The rapid proton-capture process is one of the primary, explosive thermonuclear burning processes that drive type I x-ray bursts. A possible termination of the rapid proton-capture process at around ${}^{84}\mathrm{Mo}$ was previously suggested by the formation of a ZrNb cycle. We report here precision mass measurements at around ${}^{84}\mathrm{Mo}$ that conclude the issue regarding the possibility of the cycle. The experiment was conducted using the multireflection time-of-flight spectrograph at RIKEN Radioactive Isotope Beam Factory, and the masses of ${}^{79}\mathrm{Y}$, ${}^{83}\mathrm{Nb}$, ${}^{84}\mathrm{Mo}$, ${}^{88}\mathrm{Ru}$, and an isomer in ${}^{78}\mathrm{Y}$ were measured. For ${}^{84}\mathrm{Mo}$, ${}^{88}\mathrm{Ru}$, and the isomeric state of ${}^{78}\mathrm{Y}$, their masses were experimentally determined for the first time with uncertainties of $\delta m\approx 20\mathrm{keV}/{\mathrm{c}}^2$. The mass precisions of ${}^{79}\mathrm{Y}$ and ${}^{83}\mathrm{Nb}$ were improved to 13 and $9.6\mathrm{keV}/{\mathrm{c}}^2$, respectively. The new $\alpha$-separation energy of ${}^{84}\mathrm{Mo}$, 1.434(83) MeV, unambiguously rules out the possibility of forming the ZrNb cycle. The x-ray burst simulation with the new masses shows that our measurements effectively remove the large final abundance uncertainties in the $A=80–90$ mass region. The new mass values improve the prediction power for the composition of the nuclear ashes in x-ray bursts.

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• Binding energy & masses
• rp process