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We present the mass excesses of 52-57Sc, obtained from recent time-of-flightnuclear mass measurements at the National Superconducting Cyclotron Laboratoryat Michigan State University. The masses of 56Sc and 57Sc were determined forthe first time with atomic mass excesses of -24.85(59)(+0 -54) MeV and-21.0(1.3) MeV, respectively, where the asymmetric uncertainty for 56Sc wasincluded due to possible contamination from a long-lived isomer. The 56Sc massindicates a small odd-even mass staggering in the A = 56 mass-chain towards theneutron drip line, significantly deviating from trends predicted by the globalFRDM mass model and favoring trends predicted by the UNEDF0 and UNEDF1 densityfunctional calculations. Together with new shell-model calculations of theelectron-capture strength function of 56Sc, our results strongly reduceuncertainties in model calculations of the heating and cooling at the 56Tielectron-capture layer in the outer crust of accreting neutron stars. We foundthat, in contrast to previous studies, neither strong neutrino cooling norstrong heating occurs in this layer. We conclude that Urca cooling in the outercrusts of accreting neutron stars that exhibit superbursts or high temperaturesteady-state burning, which are predicted to be rich in A=56 nuclei, isconsiderably weaker than predicted. Urca cooling must instead be dominated byelectron capture on the small amounts of adjacent odd-A nuclei contained in thesuperburst and high temperature steady-state burning ashes. This may explainthe absence of strong crust Urca cooling inferred from the observed coolinglight curve of the transiently accreting x-ray source MAXI J0556-332.

Mass measurement of 56Sc reveals a small A=56 odd-even mass staggering, implying a cooler accreted neutron star crust