Precision Mass Measurements around Mo 84 Rule Out ZrNb Cycle Formation in the Rapid Proton-Capture Process at Type I X-Ray Bursts

Le résumé fourni par la source

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 $\ensuremath{\delta}m\ensuremath{\approx}20\text{ }\text{ }\mathrm{keV}/{\mathrm{c}}^{2}$. The mass precisions of $^{79}\mathrm{Y}$ and $^{83}\mathrm{Nb}$ were improved to 13 and $9.6\text{ }\text{ }\mathrm{keV}/{\mathrm{c}}^{2}$, respectively. The new $\ensuremath{\alpha}$-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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