Single particle nature of the yrast line at high angular momentum in 152Dy

Linear polarization and Doppler shift recoil-distance decay curves for γ-ray transitions deexciting very high spin yrast states up to spin 36? in ¹⁵²Dy were measured with a delayed-promt γ-γ coincidence technique via the reactions ¹²²Sn (³⁴S, 4n) and ¹²⁴Sn(³²S, 4n), at 150 MeV incident beam energy. The deduced experimental transition probabilities are consistent with those obtained for non-rotational nuclei indicating that the yrast angular momentum originates mainly from the alignment of individual particle spins.     

Yrast isomers and very high spin states in 148, 149, 151, 152Dy and 147Gd

We have studied level schemes for the feeding and decay of high-spin isomers in the nuclei ^{148, 149, 151, 152}Dy and ¹⁴⁷ Gd. A variety of techniques involving γ-ray spectroscopy with pulsed heavy-ion beams have been applied including linear polarization, γ-γ correlations and recoil distance measurements.The general aspect of these level schemes is in accordance with the idea that the states near and along the yrast line can be described by a spheroidal shell model in which the nucleus is axially symmetric with respect to the total spin. In these cases the spin is generated not by collective rotation, but by alignment of many single-particle orbits around a common axis.     

New results in the decay of150Ho and152Tm

A new isotope¹⁵²Tm (T _1/2=5.2±0.6 s) and a new isomeric state in¹⁵⁰Ho (T _1/2=88±15 s) were observed with the on-line isotope separator ISOCELE II at Orsay. The decay of¹⁵²Tm is similar to the one of the high spin state (9⁺) in¹⁵⁰Ho. The new¹⁵⁰Ho isomer, probably with a low spin (2 or 3), decays mainly to the 2⁺ level in¹⁵⁰Dy.     

In-beam γ-ray studies of complex band structures and of isomeric states in 152, 153Dy nuclei

The high-spin level structures of ¹⁵²Dy and ¹⁵³Dy were studied experimentally with ^{154, 155}Gd(α xnγ) in-beam reactions, and for ¹⁵²Dy also with ${}^{\mathrm{144,\; 146}}\text{Nd}\text{(}{}^{12}\text{C, x}\text{n}\text{γ)}$ reactions. The experiments included measurements of singles γ-ray and conversion-electron spectra, γ-ray angular distributions and E_γ-t and E_γ-E_γ-t coincidences. A multiplicity filter set-up was used to study the feeding and decay of isomeric states in ¹⁵²Dy. In ¹⁵²Dy about twenty so far unknown levels were found, including two high-spin isomeric states with $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 60}\text{and}\text{≈ 13}\text{ns}$ at excitation energies E_x ≈ 5.04 and 6.08 MeV, respectively. These states are compared with recent calculations on yrast traps. The level scheme of ¹⁵³Dy contains 28 levels up to E_x = 4.1 MeV and $\text{J}{}^{\mathrm{\pi }}\text{= (}\text{37}\text{2}{}^{\mathrm{+}}\text{)}$. Band structures in both nuclei are discussed in comparison with other N = 86 and N = 87 isotones.     

Yrast structure of 62 139 Sm77:γ-softness of the 62 140 Sm78 core

A general survey of the octupole instability of high spin superdeformed (2∶1) and hyperdeformed (3∶1) nuclei has been performed in the cranked Nilsson-Strutinsky model. The octupole degree of freedom is predicted to play an important role in several already observed superdeformed nuclei around¹⁵²Dy, and in particular in N=114 isotones in the Hg-Pb-region. High spin reflection asymmetric hyperdeformed nuclei are predicted.     

Feeding times of high-spin states in 152, 154Dy: Probes of nuclear structure above the yrast line

Measurements of feeding times of high-spin yrast states up to spin 30 ? in ¹⁵⁴Dy and 36 ? in ¹⁵² Dy were utilized to obtain information about possible spin-dependent shape changes. The reactions ²⁵Mg(¹³⁴Xe, 5n), ¹²⁴Sn(³⁴S, 4n) and ²⁵Mg(¹³²Xe, 5n), ¹²²Sn(³⁴S, 4n) were used to populate the high-spin states in ¹⁵⁴Dy and ¹⁵²Dy, respectively. Feeding times as well as lifetimes were determined with the recoil-distance technique. In ¹⁵²Dy only long feeding times (? 10 ps) could be identified, indicating that the aligned-particle yrast states are fed through configurations of similar character, with little direct population from collective cascades in the continuum region. In ¹⁵⁴Dy discrete states with I ? 30 ? have lifetimes which are characteristically collective, whereas the preyrast cascades exhibit both fast (?1 ps) and slow (～10 ps) feeding components. The latter imply a change with increasing spin from collective to aligned-particle character, probably associated with a prolate to oblate shape transition.     

Angular momentum dependence of giant dipole vibrations in hot dysprosium nuclei

The spin dependence of the giant dipole resonance built on excited states in ¹⁵⁶Dy has been studied with a new technique using Ge-BGO spectrometers. The observed structure of the GDR and the measured γ-ray anisotropies are consistent with a change in nuclear shape from prolate to oblate with increasing spin. No evidence is found for centroid shifts nor for superdeformed shapes at high temperature.     

γ-Rays draining the superdeformed band in152Dy

Further analysis of earlier¹⁵²Gd(α,4n) in-beam data revealed several previously unplaced¹⁵²Dy γ-rays feeding the oblate yrast states, among them γ-transitions emitted in decay of the superdeformed band. We can also weakly detect the lowest two members of the superdeformed cascade, but our data were not quite good enough to make a firm connection to them.     

Beta-delayed gammas and QEC-values in the decays of150m,152mHo and149Dy

^{150m, 152m}Ho, and¹⁴⁹Dy were produced by fusion-evaporation reactions of⁴⁰Ar with targets of natural indium and¹¹⁴Cd. Theβ-decays of the isotopes were reinvestigated with emphasis on high-excitation daughter states. The decay energies for selectedβ-transitions were measured from electron-capture to positron-decay ratios. Corrections forγ-feeding from higher-lying states were taken into account. The following Q_EC-values were derived for^150mHo,^152mHo, and^149mDy of 6819 ( _?100 ⁺¹¹⁷ , 6263 ( _?80 ⁺⁸⁹ ), and 3914 ( _?52 ⁺⁷⁶ ) keV, respectively.     

Unambiguous identification and properties of high spin isomers in nuclei close to N = 82

The search for and study of high spin isomers produced by ¹²C, ¹⁴N and ¹⁶O induced reactions on targets close to N = 82 has been performed. The observed isomers were identified unambiguously by a detailed study of the excitation functions, delayed gamma-ray multiplicity and multiparametric gamma-gamma coincidences. The isomers with the highest spin above the ground state were observed in ¹⁵²Er and ¹⁵¹Ho nuclei.     

γ-Ray emission from oriented nuclei in a multiaxis nuclear spin system: 166mHo165Ho

A new method is proposed to measure multiaxis nuclear spin structures using γ-ray emission from oriented radioactive nuclei. This method, which depends only on angular momentum theory, is presented for helical spin structures. The atomic magnetic structure can also be obtained when the nuclear magnetism is produced by hyperfine interaction. Measurements on ^166mHo in a single crystal of ¹⁶⁵Ho metal show that the spin axes form a single cone with a half-angle of 80.4±0.4°.     

The structure of Yrast isomers and the nature of γ-rays from a compound nucleus

The unusually large population of yrast isomers in ¹⁵²Dy is theoretically analysed. We also discuss the relation between the structure of the yrast states and the large ?=1 component of γ-rays emitted from a compound nucleus.     

Level Schemes in Neutron-Rich 141Xe、152Ce、153Nd and 160Sm Nuclei

The level schemes of neutron-rich nuclei,¹⁴¹Xe、¹⁵²Ce、¹⁵³Nd and ¹⁶⁰Sm have been established for the first time through measuring the prompt γ-rays from spontaneous fission of ²⁵²Cf.The highest spin state is up to 16⁺.The collective band structures in these neutron-rich nuclei are discussed.     

Extreme nuclear deformations studied at the GASP spectrometer

Very exotic shapes can be assumed for the nuclei at high angular momenta. Their study has become possible with the advent of the last generation high-resolution γ-ray spectrometers. In this talk results are presented on superdeformed nuclei of theA=140 mass region where a detailed spectroscopy in the second minimum is now possible. Hyperdeformed rotational bands, expected in the third minimum, lie at even higher spins where their observation is very difficult from the experimental point of view. We have up to now only weak signals of their presence. The status of the search for hyperdeformation in¹⁵²Dy is reported.     

Gammaspektrum nach Neutroneneinfang in Eu152(12,4a) und Niveauschema von Eu153

Using a source of Eu ₂ ¹⁵¹ O₃ the (n, γ) spectrum of Eu¹⁵² and Eu¹⁵³ was investigated by means of the Risø bent crystal spectrometer. A total of 57 transitions were fitted into a level scheme of Eu¹⁵³. The rotational bands of the Nilsson states 5/2⁺ [413], 5/2^? [532], and 3/2⁺ [411] were found up to the spin values 15/2, 13/2, and 11/2 respectively. The 13/2 member of the 3/2⁺ [411] band is suggested at 716.15 keV. The interpretation of these levels is confirmed by the comparison of the experimental branching ratios with theoretical predictions and by considerations concerning the spin dependent population of these levels.     

Single-particle states at very high spin in the light dysprosium isotopes

High-spin states in ^150–152Dy have been investigated. Gamma-gamma coincidences in all three isotopes have been studied with Compton-suppressed Ge(Li) detectors. For ¹⁵¹Dy, lifetimes of the high-spin states have been measured by the recoil-distance technique. The spin and parity assignments, transition rates, and level systematics support the interpretation of the observed states as proton particle-hole excitations coupled to aligned valence neutron configurations. Shell-model calculations, based on effective interactions and a ¹⁴⁶Gd core, quantitatively reproduce the observed level patterns, and the predicted transition rates qualitatively agree with the observed values.     

A 150 ms 10+ Isomer in146Dy

In the course of systematic studies of very proton rich nuclei in theN≈82 region with Ni induced compound reactions, an unusually long-lived high spin isomer has been found. After in-and off-beamγ and conversion electron measurements, asγ excitation functions,γ andeγ coincidences, pulsed beam techniques and multi spectrum analyses of the residual activities, this isomer has been tentatively assigned to be a 10⁺ state in¹⁴⁶Dy, which decays into two 7^? states byE3 transitions. The half-life of the isomer has been measured to be 150±20ms. The isomer has been found to follow the β decay of the previously unknown 3.9 s isotope¹⁴⁶Ho. The mechanism of the appearance of such an isomer is discussed.     

Multipolarity of the continuum γ-rays in 152Dy and 160Er via linear polarisation measurements

The multipolarity of the continuum transitions in ¹⁵²Dy and ¹⁶⁰Er has been deduced from γ-ray linear polarisation and angular distribution measurements. The yrast part for ¹⁶⁰Er agrees with the predicted stretched E2 cascade. The statistical parts for both ¹⁵²Dy and ¹⁶⁰Er consist of a mixture of dipolar stretched and non-stretched transitions, predominantly of electrical character. Below 1.5 MeV, the data of ¹⁵²Dy exhibit a different structure, and reveal a stretched M1 component in the low-energy region.     

Level structure of the 89-neutron nucleus 153Gd: (I). Levels and gamma-transitions in 153Gd excited in the decay of 153Tb

Investigations of the properties of ¹⁵³Gd excited states populated in ¹⁵³Tb decay were continued. The following measurements were performed: coincidence spectra e^?γ with L41, K93 + L52, K110, K129 + L87 + L88 and K195 + L152 keV conversion electron lines, angular correlations of high energy γ-cascades going through the 109.7 keV level, delayed e^?γ and e^?e^? coincidence spectra to determine the half-lives of 41.5, 93.3, 109.7, 129.1,183.5, 212.0 and 216.1 keV states, R(135°, ± B) parameters of IPAC for the 102–110 and 83–129 keV cascades using ¹⁵³Tb sources implanted into Fe foil.A decay scheme of ¹⁵³Tb containing 50 excited levels is proposed. Their spins, parities, $\text{log}\text{?t}$ and, for low-lying levels, also the mean half-lives have been determined. An estimation of the g-factors of the 109.7 and 129.1 keV levels has been given. On the basis of half-lives of investigated states absolute values of reduced γ-transition probabilities for these states have been calculated. The structure of the ground state of ¹⁵³Gd is discussed.