The rapid proton-capture ($rp$-) process is one of the primary, explosive thermonuclear burning processes that drive type I X-ray bursts. A possible termination of the $rp$-process at around ${ }^{ 84}$Mo was previously suggested by the formation of a ZrNb cycle. We report here precision mass measurements at around ${ }^{ 84}$Mo, which have concluded the possibility of the cycle. The experiment was conducted using the multi-reflection time-of-flight spectrograph at RIKEN RI Beam Factory, and the masses of ${ }^{ 79}$Y, ${ }^{ 83}$Nb, ${ }^{ 84}$Mo, ${ }^{ 88}$Ru, and an isomer in ${ }^{ 78}$Y were measured. For ${ }^{ 84}$Mo, and ${ }^{ 88}$Ru, and the isomeric state of ${ }^{ 78}$Y, their masses are experimentally determined for the first time with uncertainties of $\delta m \approx 20~{ \rm keV/c^2}$. The mass precision of ${ }^{ 79}$Y and ${ }^{ 83}$Nb is improved to $13~{ \rm keV/c^2}$ and $9.6~{ \rm keV/c^2}$, respectively. The new $\alpha$-separation energy of ${ }^{ 84}$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.
Categories: nucl-ex astro-ph.HE nucl-th