Superdeformed Band in Asymmetric N $>$ Z Nucleus, $^{40}$Ar and High-Spin States in A $=$ 30 $\sim$ 40 Nuclei

Abstract

A rotational band with five cascade γ-ray transitions was newly found in 40 Ar. The deduced transition quadrupole moment of 1.45 +0.49 −0.31 eb has demonstrated this band as having a superdeformed shape of β 2 ∼ 0.5. The structure of the band was discussed in the framework of cranked Hartree-Fock-Bogoliubov calculations and the assignment of multiparticlemultihole configuration has been made. §1. IntroductionAfter the systematic investigation of superdeformation in various mass regions, 1) a new 'island' of superdeformed (SD) nuclei was found in the nuclear chart around A ∼ 40 (i.e., 36 Ar, 2) 40 Ca, 3) and 44 Ti 4) ). These N = Z nuclei are magic and near magic systems whose ground states have a spherical shape. In order to produce the collective degrees of freedom necessary for the formation of SD states in these nuclei, cross-shell excitations involving both the sd and pf shells are necessary. SD shell structure in this mass region plays an important role to form such large deformed structures. Woods-Saxon single-particle diagram (see Fig. 4 of Ref. 3)) shows a SD gap at β 2 ∼ 0.6 in N = Z = 20, which is associated with the SD band of 40 Ca 3) with 8 particle 8 hole (8p-8h) configuration built on the third 0 + state. Another SD gap at β 2 ∼ 0.5 at N = Z = 18 is associated with the observed SD band in 36 Ar 2) which is built on the second 0 + state with 4p-4h configuration. The existence of many deformed gaps in this region may result in the SD structures at the excited 0 + states and rotational bands built on such states are expected at high-spin levels. If such simple picture generally holds in this mass region, another SD structure in asymmetric N > Z nucleus, 40 Ar, associated with the Z = 18 and N = 22 SD shell gaps will be present.The presence of the SD band starting from the exited 0 + level is another remarkable feature in this mass region. SD bands in the other mass region were found in much higher-spin regime ( 20 ) and the observed lowest level of the band is supposed to lie ∼ 3 MeV above the yrast line. In most cases, linking transitions