Penning-Trap Mass Measurements of the Neutron-Rich K and Ca Isotopes: Resurgence of the N = 28 Shell Strength

TL;DR

This study reports precise Penning-trap mass measurements of neutron-rich potassium and calcium isotopes, revealing a resurgence of the N=28 shell strength, with significant implications for nuclear shell structure understanding.

Contribution

The paper provides more precise mass measurements of specific isotopes, showing a notable increase in the N=28 shell gap, which challenges previous assumptions about shell quenching.

Findings

More precise mass excesses for 48,49,50K and 47,50Ca.

Significant deviation from previous mass evaluations for 48,49K.

Increased N=28 shell gap energy by approximately 1 MeV.

Abstract

We present Penning-trap mass measurements of neutron-rich 44,47-50K and 49,50Ca isotopes carried out at the TITAN facility at TRIUMF-ISAC. The 44K mass measurement was performed with a charge-bred 4+ ion utilizing the TITAN EBIT, and agrees with the literature. The mass excesses obtained for 47K and 49,50Ca are more precise and agree with the values published in the 2003 Atomic Mass Evaluation (AME'03). The 48,49,50K mass excesses are more precise than the AME'03 values by more than one order of magnitude. For 48,49K, we find deviations by 7 sigma and 10 sigma, respectively. The new 49K mass excess lowers significantly the two-neutron separation energy at the neutron number N=30 compared with the separation energy calculated from the AME'03 mass-excess values, and thus, increases the N=28 neutron-shell gap energy at Z=19 by approximately 1 MeV.