Theoretical Constraints for Observation of Superdeformed Bands in the Mass-60 Region

Abstract

The lightest superdeformed nuclei of the mass-60 region are described using the projected shell model. In contrast to the heaviest superdeformed nuclei where a coherent motion of nucleons often dominates the physics, it is found that alignment of g 9͞2 proton and neutron pairs determines the high spin behavior for superdeformed rotational bands in this mass region. It is predicted that, due to the systematics of shell fillings along the even-even Zn isotopic chain, observation of a regular superdeformed yrast band sequence will be unlikely for certain nuclei in this mass region. PACS numbers: 21.10.Re, 21.60.Cs, 23.20.Lv, 27.50. + e The mass-190 nuclei are the heaviest nuclei known where long-sequence rotational bands associated with the superdeformed (SD) minimum have been observed [1]. In a recent systematic study using the projected shell model (PSM) [2], it was concluded that the role of highj intruder orbitals is suppressed in these nuclei because of strong correlations in the quadrupole field and nonnegligible correlations in the pair field [3]. This conclusion was reinforced by the demonstration that quasiparticle additivity generally does not hold [4]. Superdeformation in the mass-60 region was predicted some years ago [5] and was recently observed [6][7][8]. This is the lightest known region of SD rotational bands and these new bands show very different character from those of the mass-190 nuclei.The mass-60 SD bands are associated with the highest rotational frequencies ͑hv ഠ 1.8 MeV͒ observed so far in SD nuclear systems; in contrast, in the SD mass-190 nuclei the maximum rotational frequency is typically 0.4 MeV. However, the magnitudes of deformation and pairing appear to be comparable in the mass-60 and mass-190 regions. We may expect that the single-particle level density near the N 30 gap is much lower than for heavier nuclei. Thus, there can be substantial fluctuations in shell fillings along an isotopic chain, which could give rise to drastic changes in the single particle and collective behavior. In addition, the maximum spin within the yrast and near-yrast bands in SD mass-60 nuclei is generally much lower than in heavier nuclei (SD bands terminate earlier [9]). These new features lead us to expect complex behavior in this region relative to previously studied SD nuclei.Thus far, the SD bands of the mass-60 nuclei have been explained using mean-field theories (cranked relativistic mean-field theory and cranked Nilsson model [9], or cranked Skyrme-Hartree-Fock method [7]), with complete neglect of pairing correlations. These descriptions reproduce many of the gross features found in these nuclei. However, some interesting questions have not been discussed. For example, why does the observed SD band in 62 Zn [6] consist of only a few g rays, while popula-tion of the SD band in neighboring 60 Zn [8] extends to low spin states? And why has one not seen a SD yrast band at all in 64 Zn [10]? Can one predict spin values for these bands? Can one give a microscopic justification for the complete neg...

Single-particle and collective motion for proton-rich nuclei in the upperpfshell

“…The strength of the quadrupolequadrupole force in the Hamiltonian is determined selfconsistently, and the monopole pairing strength is the same as that in Ref. [8]. For the four nuclei calculated in this paper, the ratio of quadrupole pairing to monopole pairing strength is fixed at 0.30.…”

“…This kind of shell model truncation is highly efficient if the singleparticle (SP) basis is realistic because the basis already contains many correlations [6]. It has been shown that the PSM can describe the spectra and electromagnetic transitions in normally deformed [6], superdeformed [7,8], and transitional nuclei [9]. One can further calculate the nuclear matrix elements for astrophysical processes such as direct capture and decay rates.…”

“…This is the same size SP basis as employed in Ref. [8]. The deformation parameters are taken from Ref.…”

T=0neutron-proton pairing correlations in the superdeformed rotational bands around60Zn

“…The superdeformed (SD) bands in the A≃60 nuclei have already been studied theoretically within various approaches [13][14][15][16][17][18][19][20][21]. In the present paper we use two methods: (i) the cranked Hartree-Fock (HF) method, solved by using the HFODD (v1.75r) computer code [22], with the Skyrme SLy4 [23] effective interaction and no pairing (see Ref.…”

Particle stability of highly and superdeformed states of Ni, Cu, and Zn isotopes nearβstability in relativistic mean-field theory

“…Since the first observation of the SD band in 62 Zn [1] in the mass region Aϳ60, strongly deformed bands have been observed in a number of nuclei: 60,61,64,65,68 Zn [4][5][6][7][8], 58,59 Cu [2,9,10], 56,59 Ni [3,11], etc. It was pointed out that the SD properties in Zn isotopes is isospin-sensitive [5,12]. The highly deformed or SD secondary minima at higher spins in nuclei of Zϳ30 region is due to the particle-hole excitations from the 1f 7/2 orbital below the Z͑N͒=28 shell gap to the 1g 9/2 orbitals above the gap.…”