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

Abstract

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 BaF 2 scintillators (for lifetime measurements). In the case of the 35Al decay, ( T 1/2=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 [ T 1/2=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, ( T 1/2=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.