X線バーストでの元素合成過程の解明に前進～原子核質量の精密測定によって合成過程の終わり方の問題に結論～

2025-10-09 高エネルギー加速器研究機構,理化学研究所

高エネルギー加速器研究機構（KEK）と理化学研究所の共同研究チームは、宇宙の爆発的天体現象「I型X線バースト」における元素合成の終端過程を実験的に解明した。研究では、速い陽子捕獲過程（rp過程）の終わりを担うと考えられていた「ジルコニウム‐ニオブ（ZrNb）サイクル」の鍵核種・モリブデン84（⁸⁴Mo）の質量を高精度に測定。その結果、通常のX線バースト条件ではZrNbサイクルが成立せず、rp過程はより重い元素へ進むことが判明した。これは宇宙での元素生成メカニズムの理解を一歩前進させる成果である。

図 1 速い陽⼦捕獲過程の概略。Sαが⼩さいとアルファ崩壊を起こして閉じた反応サイクルを形成し、速い陽⼦捕獲過程の終端となる。Sαが⼤きいとベータ崩壊を起こし、より重い元素の合成に進む。

＜関連情報＞

精密質量測定84MoタイプI X線バーストにおける急速陽子捕獲過程におけるZrNbサイクル形成の排除 Precision Mass Measurements around 84Mo Rule Out ZrNb Cycle Formation in the Rapid Proton-Capture Process at Type I X-Ray Bursts

S. Kimura, M. Wada, C. Y. Fu, N. Fukuda, Y. Hirayama, D. S. Hou, S. Iimura, H. Ishiyama, Y. Ito et al.
Physical Review Letters Published: 8 October, 2025
DOI: https://doi.org/10.1103/2dyn-q7wp

Abstract

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 ⁢≈20 keV/c². The mass precisions of ⁷⁹Y and ⁸³Nb were improved to 13 and 9.6 keV/⁢c², respectively. The new -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 =80–90 mass region. The new mass values improve the prediction power for the composition of the nuclear ashes in x-ray bursts.