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

Abstract

<p>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 84Mo was previously suggested by the formation of a ZrNb cycle. We report here precision mass measurements at around 84Mo 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 79Y, 83Nb, 84Mo, 88Ru, and an isomer in 78Y were measured. For 84Mo, 88Ru, and the isomeric state of 78Y, their masses were experimentally determined for the first time with uncertainties of 𝛿⁢𝑚≈20 keV/c2. The mass precisions of 79Y and 83Nb were improved to 13 and 9.6 keV/⁢c2, respectively. The new 𝛼-separation energy of 84Mo, 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.<br></p>