R.M. Lieder

Study of band crossings in 182Os

High-spin states in 182Os have been populated by the (18O, 4n) reaction and studied using in-beam spectroscopy methods. Seven side bands have been established for the first time. Three of the side bands show a band crossing. The features of the bands have been interpreted in the framework of the cranking model considering the motion of independent particles in a rotating deformed potential. The band crossings of the yrast band and of these three side bands can be explained by the rotation-alignment of a pair of i132 quasineutrons.

Study of high-spin states in 181, 182Os

Abstract High-spin states in the nuclei 181, 182Os have been populated in the 150 Nd( 36 S ,x n ) reactions and studied with the ESSA30 array. The nucleus 181Os has also been studied at the NBI tandem accelerator using the 167Er(18O,4n) reaction. The previously known bands in both nuclei have been extended to higher spins and two new side bands have been found in 181Os. In the latter nucleus the ground state has been established to have I π = 1 2 − . The extraction of the ratios of reduced transition probabilities B(M1) B(E2) from branching and E2 M1 mixing ratios permitted configuration assignments for most of the bands in both nuclei. The analysis has been carried out within the semiclassical vector model for M1 radiation. The positive-parity yrare sequences in 182Os and the band based on the I π = K π = 23 2 − state in 181Os have been interpreted as t-bands arising from a rotation about a tilted axis. The alignment behaviour and the crossing frequencies are for most of the bands consistent with predictions of the cranked shell model.

THE DISTRIBUTION OF THE ROTATIONAL TRANSITION STRENGTH IN WARM NUCLEI STUDIED THROUGH GAMMA-RAY CORRELATIONS

Abstract The study of damping of rotational motion applying te rotational plane mapping (RPM) method is presented and discussed. The aim of this technique is to extract the distribution of the rotational transition strength from an analysis of the shape of the “central valley” of two- and three-dimensional γ-ray spectra. The method is applied to a triple γ-coincidence data set of 162,163Tm nuclei formed in 37Cl+130Te reactions. The rotational transition strength is obtained as a function of rotational frequency for selected regions of entry states, and the width is found to be rather constant and approximately equal to 80 keV. This value is significantly smaller than the value predicted theoretically for the rotational damping width Γrot. Also the ratio between the observed depth and width of both the 2D and 3D valleys does not agree with the simple model adopted in the RPM method. These discrepancies point to the presence of both a wide and a narrow component in the distribution of rotational strength as extracted by the RPM method. The analysis of simulated spectra obtained on the basis of realistic band-mixing calculations, including residual interactions, confirms this behaviour.

Study of band structures in the γ-unstable nucleus 186Pt

Abstract High-spin states in the transitional nucleus 186Pt have been populated via the 188Os(α, 6n) reaction and studied with the OSIRIS spectrometer. The yrast band and the γ-band have been extended to Iπ = (26+) and 10+, respectively, and nine new side bands have been observed. The features of the γ-band suggest that the nucleus is extremely γ-soft. A coexistence of prolate, oblate and triaxial band structures has been established. The crossing observed in the bands built on the prolate nuclear shape is considered to result from the ( πh 9 2 ) 2 alignment. For oblate shapes a ( v i 13 2 ) 2 alignment has been found.

A finite number of regular rotational bands in the superdeformed well of 143Eu

Abstract The number of excited superdeformed bands in 143 Eu is measured by use of the Fluctuation Analysis Method. Between 10 and 40 rotational bands, displaying typical rotational energy correlations over two consecutive transitions, are populated within a rather narrow range in transition energy, E γ ≈ 1300–1500 keV. These numbers are close to the values found for normally deformed nuclei and agree with microscopic cranking + band mixing calculations for the specific superdeformed nucleus, which predict the onset of rotational damping to occur at the excitation energy U 0 = 1.3–1.6 MeV above the yrast line.

Proton excitations across the Z = 64 gap in the doubly magic superdeformed nucleus 144Gd

Five superdeformed (SD) bands have been identified in 144Gd with the GASP array using the reaction 100Mo + 48Ti. The J(2) moments of inertia of the four excited SD bands are flat as a function of rotational frequency, indicating the blocking of the N = 6 proton crossing observed in the yrast SD band. According to cranked shell model calculations, the excited bands have two-quasiproton configurations with one proton in the N = 6 orbital and the other in each signature partner of the [404]92+ or [411]32+ orbitals, lying above the Z = 64 gap. The interpretation of the excited SD bands in terms of proton excitations points to the high stability of the N = 80 shell closure at SD shapes. The population of the yrast SD band as a function of the beam energy has been also investigated. Our data shows that the intensity of the yrast SD band is not configuration dependent, being comparable to that of yrast SD bands in the neighboring nuclei. By comparing experimental and theoretical alignments, spins and configurations are assigned to all observed SD bands in the A = 140–150 region.

Interaction strength and shape difference for the h9/2 and h11/2 configurations in163Tm

The strongly shape driving πh9/2[541]l/2− configuration with α=+1/2 exhibits some anomalous, and so far unexplained, features concerning the crossing frequency, ħωc, the aligned angular momentum, ix, and interaction strength, at the alignment of the first pair of i13/2 quasineutrons in several odd-Z rare earth-nuclei. The h9/2[541]1/2− and h11/2[523]7/2− bands have been studied in the stably deformed rare-earth nucleus163Tm to investigate these features. A difference in band crossing frequency of ∼ 80 keV between the two bands is found. Rotational bands built on these two configurations have been found to cross in the spin range I=25/2–29/2 ħ. Theγ-decay pattern between the two bands is established in the crossing region and analysed in terms of a moderate shape difference between them. A theoretical estimate of the size of the interaction strength between the two bands is presented and compared to the experimental value. The observed band structure in163Tm is very similar to that of167Lu which has 2 protons and 2 neutrons in addition. This observation is discussed in relation to the similarity of the yrast bands of the two even-even “core” nuclei162Er and166Yb, for which theγ-transition energies are identical within ∼0.2 keV below the vi13/2 crossing.

Origin of the backbending in theπh9/2 band of185Ir

Highspin states of185Ir were excited in the187Re(α,6n) reaction and measured with the OSIRIS spectrometer. The first backbending in theπhx9/2 band was found at a rotational frequency of ħω=0.40 MeV. Blocking arguments suggest that this is due to the alignment of an h9/2 quasiproton pair.

Search for superdeformation in180Os

Two-dimensional γ-ray energy correlations have been measured at high spins in1800s with the TESSA2 spectrometer. The Eγ − Eγcorrelation matrix displays ridges characteristic of rotational bands with different energy separations of 96± and 72± keV for the energy regions of 660 to 840 keV and 975 to 1170 keV, respectively. The separation of these ridges remains constant in both energy regions. The dynamical moments of inertia are θ(2) =83±3 and 111±5 ħ2/MeV, respectively. The latter value suggests superdeformation in1800s with an axis ratio c/a=1.76 (β2 ≅0.64) at high spins.