Speaker
Description
Shell evolution [1,2] is one of the most discussed topics in the last two-decades in nuclear structure physics. It reveals that the traditional shell structure known for stable nuclei changes when we go towards the neutron-drip line. Recently, the low-energy states of neutron-rich $^{55}$Sc has been populated in an experiment [3], in which the first excited state $(3/2)^-$ has been found near 0.5 MeV. Since the structure of $^{55}$Sc includes one proton above the closed Z = 20 core, the low-excitation energy of this state points to the disappearance of the traditionally large proton $1p_{3/2}$-$0f_{7/2}$ energy gap in the neutron-rich region. In order to into look this point in detail, we have performed theoretical calculations within the shell-model framework [4]. We have found out that the proton $1p_{3/2}$-$0f_{7/2}$ energy gap reduces at $^{55}$Sc but the $(3/2)^-$ state does not gain its excitation energy from this gap. In fact, this state mainly originates from the transition of a neutron across the N = 34 semi-magic shell gap. Thus, its low-excitation energy indicates the weakening of this gap above Ca.
References:
[1] T. Otsuka et al., Phys. Rev. Lett. 95, 232502 (2005).
[2] T. Otsuka et al., Rev. Mod. Phys. 92, 015002 (2020).
[3] D. Steppenbeck et al., Phys. Rev. C 96, 064310 (2017).
[4] P. Kumar et al., Phys. Rev. C 100, 024328 (2019).
