Time-of-Flight Mass Measurements for Nuclear Processes in Neutron Star Crusts

We present results from time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory that are relevant for neutron star crust models. The masses of 16 neutron-rich nuclei in the scandium–nickel range were determined simultaneously, with the masses of ${}^{61}\mathrm{V}$, ${}^{63}\mathrm{Cr}$, ${}^{66}\mathrm{Mn}$, and ${}^{74}\mathrm{Ni}$ measured for the first time with mass excesses of $-30.510(890)\mathrm{MeV}$, $-35.280(650)\mathrm{MeV}$, $-36.900(790)\mathrm{MeV}$, and $-49.210(990)\mathrm{MeV}$, respectively. With these results the locations of the dominant electron capture heat sources in the outer crust of accreting neutron stars that exhibit super bursts are now experimentally constrained. We find the experimental $Q$ value for the ${}^{66}\mathrm{Fe}\to {}^{66}\mathrm{Mn}$ electron capture to be 2.1 MeV ($2.6\sigma$) smaller than predicted, resulting in the transition occurring significantly closer to the neutron star surface.

Figure 1

Integral of the heat deposited in the neutron star crust by electron captures as a function of depth (indicated through ${\mu }_e$). Shown are results for masses from the FRDM [28] only (red, dot-dashed line), for implementing previously published experimental masses (green, solid line), and for implementing in addition our new mass results (black, dashed line).

Figure 2

Black diamonds show fit residuals for the reference isotopes as a function of their mass-to-charge ratio (${}^{51}\mathrm{Sc}$, ${}^{52}\mathrm{Sc}$, ${}^{54}\mathrm{Ti}$, ${}^{55}\mathrm{Ti}$, ${}^{56}\mathrm{Ti}$, ${}^{56}\mathrm{V}$, ${}^{57}\mathrm{V}$, ${}^{58}\mathrm{V}$, ${}^{59}\mathrm{V}$, ${}^{60}\mathrm{Cr}$, ${}^{61}\mathrm{Cr}$, ${}^{62}\mathrm{Cr}$, ${}^{61}\mathrm{Mn}$, ${}^{63}\mathrm{Mn}$, ${}^{64}\mathrm{Mn}$, ${}^{64}\mathrm{Fe}$, ${}^{66}\mathrm{Fe}$, ${}^{71}\mathrm{Ni}$, ${}^{72}\mathrm{Ni}$, ${}^{73}\mathrm{Ni}$, ${}^{73}\mathrm{Cu}$, ${}^{74}\mathrm{Cu}$, ${}^{75}\mathrm{Cu}$, ${}^{75}\mathrm{Zn}$, ${}^{76}\mathrm{Zn}$, and ${}^{79}\mathrm{Zn}$). Blue circles are the isotopes for which we present an improved mass value (see Table ).

Figure 3

Two neutron separation energies as a function of neutron number. $S_{2n}$ values calculated with our mass results are shown as filled symbols (red). Open symbols show $S_{2n}$ values from the literature [15, 16, 17, 18].