Time-of-flight mass measurements of neutron-rich chromium isotopes up to N = 40 and implications for the accreted neutron star crust

TL;DR

This study measures the masses of neutron-rich chromium isotopes up to N=40, compares results with shell-model calculations, and explores implications for neutron star crust heating, revealing a cooler crust than previously modeled.

Contribution

First measurement of 64Cr mass and analysis of its impact on neutron star crust heating using new experimental data and shell-model comparisons.

Findings

Mass of 64Cr determined for the first time.

Significant change in two-neutron separation energy trend at N=38.

Reduced electron-capture heating in neutron star crust due to lower 64Cr binding energy.

Abstract

We present the mass excesses of 59-64Cr, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of 64Cr is determined for the first time, with an atomic mass excess of -33.48(44) MeV. We find a significantly different two-neutron separation energy S2n trend for neutron-rich isotopes of chromium, removing the previously observed enhancement in binding at N=38. Additionally, we extend the S2n trend for chromium to N=40, revealing behavior consistent with the previously identified island of inversion in this region. We compare our results to state-of-the-art shell-model calculations performed with a modified Lenzi-Nowacki-Poves-Sieja interaction in the fp shell, including the g9/2 and d5/2 orbits for the neutron valence space. We employ our result for the mass of 64Cr in accreted neutron star crust network calculations and find a reduction in the strength and depth of electron-capture heating from the A=64 isobaric chain, resulting in a cooler than expected accreted neutron star crust. This reduced heating is found to be due to the >1-MeV reduction in binding for 64Cr with respect to values from commonly used global mass models.