No abstract

No abstract

No abstract

Level energies of negative parity yrast bands (NPB) have been studied with a recently proposed, sensitive method. The irregular behaviour of the NPB's in the N=88 nuclei around spin 9-11 and the smooth behaviour in 156Dy (up to I=13) and in Z38U (up to I = 19) support calculations by Vogel, where NPB's are described as aligned octupole bands up to a critical spin where intersection with two-quasiparticle bands takes place. Considering the NPB levels (I< 13) in 156Dy as members of an aligned octupole band, we obtain a remarkably good reproduction of their decay properties and energies using the VMI model. Considerable interest exists in the structure of negative parity bands (NPB) in doubly even nuclei. These bands have been observed in the deformed nuclei 238U [1] and 16ZEr [2,3] [-11] obtain a set of negative parity levels by coupling quadrupole and octupole bosons of which the lowest sequence is an aligned one starting at U= 3-. The spacing is equal to that of the ground state band apart from an extra term proportional to the (quadrupole) boson quantum number ne. In the deformed region Neerg~rd and Vogel [12] obtain a similar set of negative parity states, now grouped into well defined octupole vibrational bands with K =0-, 1-, 2-and 3-, though these bands are strongly perturbed by Coriolis interaction. If the strength of this interaction reaches its spherical limit and the unperturbed band head energies are degenerate, the lowest octupole band, starting at 3-, becomes an aligned one. An alternative approach to negative parity bands has been applied in the cases of l~[5], 126. tZSBa [6] and 156Er [10], where the NPB's are interpreted as rotational bands built upon two-quasiparticle states. The NPB levels in t52Gd were reproduced by Zolnowski et al. [7] and by Nomura [8], who coupled octupole phonons to the transitional core. Recently, Vogel [13] showed that in deformed nuclei the "yrast" NPB should consist of two branches: an aligned octupole band up to a critical spin value (Iorit) at which band crossing takes place with an aligned two-quasiparticle (2-q.p.) band. The transition to the aligned 2-q.p. situation occurs, according to Vogel, when the octupole correlation energy amounts (according to Vogel R ~ I-Jl), the critcal spins become I = 25-27 and I = 9-11 respectively. Negative parity bands in N = 88 nuclei are known up to high spin values. We have investigated these bands with a new sensitive method [14], by plotting the second differentials of the level energies as a function of I. In this way band irregularities are very clearly demonstrated, as is shown in Figure 1.

No abstract