L. Funke

In-beam study of 80Kr; Quasiparticle excitations in nuclei around mass 80

Abstract The excited states in 80 Kr have been studied in the reactions 77 Se(α, n), 78 Se(α, 2n), 80 Se(α, 4n) and 65 Cu( 18 O, p2n) by using in-beam γ-ray spectroscopy. In addition to γγ-coincidences, excitation functions and angular distributions, linear polarization of γ-rays and conversion electrons were also measured. All together, 32 levels have been identified up to spin 14 at an excitation energy of 6.7 MeV in 80 Kr. For 21 of these levels the mean lifetime could be determined by Doppler shift methods and by the pulsed-beam γ-timing method. The B (E2) values of 30–60 W.u., derived for many transitions, indicate strong collectivity and the existence of several band structures is suggested. Above 2.5 MeV 2-quasiparticle (qp) excitations become important. The excitation energies of 80 Kr and its neighbours 77, 78, 79 Kr, 77 Br and 81 Rb have been analysed in terms of the cranked shell model. In 78,80 Kr two-proton excitations have been found to be responsible for the observed band crossing. Quasiparticle excitations strongly influence the pairing and stabilize the deformation. The anomalies in the negative-parity bands of 81 Rb and 77 Br are interpreted as a crossing of a 3qp and a 1qp band and the relatively low frequency of the crossing point is ascribed to the blocking effect.

Two-quasiparticle and rotation-aligned structures in 190Pt, 192Pt and 194Pt

Abstract The γ-decay of high-spin levels in 190, 192, 194 Pt is observed when Os targets are bombarded with α-particles with energies between 23 and 51 MeV. Energies, intensities, angular distributions, lifetimes of isomeric levels, as well as coincidence relations and yield of conversion electrons are measured. In this way, levels up to spin 18, 20 and 12 are established in 190 Pt, 192 Pt and 194 Pt, respectively. In 190, 192 Pt the separation of the 10 + and 12 + members of the ground band is very small and the B (E2) value of the 12 + to 10 + transition in 192 Pt is determined to be 50 s.p.u. These observations are interpreted as a band crossing phenomenon within the rotation-alignment model. Explicit calculations within such a model give a fair reproduction of the experimental energies in the ground band if the nucleus has oblate deformation and if alignment of i 13 2 neutrons is assumed. In both nucleides additional high-spin levels of both even and odd spins and of positive parity are observed to come close in energy to the 10 + and 12 + members of the ground band. Such additional levels are also predicted by the calculations. In 194 Pt the 10 + to 8 + energy separation is remarkably small and the corresponding B (E2) value is believed to be smaller than the single-particle estimate. This discontinuity may be associated with a change to a two-proton-quasiparticle excitation. In addition to the ground band, a negative-parity “band” with spins ranging from 5 to 13 is strongly populated in all three isotopes. A qualitative explanation of this negative-parity “band” in terms of a generalized particle-rotor model is discussed. Finally, a 10 − isomer and a band structure built on this isomer are observed in 190 Pt and 192 Pt.

Shape change and fast M1 transitions in 81Kr

Abstract A shape transition from a probably asymmetric shape at low excitation to a more axial-symmetric shape above 21 2 + has been found in 81 Kr. This shape change and the drastic increase of the M1 transition probabilities above spin 21 2 are attributed to the alignment of two g 9 2 protons.

Rotation alignment in 190Pt and 192Pt

Abstract Levels with spins up to 18 (20) in the ground-state bands of 190Pt and 192Pt have been observed. The very small energy spacing between the 12+ and 10+ levels in both nuclides is interpreted as a sudden decoupling of two i 13 2 neutrons from the collective core.

On the interaction of the s-band with the ground state and gamma band in nuclei around mass 80

Abstract A comparison of experimental data on high-spin states in 80Kr, 82Sr, 81Kr and 81Rb with cranked shell model calculations reveals first evidence for the predicted oscillations of the interaction strength between the ground state band and the g 9 2 - proton s-band. Similar oscillations of the interaction strength between the gamma and s-band are suggested.

Evidence for rotational band structures in the N = 88 nuclide 153Tb

Excited states in 15365Tb88 have been studied in the reactions 151Eu(α, 2n) and153Eu(α,4n) using in-beam γ-ray spectroscopy. Levels of positive parity are identified up to spin 272. Most of these states are interpreted as members of rotational bands characterized by the Nilsson configurations 32+[411], 52+[402] and 72+[404]. Furthermore, a sequence of negative-parity states is observed up to spin 312 (352). The negative-parity band is remarkably well described in a Coriolis coupling calculation including all configurations of the h112 Nilsson multiplet. In this calculation the moment of inertia parameter varies with collective angular momentum in the same way as in the ground-state band of 152Gd. Using the same prescription for calculating rotational energies, the irregular level spacings within the 52+[402] and 72+[404] bands are also well explained.

Evidence for shape coexistence from few-quasiparticle excitations in 83Kr

Excited states in 83Kr were studied in the 82Se(α, 3n) reaction using in-beam γ-ray spectroscopy. Measurements of γγ coincidences, excitation functions, angular distributions and linear polarization of the γ-rays were carried out and levels were established up to I = 292. Most of the levels above 2.5 MeV could be grouped into 3 band-like sequences. Lifetimes or limits of lifetimes were determined for 8 members of these bands using the DSA method. The positive-parity 3qp band, I = 212 to 292, indicates a prolate deformation similar to the corresponding band in 81Kr. In contrast, only low collectivity was found for a ΔI = 2 sequence built on the 172− yrast state and reaching up to I = (292). From similarities of the level spacings with the ground state bands and 2qp bands of the even-mass neighbours, the 3qp configuration ν(g92)−2νp12 is proposed for this sequence. Another band, comprising levels with Iπ = 132− to (272−) in a ΔI = 1 sequence, displays a sharp decrease of the intraband M1 transition rates with increasing spin.

In-beam study of high-spin few-particle states and the decay of an 112 isomer in 151Tb

Abstract Levels in 151Tb have been populated in (α, 4n) and (τ, 3n) reactions on an 151Eu target and the decay of these levels has been studied in-beam. Excitation functions, angular distributions and γγ coincidences are measured using Ge(Li) spectrometers. Also, conversion electrons and delayed γ-rays have been recorded. The measurements have revealed a 25 s isomeric 11 2 − level decaying via levels with spin and parity 5 2 + and 3 2 + to the 1 2 + ground state. The isomer and a 15 2 + state are fed through two similar level sequences, which have been established up to levels with spin and parity Equal to 31 2 − ( 35 2 − and 35 2 + ( 39 2 + ) , respectively. Another level sequence ranges from 33 2 (+) to ( 45 2 + ) . The levels with spin below 11 2 are interpreted as shell-model excitations of the odd proton. The high-spin states are ascribed to excitations of the five valence nucleons outside the magic 64146Gd82 core. In particular, a reasonable interpretation of the high-spin level sequences is obtained if the three-particle clusters (π h 11 2 vf 7 2 vh 9 2 ) 27 2 ) − and (π h 11 2 vh 9 2 vi 13 2 ) 33 2 + are assumed topersist as stable components within the sequences. The stability of these clusters is explained as a result of the maximization of the overlap of nucleonic wave functions that is obtained by alignment of angular momentum (MONA).

On the irregularities in the band structures of 78Kr

Using in-beam gamma -ray spectroscopy techniques experimental information on the presence of two-quasiparticle components in the band structures of 78Kr has been derived. The observation of a second 10+ state decaying by a fast M1 transition of B(M1, 102+ to 101+)=0.8(4) Wu to the 10+ yrast level gives strong evidence for a band crossing between the collective ground-state band and a two-quasiparticle band. The residual interaction between these configurations has been estimated to be V(10+)=130+or-10 keV. The configuration mixing also provides an explanation for the decrease of the B(E2) values in the yrast sequence above the 8+ level.

High-spin two-quasiparticle bands in76Br

Excited states of the doubly-odd nucleus V6Br have been studied with in-beam γ-ray spectroscopy techniques. In addition to the positive parity band that has been extended up to I=(13) a Iπ=4− isomer (T1/2=0.5±0.2 ns) and two bands of negative parity have been identified. The bands are discussed in terms of two-quasiparticle configurations. For the band built on the 4− isomer the configuration πg9/2⊗νf5/2 or p3/2 is proposed.