Study of the neutron-rich nuclei with N=21, sup 3 sup 5 Si and sup 3 sup 3 Mg, by beta decay of sup 3 sup 5 Al and sup 3 sup 3 Na

The first information on the level structure of the N=21 nuclei, sup 3 sup 5 Si and sup 3 sup 3 Mg, has been obtained by the beta decay study of sup 3 sup 5 Al and sup 3 sup 3 Na, produced by fragmentation of an UC target with 1.4 GeV protons at CERN/ISOLDE. The experimental technique involved beta-gamma, beta-gamma-gamma, and beta-n-gamma coincidences, neutron spectra being obtained by time of flight measurements. Gamma detection was made either using large Ge counters or small BaF sub 2 scintillators (for lifetime measurements). In the case of the sup 3 sup 5 Al decay, (T sub 1 sub / sub 2 =41.6(2.2) ms), a simple structure has been found for the level scheme of sup 3 sup 5 Si (Z=14, N=21) which has been interpreted with the level sequence : 7/2 sup - , 3/2 sup - and 3/2 sup + corresponding respectively to the ground state and the states at 910 and 974 keV. The life-time of the 974 keV [T sub 1 sub / sub 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 sup 3 sup 3 Mg resulting from the sup 3 sup 3 Na decay, (T sub 1 sub / sub 2 =8.0(6) ms). In this case also, a level scheme could be obtained for the first time, with 5 bound states in sup 3 sup 2 Mg and sup 3 sup 1 Mg 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.