Crossing of negative parity bands in even-even nuclei around A?150

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.

Frequency and Alignment Classification of Multiple Band Crossings

“…7 The resulting powerseries expansions are analyzed using techniques developed for the study of singularities in critical phenomena. 8 The initial conditions in real space are 5 ' 7 * 9 v 1 (x l ,x 2f x 3 ; t = 0) ^cos^sin^cosATg,…”

“…This gain in i probably results from the crossing of a if =0 octupole band with a two-quasiparticle band of larger i 9 similar to that observed at / = 11" in the i ST =88 nuclei . 8 A remarkable fact is that these three sidebands experience a band crossing around ^C2 = 0.36 MeV, between the frequencies of the first (0.27 MeV) and second (0.42 MeV) yrast backbends. Furthermore, the yrast i 13 / 2 band in 161 Yb shows a crossing at a very similar frequency; in this case, an estimate of the alignment gain in the crossing (Az« 6.5/z) can be extracted.…”

Boson description of nuclear collective motion

“…According to the argument by de Boer et al [12], irregular behaviors of A2E(I) suggest that levels above 11-in X~176 and aS~ belong to the intersecting two-quasiparticle band. In order to treat such a situation it is necessary to extend the boson expansion technique to the case of coupled system of quadrupole phonon and a selected twoquasiparticle state.…”

“…As for the levels other than the yrast bands, calculation gives E(0~-) agreeing with experiment but E(2 +) and E(3/) at much higher positions than the latter. According to de Boer et al [12], the members with spins higher than 11-should be regarded to belong to the crossing two-quasiparticle band. We can reproduce rather well the GSB up to 6 +, and the positions of the 2+, 3[, 42 states belonging to the quasi-y-band.…”

“…It is to be noted that A2E(I) of both the GSB and NPB takes plus or minus signs alternatively with I, and so the argument concerning the band structure due to de Boer et al [12] is difficult to apply. …”