PDMATHAAB同量异位素中${Z=28}$壳层间隙的持久性：在${^{75}}$ Co中可能的${(1/2^-)}$ $μ$ s异构体的鉴定和$β$衰变为${^{75}}$ Ni
Persistence of the ${Z=28}$ shell gap in ${A=75}$ isobars: Identification of a possible ${(1/2^-)}$ $μ$s isomer in ${^{75}}$Co and $β$ decay to ${^{75}}$Ni
摘要：
背景：双幻奇异核壳层结构的演化是核物理和天体物理研究的热点。在$^{78}$ Ni的“西南”区域，变形的发展可能会引发我们对爆炸性核合成的理解的重大转变。为此，新的光谱信息的关键密切相关的核是非常有价值的。目的：我们打算测量$^{75}$ Co的同分异构和$β$衰变，相对于$^{78}$ Ni具有一个质子和两个中子空穴，以获得$^{75}$ Co及其$β$ -衰变子体$^{75}$ Ni和$^{74}$ Ni的新的核结构信息。研究方法：核$^{75}$ Co是在RIKEN Nishina中心放射性离子束工厂的$^{238}$ U的相对论飞行裂变反应中产生的。利用BigRIPS和EURICA装置研究了其异构体和$β$衰变。结果如下：我们获得了$^{75}$ Ni和$^{74}$ Ni的部分$β$ -衰变谱，并报道了$^{75}$ Co中一个新的同分异构跃迁.延迟的$γ$射线的能量[ $E_γ=1914(2)$ keV]和半衰期[ $t_{1/2}=13(6)$ $μ$ s]支持在1914（2）keV处存在一个$J^π=(1/2^-)$同分异构态.与PFSDG-U壳模型计算的比较提供了一个很好的帐户中观察到的状态$^{75}$镍，但第一个计算的$1/2^-$水平$^{75}$ Co，一个长形$K=1/2$状态，预测约1兆电子伏以下观察到的$(1/2^-)$水平。结论：证明了$^{75}$ Ni的最低激发态为类球形结构。在$^{75}$ Co的情况下，结果表明，球形配置的优势，变形的可能比预期的$^{78}$镍以下。需要进一步的实验努力来辨别$J^π=(1/2^-)$异构体的性质。
英文摘要：
Background: The evolution of shell structure around doubly magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the `southwest' region of $^{78}$Ni, the development of deformation might trigger a major shift in our understanding of explosive nucleosynthesis. To this end, new spectroscopic information on key close-lying nuclei is very valuable. Purpose: We intend to measure the isomeric and $β$ decay of $^{75}$Co, with one-proton and two-neutron holes relative to $^{78}$Ni, to access new nuclear structure information in $^{75}$Co and its $β$-decay daughters $^{75}$Ni and $^{74}$Ni. Methods: The nucleus $^{75}$Co is produced in relativistic in-flight fission reactions of $^{238}$U at the Radioactive Ion Beam Factory in the RIKEN Nishina Center. Its isomeric and $β$ decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial $β$-decay spectra for $^{75}$Ni and $^{74}$Ni, and report a new isomeric transition in $^{75}$Co. The energy [$E_γ=1914(2)$ keV] and half-life [$t_{1/2}=13(6)$ $μ$s] of the delayed $γ$ ray lend support for the existence of a $J^π=(1/2^-)$ isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in $^{75}$Ni, but the first calculated $1/2^-$ level in $^{75}$Co, a prolate $K=1/2$ state, is predicted about 1 MeV below the observed $(1/2^-)$ level. Conclusions: The spherical-like structure of the lowest-lying excited states in $^{75}$Ni is proved. In the case of $^{75}$Co, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below $^{78}$Ni. Further experimental efforts to discern the nature of the $J^π=(1/2^-)$ isomer are necessary.
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