J. Döring

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.

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.

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.

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.

Evidence for deformed states in 75Br

Abstract The excited states in 75 Br have been studied via the reactions 74 Se(p, γ), 74 Se(d, n), 74 Se( 3 He, pn) and 74 Se(α, p2n) by using in-beam γ-ray spectroscopy. In addition to measurements of γ-γ coincidences, excitation functions and angular distributions of γ-rays, ns lifetime measurements have also been carried out. As a result 19 levels have been identified up to spin ( 17 2 ) and excitation energies up to 2.6 MeV. The B (E2) value of 88 W.u. derived for the 88.4 keV γ-ray indicates strong collectivity within a positive-parity band. A comparison of the excitation energies of the unique-parity states in 75 Br and 77 Br with those in 153 Tb and 155 Tb reveals that the average deformation increases when going from 77 Br ( N = 42) to 75 Br ( N = 40).

Yrast spectroscopy of the N=48 nucleus 84Kr

Abstract More than 20 new levels of 84 Kr populated via the 82 Se(α, 2nγ) reaction have been established up to spin J =14ℏ at an excitation energy of E x = 7.65 MeV by in-beam γ-ray and conversion-electron spectroscopy. For 31 excited states the mean lifetimes have been measured by applying Doppler-shift and pulsed-beam γ-ray timing methods. The dominant structure of the J π = 12 + isomer at 5373 keV (T 1 2 = 44 ± 2 ns ) has been inferred from the comparison of the experimental g -factor of g (12 + ) = +0.17 ± 0.02 with estimates for different 4qp configurations to be the stretched 4qp configuration π( f 5 2 −1 , p 3 2 −1 )ν( g 9 2 −2 ) . This interpretation of the 12 + state has been confirmed by shell-model calculations with 88 Sr as the core. Some enhancement of the E2 strength observed for the 10 2 + → 8 2 + transition might point to the alignment of two g 9 2 protons. Most of the negative-parity states could be grouped into two band-like sequences, built on top of the two lowest-lying 5 − states, with ΔJ = 2 and ΔJ = 1, respectively, and B (E2) values of about 10 W.u. Some arguments allow the two-neutron hole configuration ν( g 9 2 −1 , p 1 2 −1 ) to be ascribed to the 5 1 − state and the ΔJ = 2 level sequence, whereas for the 5 2 − state and the ΔJ = 1 level pattern a two-proton configuration π( f 5 2 −1 or p 3 2 −1 , g 9 2 ) might be predominant. The highest-lying levels of 84 Kr observed up to spin 14ℏ at an excitation energy of 7.65 MeV were found to have odd parity and to be connected by fast M1 transitions.

Transition probabilities in the band crossing region of 79Kr

A change of the M1 transition probabilities is observed in the band crossing region in the positive-parity yrast band of 79Kr. This change is compared with that observed in the neighbouring nucleus 81Kr and is interpreted on the basis of a semiclassical coupling scheme of particle angular momenta.

Evidence for a four-quasiparticle isomer in 84Kr

Abstract A 12+ isomer has been found in 84Kr at 5373 keV with a half-life of T 1 2 =45(5) ns . For the new isomer the aligned 4qp configuration v( g 9 2 ) 8 + −2 π ( p 3 2 , f 5 2 ) 4 + is strongly suggested on the ba of the measured g-factor of g=+0.14(3) and from the sum of the corresponding 2qp energies.

Collective and two-particle excitations in78Se

Excited states in78Se have been studied up to spin (12)ℏ at about 5.8 MeV in the76Ge(α, 2n) reaction using in-beamγ-ray spectroscopy. Mean lifetimes could be determined for 27 of the 33 levels observed by applying Doppler shift and pulsed-beam timing methods. According to theB(E2) values most of the levels have been grouped into collective bands. Irregularities in the level spacings of the yrast band above spin 6ℏ are interpreted to be due to the interaction of the ground state band withg9/2 two-proton and two-neutron excitations. The mutual mixing of these configurations is reflected by strongM 1 transitions between the mixed states. The interaction strengths between the configurations involved have been estimated from three-band mixing calculations.