11/08/2023
By Yiyi Zhu

The Kennedy College of Sciences, Department of Physics and Applied Physics, invites you to attend a doctoral dissertation defense by Yiyi Zhu.

Candidate Name: Yiyi Zhu
Degree: Doctoral
Defense Date: Wednesday, Nov. 15, 2023
Time: 2 p.m.
Location: Pinanski 202
Thesis Title: The β-Decay of Isotopes in the N = 20 “Island of Inversion”

Committee:

  • Advisor: Peter Bender, Physics Department, UMass Lowell
  • Partha Chowdhury
  • Viktor Podolskiy

Abstract:
The structure of nuclei along the valley of stability is well understood through the observation of traditional magic numbers. These “magic numbers” correspond to significant gaps in the single-particle energy levels of nucleons within realistic mean-field theories. However, as more exotic nuclei from stability have been probed over the past few decades, these magic numbers appear to evolve, create novel ground-state configurations. One such area of the nuclear chart where this evolution is observed is known as the “Island of Inversion” (IoI).

The present work revolves around the β-decay of extremely neutron-rich isotopes at the edge of the IoI. The experiment used fast, fragmented ions produced by projectile 48Ca primary beam at National Superconducting Cyclotron Laboratory (NSCL) and implanted them into a Silicon detector array known as the Beta Counting System (BCS). Implanted isotopes of interest were identified and analyzed event-by-event, correlated to subsequent γ-emission which was detected by an array of 16 clover style, High Purity Germanium Detectors (HPGe).

The β-decay of 33,32Na and 31,30Ne in the N = 20 IoI was investigated in the present work, with a focus on their half-lives and the construction of level schemes for the decay daughters. The level schemes were constructed based on the analysis of γ- γ coincidences and γ singles. The number of implanted ions combined with observed emitted γ radiation was used to determine logft values. Tentative spin-parity are assigned to each level in 33,32,31Mg and 31,30Na produced following the β or β-n decay. These assignments are compared to the shell model calculations and the adopted levels from the literature, updating and expanding the various level schemes. This new information allows for a deeper interpretations of the various studied daughter nuclei as well as the ground-state configurations of the decay parents, 33,32Na and 31,30Ne.

Notably, in 33Na and 31Ne β- and β-n decay, non-negligible branching ratios to the 33Mg ground state (J^π= 3/2-) and 31Na ground state ( J^π= 3/2+) suggest the spin-parity assignments of 3/2- for 33Na and 3/2+ for 31Ne ground state, respectively. These results illustrate the presence of significant 1p1h and/or 3p3h configurations in the ground states of these isotopes, rather than the primary 2p2h configuration as has been previously suggested. Comparing these states to the 0p0h or 2p2h structures, the presence of 1p1h or 3p3h configurations provides more direct information about the effects of intruder orbitals and cross-shell interactions.