J.K. Johansson

Gamma-ray spectroscopy of 126Ba

Abstract States of 126Ba up to spin 36+ were populated in the reaction 96Zr(34S, 4n)126Ba at 155 MeV and up to spin 20+ in the reaction 116Sn(13C, 3n)126Ba at 56 MeV. Gamma-ray spectroscopy was performed with the 8π spectrometer, an instrument comprising 20 Campton-suppressed HPGe detectors and 71 BGO ball elements. A level scheme organized into fifteen rotational bands is proposed on the basis on the γγ-coincidence and γ-ray angular correlation data. The competing π h 11 2 and v h 11 2 band crossings are investigated and interpreted in terms of cranked shell model and total routhian surface calculations. Amongst the topics discussed are (i) a comparison of DCO and spin orientation techniques for determining spins and multipolarities. (ii) the addivity property of quasiparticle energies or routhians, and (iii) analysis of B(M1)/B(E2, ratios between signature partner bands. The possibility of couplings between vibrational and aligned quasiparticle structures is explored.

Multiple band structure and band termination in 157Ho towards complete high-spin spectroscopy

Abstract Rotational bands of 157 Ho have been populated via the 124 Sn( 37 Cl, 4n) reaction at beam energies of 155 and 165 MeV. Gamma-ray spectroscopy was performed using the 8 π spectrometer at Chalk River. Many rotational bands have been observed for the first time. A detailed level scheme is presented, containing approximately 380 transitions, and the quasiparticle structure of the various bands is discussed. Band termination has been observed in the yrast states. For strongly coupled bands, B (M1)/ B (E2) transition strength ratios are extracted and compared with previous measurements and theoretical expectations. Branching ratios for out-of-band E2 transitions are analysed to extract band mixing interaction strengths. Implications for rotational damping are considered. The interaction at the first backbend in the ground band is found to be strongly signature dependent; this is evidence for a signature-dependent triaxial shape of the nucleus.

Alignment processes and shape variations in 184Pt

Abstract High-spin states in the transitional nucleus 184Pt were populated via the reactions 154Sm(34S, 4n)184Pt and 172Yb(16O, 4n)184Pt. The yrast band was extended up to I = 28 ħ and six new side bands built on both neutron and proton quasiparticle configurations were observed. Shell correction-type calculations indicate variations of the nuclear shape in different bands, especially as a result of band crossings due to the process of angular momentum alignment. Comparison of the band characteristics are made between 184Pt and eight adjacent nuclei. The pattern of band crossings in these nine nuclei is considered from the viewpoint of blocking comparisons and of theoretical calculations. The competition between low-frequency vi 13 2 and πh 9 2 band crossings is discussed.

Rotational bands and shape changes in 124Ba

Abstract High-spin states in 124 Ba were populated via the 94 Zr( 34 S, 4n) 124 Ba and 110 Cd( 16 O, 2n) 124 Ba reactions at 145 MeV and 60 MeV, respectively. The yrast band has been extended to 34 ħ and seven side bands have been observed. Gamma-ray angular distributions with respect to the nuclear spin axis have been used along with directional correlation (DCO) ratios to establish spins and multipolarities. The competing π h 11 2 and ν h 11 2 crossings in this region are investigated and an interpretation of the observed crossings is proposed within the framework of the cranked shell model (CSM) and total routhian surface (TRS) calculations.

Gamma-ray spectroscopy of 127Ba at moderate spins

Abstract The near-yrast spectroscopy of 127Ba has been examined in detail up to spin 39 2 with the reaction 117Sn(13C, 3n) at 60 MeV incident energy. Gamma-ray spectroscopy was performed with the 8 π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. Evidence is presented for the identification of single-neutron quasiparticle bands corresponding to Nilsson orbitals [532] 5 2 , [523] 7 2 , [411] 1 2 and [402] 5 2 . The properties of the [523] 7 2 band suggest that the nucleus has a triaxial shape when that orbital is occupied. We draw attention to: (i) the signature splitting; (ii) staggering of B(E2) ratios; (iii) the mean Q 1 Q 2 ratio; and (iv) the comparison of the staggering in the B( M 1) B( E 2) ratios with the signature splitting. All these factors point to a γ-deformation in the neighbourhood of γ∼−20°. Based on the large B( M 1) B( E 2) ratios, the observed three-quasiparticle bands involve proton excitation. Of these bands, two have clear-cut structural assignments ν h 11 2 π( h 11 2 ) 2 and ν d 5 2 π( h 11 2 ) 2 . However, the third band presented difficulties and no assignment was consistent with all the data. The shift in crossing frequency for the π( h 11 2 ) 2 with a ν h 11 2 spectator is very large and corresponds to a shift in the routhian of about 220 keV. This we interpret to be due to a negative γ-deformation for bands involving ν h 11 2 . The opposite shift of ∼300 keV for the same crossing with a ν d 5 2 spectator cannot be explained as a shape polarization effect.

Additional superdeformed states in the continuum of 149Gd

Abstract The analysis of γ -ray energy-energy correlations measured in the 124 Sn + 30 Si reaction at 150 MeV beam energy reveals that the known discrete line superdeformed band accounts for only (19–48)% of all possible superdeformed states populated in 149 Gd in the γ -ray energy range of 1263–1433 keV. The ridge structure was examined for possible additional discrete line bands.

Discrete-Line Spectroscopy in Superdeformed Nuclei

The discrete-line spectroscopy of collective structures in nuclei having superdeformed shapes has progressed rapidly over the past three years, and we are now in a position to test the present nuclear models in a systematic manner. This talk will focus on developments from the 8π Collaboration, including the discovery of multiple superdeformed bands within the same nucleus and the use of high-resolution two-and three-dimensional γ-ray correlation techniques to extract band structures having very low intensity.