  The location of electron capture heat sources in the crust of accreting
neutron stars depends on the masses of extremely neutron-rich nuclei. We
present first results from a new implementation of the time-of-flight technique
to measure nuclear masses of rare isotopes at the National Superconducting
Cyclotron Laboratory. The masses of 16 neutron-rich nuclei in the scandium --
nickel range were determined simultaneously, improving the accuracy compared to
previous data in 12 cases. The masses of $^{61}${V}, $^{63}${Cr}, $^{66}${Mn},
and $^{74}${Ni} were measured for the first time with mass excesses of
$-30.510(890)$ MeV, $-35.280(650)$ MeV, $-36.900(790)$ MeV, and $-49.210(990)$
MeV, respectively. With the measurement of the $^{66}$Mn mass, the locations of
the two dominant electron capture heat sources in the outer crust of accreting
neutron stars that exhibit superbursts are now experimentally constrained. We
find that the location of the $^{66}$Fe$\rightarrow^{66}$Mn electron capture
transition occurs significantly closer to the surface than previously assumed
because our new experimental Q-value is 2.1 MeV (2.6$\sigma$) smaller than
predicted by the FRDM mass model.
