Direct Mass Measurements of Short-Lived $A=2Z-1$ Nuclides ${}^{63}\mathrm{Ge}$, ${}^{65}\mathrm{As}$, ${}^{67}\mathrm{Se}$, and ${}^{71}\mathrm{Kr}$ and Their Impact on Nucleosynthesis in the $rp$ Process

Mass excesses of short-lived $A=2Z-1$ nuclei ${}^{63}\mathrm{Ge}$, ${}^{65}\mathrm{As}$, ${}^{67}\mathrm{Se}$, and ${}^{71}\mathrm{Kr}$ have been directly measured to be $-46921(37)$, $-46937(85)$, $-46580(67)$, and $-46320(141)\mathrm{keV}$, respectively. The deduced proton separation energy of $-90(85)\mathrm{keV}$ for ${}^{65}\mathrm{As}$ shows that this nucleus is only slightly proton unbound. X-ray burst model calculations with the new mass excess of ${}^{65}\mathrm{As}$ suggest that the majority of the reaction flow passes through ${}^{64}\mathrm{Ge}$ via proton capture, indicating that ${}^{64}\mathrm{Ge}$ is not a significant $rp$-process waiting point.

Figure 1

The revolution time spectrum zoomed in on a subset of the $A=2Z-1$ nuclides. The inset shows the well-resolved ground and isomeric [$T_{1/2}=247\mathrm{ms}$, $E^{*}=3174.3(1.0)\mathrm{keV}$ [21] ] states of ${}^{53}\mathrm{Co}$. Our result $E^{*}({}^{53}\mathrm{Co}^m)=3202(36)\mathrm{keV}$ is in good agreement with the literature value.

Figure 2

Regions in the temperature and density plane where ${}^{64}\mathrm{Ge}$ is a waiting point (WP) with an effective lifetime longer than 50% of the $\beta$-decay lifetime. Shaded areas are deduced assuming a $1\sigma$ variation of the AME03 mass for ${}^{65}\mathrm{As}$ (gray) and the ${}^{65}\mathrm{As}$ mass from this work (black). Results are given for a proton mass fraction of 0.7.

Figure 3

Calculated x-ray luminosity in a one-zone x-ray burst model varying $S_p({}^{65}\mathrm{As})$ within $2\sigma$ for masses from AME03 [22] (dashed black lines) and from this work (solid red lines).

Figure 4

The calculated abundance distribution of the x-ray burst ashes using $S_p({}^{65}\mathrm{As})$ from AME03 (black filled circles) and from this work (red open squares). Error bars indicate uncertainties induced by the $1\sigma$ uncertainty of $S_p({}^{65}\mathrm{As})$. Except for $A=64$, the error bars of our new results are within the symbol size.