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/