keV [T1/2=5.9(6) ns] is found consistent with the proposed level scheme and multipolarities. The investigation of the N=21, very neutron rich isotones, has been carried on with the study of 33Mg resulting from the 33Na decay, (T1/2=8.0(6) ms). In this case also, a level scheme could be obtained for the first time, with 5 bound states in 32Mg and 31Mg levels, populated in the 1n and 2n channels. These results are compared with sd–fp shell model calculations which give a fair account of the Gamow–Teller distribution and a reasonable explanation of the lowest levels with predominant contribution of 1p–1h and 2p–2h excitations.