The location of electron capture heat sources in the crust of accretingneutron stars depends on the masses of extremely neutron-rich nuclei. Wepresent first results from a new implementation of the time-of-flight techniqueto measure nuclear masses of rare isotopes at the National SuperconductingCyclotron Laboratory. The masses of 16 neutron-rich nuclei in the scandium --nickel range were determined simultaneously, improving the accuracy compared toprevious 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 ofthe two dominant electron capture heat sources in the outer crust of accretingneutron stars that exhibit superbursts are now experimentally constrained. Wefind that the location of the $^{66}$Fe$\rightarrow^{66}$Mn electron capturetransition occurs significantly closer to the surface than previously assumedbecause our new experimental Q-value is 2.1 MeV (2.6$\sigma$) smaller thanpredicted by the FRDM mass model.
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