Precision Mass Measurements around ^{84}Mo Rule Out ZrNb Cycle Formation in the Rapid Proton-Capture Process at Type...

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}Mo was previously suggested by the formation of a ZrNb cycle. We report here precision mass measurements at around ^{84}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}Y, ^{83}Nb, ^{84}Mo, ^{88}Ru, and an isomer in ^{78}Y were measured. For ^{84}Mo, ^{88}Ru, and the isomeric state of ^{78}Y, their masses were experimentally determined for the first time with uncertainties of δm≈20  keV/c^{2}. The mass precisions of ^{79}Y and ^{83}Nb were improved to 13 and 9.6  keV/c^{2}, respectively. The new α-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.