Shape evolution in self-conjugate nuclei, and the transitional nucleusSe68

Abstract

We report on the first measurement of the absolute transition strength B(E2; 0 + 1 → 2 + 1 ) in the self-conjugate nucleus 68 Se. It is found that the 0 + 1 → 2 + 1 transition displays a strength similar to that for the triaxial 64 Ge nucleus, in sharp contrast to the much stronger collectivity observed for the oblate 72 Kr nucleus. Shape evolution along the N = Z line from zinc to strontium is analyzed through beyond-mean-field calculations using the D1S force. Over this narrow mass region the nuclear structure shows clear-cut transition from prolate to oblate and back to prolate, with 68 Se standing as pivotal between triaxial and oblate deformations.Finite quantum systems may exhibit sudden shape transitions by changing their number of constituants, as observed in semiconductor atomic clusters [1]. In atomic nuclei, the ground state generally has a nonspherical symmetry and the deformation evolves with the number of nucleons due to both single-particle properties and quantum mechanical correlations. Steep shape transitions are expected in regions where several energy minima located at different deformations compete to stabilize the nucleus. This competition is enhanced in self-conjugate nuclei with ∼70 nucleons, in which neutrons and protons experience the same large deformed singleparticle shell gaps at both elongated (prolate) and flattened (oblate) quadrupole deformation. The sign of the spectroscopic quadrupole moment of the first 2 + state is hardly reachable in these neutron-deficient nuclei. It is experimentally known only in very few cases, such as 74,76 36 Kr [2] and 70 34 Se [3]. The B(E2; 0 + 1 → 2 + 1 ) transition strength from the ground state (gs) to the first 2 + excited state, quoted B(E2↑) in the following, thus remains a unique and reachable information to evaluate collectivity at low excitation energy. The low-lying spectroscopy of the N = Z nucleus 64 32 Ge, four nucleons away from 68 Se, shows strong similarities with predictions for a triaxial nucleus. Recently, its B(E2↑) transition strength has been measured and was found consistent with shell-model predictions for a triaxial shape [4]. In the case of the selenium isotopes, it has been concluded from recent low-energy Coulomb-excitation and 2 + 1 -lifetime measurements, that 70 Se is oblate in its gs [3]. The moment of inertia extracted from the ground-state band of 68 Se is similar to that of 70 Se and suggests a collective oblate deformation [5], but no direct information on the collectivity of 68 Se has been measured yet. In the krypton isotopes, a shape transition from a prolate-deformed gs in 76 Kr to an oblate shape in 72 Kr has been established [2,6]. The N = Z nucleus 72 Kr is understood as having an almost pure oblate ground state with very low mixing with the intrinsic prolate configuration. The absolute transition strength B(E2↑) of 72 Kr shows consistency with predictions of an oblate ground state [7]. However, triaxiality has been suggested to be a key degree of freedom to understand the shape transition in light krypton is...

Simultaneous investigation of the T=1(Jπ=0+) and T=0(J

“…It has been shown that the ground states of the even-even 74−82 Se are prolate-deformed [21,22]. However, for the lighter 68,70,72 Se isotopes, there is evidence for oblatedeformed ground states [23][24][25], but this has been called into question by other studies [26,27]. Theoretically, a number of microscopic approaches support an oblate ground-state shape that evolves rapidly into a prolate collective rotation for spin J 6 [24,[28][29][30][31][32][33].…”

Collectivity atN=50:Ge82andGade1, Baugher2, Bazin3 et al. 2010Phys. Rev. CSelf Cite619810View full textAdd to dashboardCite

“…The need for fairly high effective charges of e p ≈ 2 illustrates the importance of core excitations across the N = 50 shell gap which are outside of the employed model space. The experimental data stems from the present work and references [6,7,17,33,35,36]. …”

“…Their properties are driven by shape coexistence and rapid shape changes all the way from the N = Z line into the A ≈ 70 mass region [1][2][3][4][5][6][7][8][9][10][11][12][13]. On the other side of the nuclear chart, the most neutron-rich selenium and germanium isotopes accessible for experiments are around the magic neutron number N = 50.…”

Triaxial angular momentum projection and configuration mixing calculations with the Gogny force

“…This restriction is one of the major drawbacks of the method because it limits its applicability to systems where triaxiality does not play an important role. However, many exciting experimental and theoretical phenomena are closely related to the triaxial degree of freedom, for instance: the presence of γ-bands in the low lying energy spectra and γ-softness, shape coexistence and shape transitions in transitional regions [13,[15][16][17][18][19][20]; the lowering of fission barriers along the triaxial path [21][22][23]; the influence of triaxial deformation in the ground state for the mass models [24,25]; triaxiality at high spin [26][27][28]; the observation of K-bands and isomeric states in the Os region [29][30][31]; or some other exotic excitation modes such as wobbling motion and chiral bands [32][33][34].…”