Structure of Si 33 and the magicity of the N=20 gap at Z=14

Abstract

The structure of Si33 was studied by a one-neutron knockout reaction from a Si34 beam at 98.5 MeV/u incident on a Be9 target. The prompt γ rays following the de-excitation of Si33 were detected using the GRETINA γ-ray tracking array while the reaction residues were identified on an event-by-event basis in the focal plane of the S800 spectrometer at the National Superconducting Cyclotron Laboratory. The presently derived spectroscopic factor values, C2S, for the 3/2+ and 1/2+ states, corresponding to a neutron removal from the 0d3/2 and 1s1/2 orbitals, agree with shell model calculations and point to a strong N=20 shell closure. Three states arising from the more bound 0d5/2 orbital are proposed, one of which is unbound by about 930 keV. The sensitivity of this experiment has also confirmed a weak population of 9/2- and 11/21,2- final states, which originate from a higher-order process. This mechanism may also have populated, to some fraction, the 3/2- and 7/2- negative-parity states, which hinders a determination of the C2S values for knockout from the normally unoccupied 1p3/2 and 0f7/2 orbits.