$^{78}$ Ni：$N=50$幻数和高能非唯一第一禁戒跃迁的半衰期突然缩短
Suddenly shortened half-lives beyond $^{78}$Ni: $N=50$ magic number and high-energy non-unique first-forbidden transitions
摘要：
$β$ -衰变率在理解重元素的核合成过程中起着决定性的作用，并受微观核结构信息的控制。在RIKEN-RIBF上观察到Ni同位素的半衰期突然缩短，超过$N=50$。这被认为是由于中子幻数$N=50$在非常富中子的Ni同位素中的持久性。通过系统研究$N=50$周围富中子Ni同位素的$β$ -衰变率和强度分布，试图理解所观察到的半衰期突然缩短的微观机制。在核密度泛函理论的框架下描述了丰中子核中的$β$强度分布。在基态的Hartree-Fock-Bogoliubov计算和质子-中子准粒子无规相近似（pnQRPA）的跃迁计算中，我采用了Skyrme能量密度泛函（EDF）。不仅被允许，但第一禁止（FF）的过渡。通过使用Skyrme SkM* 和SLy 4泛函的计算，很好地再现了实验观察到的超过$N=50$的半衰期突然缩短。半衰期的突然缩短是由于$N=50$处的壳层间隙以及与子核中的低能$0^-$和$1^-$态的高能跃迁的协同作用。在$N=82$和126周围指出的FF转变的开始被保存在$N=50$周围的低质量核中。本文的研究表明，在预测未知区域奇异核的$β$ -衰变率时，需要自洽地考虑丰中子核壳层结构及其对FF跃迁的影响的微观计算.
英文摘要：
$β$-decay rates play a decisive role in understanding the nucleosynthesis of heavy elements and are governed by microscopic nuclear-structure information. A sudden shortening of the half-lives of Ni isotopes beyond $N=50$ was observed at the RIKEN-RIBF. This is considered due to the persistence of the neutron magic number $N=50$ in the very neutron-rich Ni isotopes. By systematically studying the $β$-decay rates and strength distributions in the neutron-rich Ni isotopes around $N=50$, I try to understand the microscopic mechanism for the observed sudden shortening of the half-lives. The $β$-strength distributions in the neutron-rich nuclei are described in the framework of nuclear density-functional theory. I employ the Skyrme energy-density functionals (EDF) in the Hartree-Fock-Bogoliubov calculation for the ground states and in the proton-neutron Quasiparticle Random-Phase Approximation (pnQRPA) for the transitions. Not only the allowed but the first-forbidden (FF) transitions are considered. The experimentally observed sudden shortening of the half-lives beyond $N=50$ is reproduced well by the calculations employing the Skyrme SkM* and SLy4 functionals. The sudden shortening of the half-lives is due to the shell gap at $N=50$ and cooperatively with the high-energy transitions to the low-lying $0^-$ and $1^-$ states in the daughter nuclei. The onset of FF transitions pointed out around $N=82$ and 126 is preserved in the lower-mass nuclei around $N=50$. This study suggests that needed is a microscopic calculation where the shell structure in neutron-rich nuclei and its associated effects on the FF transitions are selfconsistenly taken into account for predicting $β$-decay rates of exotic nuclei in unknown region.
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