Lifetime of the31−state and octupole collectivity inZr

Abstract

The half-life of the 1897.2-keV, 3l state in Zr has been measured as (67.8+4.3) ps using the recoildistance technique following inelastic excitation by 105-MeV S ions. This is equivalent to a lifetime which is about 40% longer than those reported from two recent centroid-shift measurements. Our lifetime implies B(E3)f =(0.180+0.018) e b', i.e. , (47. 1+4.7) W.u. , which is the most enhanced onephonon g.s. -+3& transition strength observed in nuclei. A recent quasiparticle random-phase approximation (QRPA) calculation reproduces our result. Serious doubts are raised about an earlier conclusion that the reduced octupole transition probabilities of "mirror" nuclei correspond to the harmonic low for small amplitude vibrations. The enhanced B(E1:3&~2&+)=(1.71+0.11) 10 ' e b deduced from the present work is consistent with appreciable octupole deformation. The measured B(E3)$ resolves problems in the interpretation of the isospin character of the E3 transition derived from inelastic scattering data.

Tensor part of the Skyrme energy density functional. II. Deformation properties of magic and semi-magic nuclei

“…The latter is not large enough to suppress pairing correlations, such that the spin-saturation is broken and the neutron spin-current density is non-negligible at spherical shape, in particular for the T26 parameterization predicting the smallest N = 40 gap. The main consequence is that for 68 Ni all contributions to the tensor energy at spherical shape are nonzero, unless suppressed by their coupling constant, see Fig. 15.…”

“…Both nuclei have, indeed, similar single-particle configurations. The 1f 7/2 − subshell is completely filled for protons in 68 Ni and for neutrons in 48 Ca, and both nuclei are spin-saturated for the other type of nucleons. However, the comparison of both nuclei in fact indicates large differences.…”

“…Indeed, the odd-parity pf shell is completely filled and the 1g 9/2 + orbital empty [49,50] and at least a 2p-2h excitation is needed to construct a positive-parity state. The interpretation of 68 Ni as a doubly-magic nucleus has also been questioned by shell model and QRPA calculations [51]. Let us mention, finally, that the first excited state of 68 Ni is a 0 + level with a small B(E0) value to the ground state [52], pointing to a possible shape coexistence with weak mixing.…”

“…However, while the Z = 28 proton shell closure is clearly visible in the mass systematics along the N = 40 isotonic chain [47], there is no pronounced discontinuity in the masses of Ni isotopes when crossing N = 40 [47,48], which hints at a more complex situation. An alternative explanation of the properties of the first 2 + 1 state in 68 Ni is based on the impossibility to construct the first 2 + state as a simple neutron 1p-1h excitation. Indeed, the odd-parity pf shell is completely filled and the 1g 9/2 + orbital empty [49,50] and at least a 2p-2h excitation is needed to construct a positive-parity state.…”

“…It is usually assumed from its spectrum that 68 Ni is doubly-magic [43,44,45]. The excitation energy of its first 2 + state is, indeed, large and the B(E2; 0 + gs → 2 + 1 ) value small [46].…”