Ong W-J, Brown E F, Browne J, Ahn S, Childers K, Crider B P, Dombos A C, Gupta S S, Hitt G W, Langer C, Lewis R, Liddick S N, Lyons S, Meisel Z, Möller P, Montes F, Naqvi F, Pereira J, Prokop C, Richman D, Schatz H, Schmidt K, Spyrou A
Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA.
Phys Rev Lett. 2020 Dec 31;125(26):262701. doi: 10.1103/PhysRevLett.125.262701.
The interpretation of observations of cooling neutron star crusts in quasipersistent x-ray transients is affected by predictions of the strength of neutrino cooling via crust Urca processes. The strength of crust Urca neutrino cooling depends sensitively on the electron-capture and β-decay ground-state-to-ground-state transition strengths of neutron-rich rare isotopes. Nuclei with a mass number of A=61 are predicted to be among the most abundant in accreted crusts, and the last remaining experimentally undetermined ground-state-to-ground-state transition strength was the β decay of ^{61}V. This Letter reports the first experimental determination of this transition strength, a ground-state branching of 8.1_{-3.1}^{+4.0}%, corresponding to a log ft value of 5.5_{-0.2}^{+0.2}. This result was achieved through the measurement of the β-delayed γ rays using the total absorption spectrometer SuN and the measurement of the β-delayed neutron branch using the neutron long counter system NERO at the National Superconducting Cyclotron Laboratory at Michigan State University. This method helps to mitigate the impact of the pandemonium effect in extremely neutron-rich nuclei on experimental results. The result implies that A=61 nuclei do not provide the strongest cooling in accreted neutron star crusts as expected by some predictions, but that their cooling is still larger compared to most other mass numbers. Only nuclei with mass numbers 31, 33, and 55 are predicted to be cooling more strongly. However, the theoretical predictions for the transition strengths of these nuclei are not consistently accurate enough to draw conclusions on crust cooling. With the experimental approach developed in this work, all relevant transitions are within reach to be studied in the future.
通过地壳乌卡过程中微子冷却强度的预测，影响了对类持久X射线瞬变中冷却中子星地壳观测结果的解释。地壳乌卡中微子冷却强度敏感地取决于富含中子的稀有同位素的电子俘获和β衰变基态到基态跃迁强度。预测质量数A = 61的原子核是吸积地壳中最丰富的原子核之一，而最后一个实验上尚未确定的基态到基态跃迁强度是(^{61}V)的β衰变。本快报报道了该跃迁强度的首次实验测定，基态分支比为(8.1_{-3.1}^{+4.0}%)，对应log ft值为(5.5_{-0.2}^{+0.2})。该结果是通过在密歇根州立大学国家超导回旋加速器实验室使用全吸收谱仪SuN测量β延迟γ射线以及使用中子长计数器系统NERO测量β延迟中子分支得到的。这种方法有助于减轻极富含中子核中的混乱效应对实验结果的影响。结果表明，A = 61的原子核并不像一些预测所预期的那样在吸积中子星地壳中提供最强的冷却，但与大多数其他质量数相比，它们的冷却仍然更大。预计只有质量数为31、33和55的原子核冷却更强。然而，这些原子核跃迁强度的理论预测不够一致准确，无法就地壳冷却得出结论。利用本工作中开发的实验方法，未来所有相关跃迁都可进行研究。
