Interpretation of observed superdeformed and other collective bands in 152Dy

The shell effects responsible for superdeformation in the ¹⁵²Dy region are discussed. Why is only one band observed in ¹⁵²Dy and why is there another coexistent collective band? How does the superdeformed band decay?     

Damping of the giant dipole resonance in hot,strongly rotating nuclei

The isovector dipole density-density response of hot rotating nuclei is calculated applying a cranked deformed Nilsson potential together with a separable dipole-dipole residual interaction. The transformation of the response function from the internal rotating coordinate frame to the laboratory frame is discussed and illustrated by classical results for a charged particle moving in a harmonic-oscillator potential. Calculations for ¹⁰⁸Sn, ¹⁵²Dy and ¹⁹⁶Pb are presented. For ¹⁰⁸Sn at high excitation energy thermal fluctuations of the shape gives rise to a rather structureless strength function with a considerable width. For ¹⁵²Dy and ¹⁹⁶Pb superdeformed minima of the potential surface are predicted. The coupling of the giant dipole resonance to the shape degrees of freedom of superdeformed nuclei can split the vibration by ≈ 10 MeV, the lowest peak being expected at an excitation energy of ≈ 7–8 MeV and carrying ≈ 30% of the energy-weighted sum rule.     

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.     

Shell effects on rotational damping in superdeformed nuclei

Damping of collective rotational motion in A ∼ 190 and A ∼ 150 superdeformed nuclei is studied by means of the cranked shell model with two-body residual force. Numerical calculations predict that in a typical A ∼ 190 superdeformed nucleus, ¹⁹²Hg, the rotational damping width is significantly small, Γ_rot ∼ 30 keV, and that the number of superdeformed bands in the offyrast region amounts up to 150 at a given rotational frequency. These features are quite different from the prediction for A ∼ 150 superdeformed nuclei and rare-earth normally deformed nuclei. It is shown that the single-particle alignments of the cranked Nilsson orbits have strong shell oscillation. It affects significantly the properties of rotational damping in superdeformed ¹⁹²Hg.     

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.     

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.     

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.     

Properties of unbound high-spin states in27Al

Selectively excited high-spin states in²⁷Al have been located by the reaction¹²C(¹⁶O,p)²⁷Al. The excitation functions of these states have been measured at incident energiesE _cm=18.7 to 30.1 MeV at intervals of 150 keV. They exhibit maxima of presumably non-statistical origin. At four different energies the subsequent decay of the²⁷Al states has been studied by detecting the final heavy recoils with the Munich recoil spectrometer in coincidence with the proton emitted in the first stage of the reaction. Using this new method branching ratios of theγ-, α-, andn-decay have been measured. Based on angular correlation arguments spins up to 27/2? have been determined within the experimental accuracy of 1–2?. The observations suggest a superdeformed shape of²⁷Al at least in some of the states.     

转动谱的若干理论及其对超形变带的研究

简述了若干转动谱理论及其对超形变带的应用 ,并用 Bohr- Mottelson的 I(I+1 )展开公式分析了 A=1 90区超形变带的性质和指定了它们的能级自旋 ,用 Harris的ω2 展开公式 J(1) =2α+(4/3 )βω2 +(6 /5 )γω4分析了 A=1 5 0区 Tb和 Dy同位素 2 0条超形变带的性质 ,指定了它们的能级自旋 .对于首次发现的152 Dy(1 )超形变带 ,带首自旋指定为 2 6 h,与实验结果更加符合. The recent developments of rotational spectral theories and its application to superdeformed bands were briefly reviewed. The superdeformed bands in A ≈190 region were analyzed and the spins of energy level were determined by the least square fitting experimental transition energy with the formula of Bohr Mottelson’s I(I+1) expansions. The superdeformed bands in A ≈150 region were analyzed by using the kinematic moments of inertia formula J (1) =2α+(4/3)βω 2+(6/5)γω 4 in...     

Discrete levels in 154Dy above spin 30ħ: Evidence for terminating band structures

High-spin states in ¹⁵⁴Dy have been studied using the TESSA2 γ-rays spectrometer following the ¹¹⁰Pd(⁴⁸Ca,4n)¹⁵⁴Dy reaction at a beam energy of 210 MeV. States up to 44⁺ and 37⁻ have been observed. Below spin 30 the data display regular rotational behaviour which can be interpreted in terms of the cranked shell model. Above spin 30, sequences of levels connected by stretched E2 transitions, which show large gains in energy when compared to a rotating liquid drop reference, are lowest in energy for both parities. Particularly low energy levels are observed for spin I^π = 36⁺ and 42⁺ and in addition dipole transitions are found connecting negative-parity states around spin I = 35. The experimental data for I ≳ 30 are compared with calculations, based on the Nilsson-Strutinsky cranking formalism, in which it is possible to trace fixed configurations through a sequence of spins. For the high-spin positive-parity sequence, the similarity with the ¹⁵⁶Er spectrum is discussed.     

Nuclear level densities and level spacing distributions from 20F to 244Am

The parameters of nuclear level density formulae have been determined from extensive and complete level schemes and neutron resonance densities for 24 nuclei between ²⁰F and ²⁴⁴Am. The constant temperature level-density formula and the Bethe formula are compared with the experimental level densities. Both formulae reproduce experimental densities equally well. The same nuclei have been used to obtain an A-dependent spin cut-off parameter for low-lying levels. The spacing distribution of levels with equal spins and parities at lower excitation energies is found to be much closer to an exponential distribution than to the Wigner distribution especially for even-even nuclei. This is at variance with previous theoretical expectations and interpretations of nuclear data compilations. It gives evidence for a further good quantum number at low excitation energies in addition to spin and parity or for very different structures.     

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.     

Behavior of shell effects with the excitation energy in atomic nuclei

We study the behavior of shell effects, like pairing correlations and shape deformations, with the excitation energy in atomic nuclei. The analysis is carried out with the finite temperature Hartree-Fock-Bogoliubov method and a finite range density dependent force. For the first time, properties associated with the octupole and hexadecupole deformation and with the superdeformation as a function of the excitation energy are studied. Calculations for the well quadrupole deformed 164Er and 162Dy, superdeformed 152Dy, octupole deformed 224Ra, and spherical 118Sn nuclei are shown. We find, in particular, the level density of superdeformed states to be 4 orders of magnitude smaller than for normal deformed ones.     

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.     

Cascade γ decay of the 191Os compound state

The intensities of two-step cascades to the final levels with excitation energies below 0.82 MeV have been determined from the accumulated experimental data on the γ-γ coincidences at thermal neutron capture into ¹⁹⁰Os. These intensities made it possible to establish the decay scheme for the compound nucleus to excitation energies of about 3 MeV. The intervals of the level densities and sums of radiative strength functions of the E1 and M1 transitions, which exactly reproduce the experimental cascade intensities, have been found from the total cascade intensities. The level density is approximated by the sum of the partial densities of levels for different numbers n of quasiparticles, with the coefficient of collective increase in the density, unambiguously determined by the accepted concepts about the energy dependence of the correlation functions of the nucleons of an excited nucleus.     

Description of high-spin isomers in theN=82 region

The high spin states and of those especially the yrast isomers are studied in the region near the rare earth nuclei. The deformation energy surface is calculated with a method modelled in principle according to a shape constrained cranked HF theory (CHF). In practice, the expectation value of the many-body Hamiltonian is calculated with cranked Nilsson functions. One finds rotation around a slightly deformed oblate or prolate symmetry axis in front or behind the rare earth region, respectively. Near the Hf isotopes strongly prolate deformed nuclei rotating around the symmetry axis are found in agreement with the knownK isomers in this region. These results are explained also qualitatively with the help of the MONA (Maximisation of theOverlap ofNucleonic wave functions byAlignment of single particle angular momenta) effect. In the second part high spin states and yrast isomers in theN=82 region are caluclated in the cranked Hartree-Fock-Bogoliubov approach with four and six quasi-particle excitations. For the excitation energies in¹⁴⁶Gd and¹⁵²Dy and a measuredg factor one finds satisfactory agreement.     

Description of high-spin isomers in theN=82 region

The high spin states and of those especially the yrast isomers are studied in the region near the rare earth nuclei. The deformation energy surface is calculated with a method modelled in principle according to a shape constrained cranked HF theory (CHF). In practice, the expectation value of the many-body Hamiltonian is calculated with cranked Nilsson functions. One finds rotation around a slightly deformed oblate or prolate symmetry axis in front or behind the rare earth region, respectively. Near the Hf isotopes strongly prolate deformed nuclei rotating around the symmetry axis are found in agreement with the knownK isomers in this region. These results are explained also qualitatively with the help of the MONA (Maximisation of theOverlap ofNucleonic wave functions byAlignment of single particle angular momenta) effect. In the second part high spin states and yrast isomers in theN=82 region are calculated in the cranked Hartree-Fock-Bogoliubov approach with four and six quasi-particle excitations. For the excitation energies in¹⁴⁶Gd and¹⁵²Dy and a measuredg factor one finds satisfactory agreement.     

Continuous gamma radiation feeding high-spin isomers in 148, 149, 151, 152Dy and 147Gd

The energy, multipolarity and multiplicity spectra of the continuum gamma radiation feeding high-spin isomers in ^{148, 149, 151, 152}Dy and ¹⁴⁷Gd have been measured at several bombarding energies. The final nuclei were selected via a delayed coincidence technique while the use of a NaI(Tl) Compton shielded crystal allowed the primary gamma-ray spectra to be generated reliably from the observed spectra.It was found that: (i) the energy of both the gamma-ray statistical cascade and the neutron cascade increases with increasing excitation energy, the latter much more rapidly than expected; (ii) the multiplicity of gamma-ray cascades, in which a statistical transition was detected, is generally lower than that of the average cascade; (iii) stretched E2 spin-correlated transitions occur above $\text{J? 39}\text{h}\text{?}\text{in}{}^{152}\text{Dy}$ and above $\text{～ 50.5}\text{h}\text{?}\text{in}{}^{147}\text{Gd}$, indicating the onset of collectivity at these spins — in addition, a region of predominantly dipole emitting states is located below $\text{T? 44}\text{h}\text{?}\text{in}{}^{147}\text{Gd}$; (iv) effective moments of inertia derived from the bump edge are 50–100% larger than those deduced from the density of stretched E2 transitions in the bump; (v) in ^149,151Dy the bump edge is very sharp but no multiplicity features are observed; (vi) although the four Dy isotopes were populated at approximately the same excitation energy, they display considerable differences in their continuum properties.Probable interpretations of these observations are discussed, in particular we have suggested that several of the observed effects are consistent with the possible presence of high-K collective bands above the yrast line.     

High spin states in the nucleus150Tb

Excited states in the nucleus¹⁵⁰Tb have been investigated up to spin 39? and 13 MeV excitation energy using the¹³⁰Te(²⁷Al, 7n) reaction and the EUROGAM array. The theoretical interpretation of the observed states has been performed in the framework of the deformed independent particle model. The analysis of the decay out of the yrast superdeformed band indicates that normal-deformed states with spins between 24? and 28? are fed.     

Lifetimes and lineshapes in superdeformed bands

The effective lifetimes and the intensity of the discrete transitions of the superdeformed band in¹⁵²Dy are described using a statistical model based on a Monte Carlo simulation. The effect of the transition strength in the continuum and the energy dependence of the barrier are investigated. The importance of the mixing of superdeformed and normal states for the resulting feeding times and intensities is emphasized. The calculations are compared to experimental data, and it is shown that the discrete line shapes are sensitive to the transition strength in the continuum.