Proton Shell Gaps in N=28 Nuclei from the First Complete Spectroscopy Study with the FRIB Decay Station Initiator

Exotic nuclei can exhibit properties different from their stable counterparts. Changes in shell structure observed in very neutron-rich nuclei give rise to the islands of inversion. Conventionally, in nuclei with N~28 and Z<20, the Fermi surfaces for neutrons and protons are located within the pf and sd shells, respectively. The selectivity of beta-decay motivates decay strength measurements to probe the nuclear shell effects of the parent and daughter nucleus. In the Gamow-Teller transitions, sd and pf neutrons transform into protons in respective spin-orbit partner orbitals. The first complete measurement of the beta-decay strength distribution of ⁴⁵Cl, performed at FRIB, exemplifies the ability of Gamow-Teller transitions to populate states associated with proton excitation across major shells, allowing for the first benchmark of the Z=20 shell gap along N=28 below ⁴⁸Ca. The measurement utilized the two focal plane system of the FRIB Decay Station Initiator (FDSi[1]), with a combination of high-resolution neutron (NEXTi) and gamma-ray (DEGAi) spectroscopy data alongside total absorption spectroscopy data (MTAS). The complete decay strength is extracted up to 8 MeV in ⁴⁵Ar and compared to large-scale shell model calculations using the SDPF-MU interaction. This sensitive approach found that a reduced Z=20 shell gap best reproduced the data.



[1] https://fds.ornl.gov/initiator/