The Structure of $^{33}$Si and the magicity of the N=20 gap at Z=14

TL;DR

This study investigates the structure of $^{33}$Si using knockout reactions, confirming a strong N=20 shell closure and providing new insights into the orbital configurations and state populations in this nucleus.

Contribution

The paper provides experimental spectroscopic factors for $^{33}$Si states, supporting the shell model and revealing details about the N=20 gap at Z=14, including the identification of unbound states.

Findings

Spectroscopic factors agree with shell model predictions.

Confirmation of a strong N=20 shell closure.

Identification of unbound states near 930 keV.

Abstract

The structure of $^{33}$Si was studied by a one-neutron knockout reaction from a $^{34}$Si beam at 98.5 MeV/u incident on a $^{9}$Be target. The prompt $\gamma$-rays following the de-excitation of $^{33}$Si were detected using the GRETINA $\gamma$-ray tracking array while the reaction residues were identified on an event-by-event basis in the focal plane of the S800 spectrometer at NSCL (National Superconducting Cyclotron Laboratory). The presently derived spectroscopic factor values, $C^2S$, for the 3/2$^+$ and 1/2$^+$ states, corresponding to a neutron removal from the $0d_{3/2}$ and $1s_{1/2}$ orbitals, agree with shell model calculations and point to a strong $N=20$ shell closure. Three states arising from the more bound $0d_{5/2}$ 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 11/2$_{1,2}^-$ 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 $C^2S$ values for knockout from the normally unoccupied $1p_{3/2}$ and $0f_{7/2}$ orbits.