Cross-shell excitations in Si 31

Author(s): Tai, PL; Tabor, SL; Lubna, RS; Kravvaris, K; Bender, PC; Tripathi, V; Volya, A; Carpenter, MP; Janssens, RVF; Lauritsen, T; McCutchan, EA; Zhu, S; Clark, RM; Fallon, P; Paschalis, S; Petri, M; Macchiavelli, AO; Reviol, W; Sarantites, DG | Abstract: © 2017 American Physical Society. The Si31 nucleus was produced through the O18(O18, αn) fusion-evaporation reaction at Elab=24MeV. Evaporated α particles from the reaction were detected and identified in the Microball detector array for channel selection. Multiple γ-ray coincidence events were detected in Gammasphere. The energy and angle information for the α particles was used to determine the Si31 recoil kinematics on an event-by-event basis for a more accurate Doppler correction. A total of 22 new states and 52 new γ transitions were observed, including 14 from states above the neutron separation energy. The positive-parity states predicted by the shell-model calculations in the sd model space agree well with experiment. The negative-parity states were compared with shell-model calculations in the psdpf model space with some variations in the N=20 shell gap. The best agreement was found with a shell gap intermediate between that originally used for A≈20 nuclei and that previously adapted for P32,34. This variation suggests the need for a more universal cross-shell interaction.

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