Results from the NSCL TOF Mass Measurement of Neutron-rich Isotopes of Argon through Iron*

A recent experiment at the National Superconducting Cyclotron Laboratory has resulted in an extension of the nuclear mass surface for neutron-rich isotopes of argon through iron. The time-of-flight technique was employed to experimentally determine the masses of several nuclei for the first time. Results include the identification of argon as the lowest even-$Z$ element exhibiting the $N=28$ shell closure, as well the uncovering of a relatively small odd-even mass stagger between $^{56}$Ti and $^{56}$Sc. The latter result yields strong Urca cooling when implemented in a state-of-the-art accreted neutron star crust reaction network. A strong $^{56}$Ti--$^{56}$Sc Urca neutrino cooling layer makes shallow neutrino cooling in the crust of accreting neutron stars a strong and robust effect due to the copious production of $A=56$ material in thermonuclear burning processes that occur at the accreted neutron star surface.