High-spin Yrast States Research Articles

High-spin intruder states in the mirror nuclei S31 and P31

Mirror energy differences (MED) between excited states in the A=31, T=1/2 mirror pair S31 and P31 have been measured up to high spin yrast states of positive and negative parity. The mirror nuclei were populated in the Mg24(C12, αn) and Mg24(C12, αp) reactions, respectively. For the first time the MED are described in the framework of the shell model in a valence space that includes three main shells. The theoretical MED values as a function of angular momentum indicate that the main contribution arises from the Coulomb interaction, however, the isospin symmetry breaking term is not at all negligible and needs to be considered to reproduce the experimental findings.Received 17 November 2020Revised 27 December 2020Accepted 1 June 2021DOI:https://doi.org/10.1103/PhysRevC.104.024309©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear spin & parityNucleon distributionShell modelProperties20 ≤ A ≤ 38Nuclear Physics

Characteristics of collectivity along the yrast line in even-even tungsten isotopes

The collective nature of high-spin yrast states in even-even $^{160\ensuremath{-}190}\mathrm{W}$ isotopes was systematically investigated by means of pairing self-consistent Woods-Saxon-Strutinsky calculations using the total Routhian surface approach in $({\ensuremath{\beta}}_{2},\ensuremath{\gamma},{\ensuremath{\beta}}_{4})$ deformation space. The calculated ground-state deformations are consistent with previous calculations and available experimental data. The deformation energy curves are presented to show the shape and softness evolutions, in particular in the triaxial direction. The backbending or upbending behavior in moment of inertia is attributed to band crossing. It is found that the neutron rotation alignment is preferred for most of the W isotopes (e.g., in $^{164\ensuremath{-}180}\mathrm{W}$), while in other nuclei the competition between the neutron and proton alignments may occur, even the proton alignment is favored in the very neutron-deficient nucleus $^{160}\mathrm{W}$. In addition, the evolution and transition between vibrational and rotational collective modes along the yrast line are investigated on the basis of the new centipedelike E-GOS (E-Gamma Over Spin) curves introduced by us, which to some extent explains the existing differences (e.g., in the moment of inertia) between theory and experiment.

Special features of isomeric ratios in nuclear reactions induced by various projectile particles

Calculations for (p, n) and (α, p3n) reactions were performed with the aid of the TALYS-1.4 code. Reactions in which the mass numbers of target and product nuclei were identical were examined in the range of A = 44–124. Excitation functions were obtained for product nuclei in ground and isomeric states, and isomeric ratios were calculated. The calculated data reflect well the dependence of the isomeric ratios on the projectile type. A comparison of the calculated and experimental data reveals, that, for some nuclei in a high-spin state, the calculated data fall greatly short of their experimental counterparts. These discrepancies may be due to the presence of high-spin yrast states and rotational bands in these nuclei. Calculations involving various level-density models included in the TALYS-1.4 code with allowance for the enhancement of collective effects do not remove the discrepancies in the majority of cases.

Electromagnetic properties of neutron-rich nuclei adjacent to the Z = 50 shell closure

Low-lying high-spin yrast states in the exotic odd–odd isotopes 124–128Sb (Z=51) and 118–128In (Z=49), studied for the first time, show a striking difference in their observed γ-ray decay. With a single valence proton particle/hole occupying the g7/2/g9/2 spin-orbit partners, dominant electric quadrupole transitions occur in Sb as opposed to magnetic dipole transitions in In. The observed properties are explained on the basis of general principles of symmetry and with large-scale shell-model calculations, and reveal novel aspects of the competition between the neutron–proton interaction and the like-nucleon pairing interaction.

Antimagnetic rotation in104Pd

The electric quadrupole transition rates for the high-spin yrast states of ${}^{104}$Pd have been measured by using the Doppler-shift attenuation method. These values decrease with the increase of angular momentum, which can be associated with the phenomenon of antimagnetic rotation. In the present work, a numerical calculation based on the semiclassical particle plus rotor model for antimagnetic rotation has been employed, giving a good description of the experimental Routhian and the transition rates and providing conclusive evidence of antimagnetic rotation in a nucleus other than cadmium.

TERMINATING STATES IN Z = 71Lu NUCLEI WITH N ≈ 90

The high-spin yrast states in 161 Lu are calculated in a Cranked Nilsson-Strutinsky approach keeping track of configurations and their evolution with spin. The outcome from calculations neglecting pairing and including pairing are compared. It is predicted that several yrast region configurations terminate at a very favored energy around I = 50 and that one of the observed bands is two transitions short of termination.

New sub-μs isomers in125,127,129Snand isomer systematics of124−130Sn

New sub-μs isomers have been observed in the neutron-rich Sn isotopes. 125,127,129Sn nuclei have been produced in a relativistic fission reaction of U238 on a Be9 target at 750 A·MeV and by the fragmentation of Xe136 at 600 A·MeV populating high-spin yrast states. In addition to the already known μs isomers, three new ones with sub-μs half-lives have been observed. These yrast isomers are the high-spin members of the ν(d3/2−1h11/2−2) and νh11/2−n, seniority v=3 multiplets leading to isomeric (23/2+) and (27/2−) states, respectively. Added to the already known 19/2+μs isomers in this region the current work completes the systematic information of neutron-hole excitations toward the filling of the last h11/2 orbital at N=82. The results are discussed in the framework of state-of-the-art shell-model calculations using realistic interactions.4 MoreReceived 27 March 2008DOI:https://doi.org/10.1103/PhysRevC.77.064313©2008 American Physical Society

Stable collective triaxial rotation in 77Kr

Based on the gamma-spectroscopic analysis of G.N. Sylvan et al., configuration-dependent cranked Nilsson-Strutinsky calculations have been performed. They describe the nucleus Kr-77 globally in good agreement with the experimental findings and the expectations derived from single-particle orbitals in a deformed potential. Within these calculations, the unambiguous assignment of a configuration with three protons and five neutrons in the g(9/2) shell to the high-spin Yrast states of negative parity indicates that the nucleus Kr-77 has stable triaxial deformation and rotates about the intermediate axis. This not only disproves the recent interpretation of an occupation of the d(5/2) orbital being responsible for the strong deformation in this band, but also represents one of the first examples in this mass region for such a rotation. (Less)

High-spin yrast structure of 43Sc

High-spin yrast states in 43Sc were investigated by using in-beam γ-ray technique with the 27Al(19F,p2n) reaction at 50 MeV. The positive-parity rotational band built on the 152-keV Jπ = 3/2+ state has been extended up to the terminating Jπ = (27/2+) state. Several fast transitions feeding to the oblate-deformed Jπ = 19/2− isomer have been also identified. the character of the observed levels and transition rates were discussed in comparison with the shell-model calculations.

New effective interaction forpf-shell nuclei and its implications for the stability of theN=Z=28closed core

The effective interaction GXPF1 for shell-model calculations in the full $pf$ shell is tested in detail from various viewpoints such as binding energies, electro-magnetic moments and transitions, and excitation spectra. The semi-magic structure is successfully described for $N$ or Z=28 nuclei, $^{53}$Mn, $^{54}$Fe, $^{55}$Co and $^{56,57,58,59}$Ni, suggesting the existence of significant core-excitations in low-lying non-yrast states as well as in high-spin yrast states. The results of $N=Z$ odd-odd nuclei, $^{54}$Co and $^{58}$Cu, also confirm the reliability of GXPF1 interaction in the isospin dependent properties. Studies of shape coexistence suggest an advantage of Monte Carlo Shell Model over conventional calculations in cases where full-space calculations still remain too large to be practical.

Lifetimes of High Spin Yrast States in Odd-Proton Nucleus131Pr

Lifetimes of the high spin yrast states in the odd-proton-nucleus 131Pr have been determined by analysing theDoppler broadened line shapes for the de-exciting γ-rays following the116Sn (19F,4n) 131Pr reaction at a bombarding energyof 95 MeV. The transition quadrupole moments extracted from themeasured lifetimes exhibit a considerable reduction near the rotationalfrequency of 0.42 MeV, at which the J(1) and J(2) moments ofinertia indicate a band crossing. The experimental result demonstratesoccurrence of the nuclear shape change induced by band crossingassociated with the alignment of a pair of h11/2 protons.

New high spin states and band termination in83Y and84Zr

The gamma decay of high spin yrast states in {sup 83}Y up to I{sup {pi}}=59/2{sup +} and 53/2{sup {minus}} have been observed using the reaction {sup 58}Ni({sup 29}Si,3p) at 110 MeV and the Gammasphere Early Implementation Array. The level scheme has been substantially extended due to the observations of several new transitions in all of the bands. A sequence of transitions feeding into the positive parity yrast band above I{sup {pi}}=47/2{sup +} seems to be consistent with a noncollective oblate structure expected at these high spins. A similar cascade is found in the data for {sup 84}Zr. A new forking of the favored negative parity band is found which may be due to neutron alignment polarizing the core to a different shape. This suggests that the {open_quotes}isomeric{close_quote}{close_quote} band in {sup 83}Y, for which one more connecting transition was found, is of a similar nature to other high-K bands found in this region. Lifetime measurements in the unfavored negative parity band are consistent with cranking calculations which predict a nearly oblate shape with a deformation parameter {beta}{sub 2}{approx}0.2. A qualitative analysis of line shapes at very high spins suggests the persistence of collectivity in the yrast sequence to the highest excitationsmore » seen. {copyright} {ital 1997} {ital The American Physical Society}« less

Lifetime measurements of the high spin yrast states in 128Ce

Lifetime measurements of the high spin states in the yrast band of 128Ce were performed using the Doppler shift attenuation method in connection with the reaction 100Ru(32S, 2p2n) 128Ce at incident energy of 141 MeV. In the vicinity of the band crossing, the previously reported anomalously high B(E2) value was not observed. The gross behavior of the normalized transition strength is approximate to symmetric rotor. It is confirmed that the trend of increase in collectivity with decreasing neutron number, which was indicated in the previous studies on 130, 132, 134Ce, continues in 128Ce. The value of quadrupole deformation parameter β2 deduced from the measured lifetimes is in good agreement with the theoretical calculation using a cranking model based on a non-axially deformed Woods-Saxon potential.

Gyromagnetic factors of the high-spin yrast states in some doubly even germanium and selenium isotopes.

The gyromagnetic factors associated with the yrast levels (with 2/sup +//le//sup /pi///le/2/sup +/) in some doubly even Ge and Se isotopes are discussed in a microscopic, parameter-free perspective in terms of the variational wave functions resulting from realistic effective interactions operating in the (2p/sub 3/2/,2p/sub 1/2/,1f/sub 5/2/,1g/sub 9/2/)/sup gr/,/nu/ configuration space. The estimates for the g factors of the yrast 2/sup +/ states are consistent with the available experimental results. The calculations reveal that the g factors for the high-spin states can provide important quantitative signatures vis-a-vis the occurrence of various structural changes in the anomalous yrast spectra in the germanium region.

Yrast feeding and multiplicity of gamma transitions in the reaction 124Sn(28Si, 5n) 147Gd at 139, 146 and 152 MeV

The population of high-spin yrast and near yrast states of 1347 Gd and the associated multiplicity of feeding transitions have been studied in the 124 Sn ( 28 Si, 5n) reaction at 139, 146 and 152 MeV. The yrast feeding is found to be independent of the initial angular momentum distribution and very weak for states of spin greater than 30 ħ. The average multiplicity of γ-transitions is found to increase with increasing beam energy. The experimental results are interpreted in terms of collective excitations within rotational bands that channel the γ- decay towards the known oblate single-particle yrast states below ~30 ħ.

Monopole and Quadrupole Giant Resonances in Rotating Triaxially Deformed Nuclei. II: A Microscopic Description of the Isoscalar and Isovector Modes

Microscopic calculations are performed for the isoscalar and isovector monopole and quadrupole giant resonances built on high-spin yrast states. The results of the calculation show that the shapes of the strength functions for both the isoscalar and isovector vibrations are determined mainly by the deformation parameters of the mean field. The effects of rotation on the isoscalar and isovector quadrupole modes are found to be quite different: The rotation-splittings of the former become significant at very high-spins, while those of the latter remain small. It is also demonstrated that the. transformation of the strength functions from the rotating frame into the laboratory frame is crucial to accurately evaluate the rotation­ splittings of the quadrupole modes.

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 ℏ in 154Dy and 36 ℏ 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 ℏ 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.

Rotation-induced shape transitions in Dy nuclei

Lifetimes of states with spins up to 30ħ have been measured in the nuclei 156Dy, 157Dy, and ll58Dy using the recoil-distance technique together with inverse reactions of the type Mg( 136 Xe, x n) . The applied method, which benefited from the high velocities of the fusion residues as well as from improvements of the recoil-distance technique, allowed us to determine lifetimes and feeding times down to 0.1 ps. Below the first backbending the resultant B(E2) values in the ground-state band of 156, 158Dy increase faster with increasing rotational frequency than expected for rigid rotors, reaching values similar to those observed for the well-deformed neutron-rich Dy isotopes. In contrast to this, the E2-transition probabilities between high-spin states are clearly retarded. The retardation gradually evolves from the rotation alignment of nucléons and indicates deformation changes most likely towards a triaxial shape. From the analysis of the side-feeding times of the high-spin yrast states it could be furthermore deduced that the E2 component of the preyrast γ-decay stems from transitions along highly collective bands.

High-spin yrast and non-yrast bands in 176Os, 178Os and 180Os

High-spin yrast and non-yrast states have been identified in 176Os, 178Os and 180Os using ( 16 O, xn) reactions, and γ-ray techniques. Band crossing anomalies are observed in each of the positive-parity yrast bands. The magnitude of these anomalies decreases with decreasing neutron number, an effect attributed to the change in the moment of inertia of the ground state rotational bands. A 23 ns isomer, predominantly K π = 7 −, is identified at 1930 keV in 180Os. The configuration of this isomer is discussed on the basis of the properties of its rotational band. Negative parity, odd and even spin, sideband sequences are observed in each isotope. Their relationship to rotation-aligned octupole and 2-quasiparticle bands is discussed from their excitation energies, band spacings, and decay properties. Detailed calculations for Coriolis mixed bands are carried out for the likely 2-quasiproton and 2-quasineutron configurations. An anomaly observed at spin 17 in the odd-spin negative-parity sequence in 180Os is attributed to a band crossing with a fourquasiparticle configuration.

Study of quasi-continuum gamma-ray deexcitation of 152–161Dy nuclei populated in α- and 12C-induced reactions

The influence of the reaction mechanism and of the nuclear structure at high angular momentum on the γ-ray deexcitation process is studied for Dy nuclei produced in a (40–80 MeV) and 12C (80–110 MeV) induced reactions on 154, 155, 158, 160Gd and 146, 150Nd targets, respectively. The quasi-continuum (q.c.) γ-ray spectra for selected exit channels are decomposed into collective and statistical parts for which the γ-ray energies, the multiplicities and partly the multi-polarities are determined. Large differences in the constitution of the q.c. γ-ray cascades and in the population strengths of high-spin yrast states in 152Dy and 156Dy are attributed to nuclear structure effects at high angular momentum. This constitution and the centroids of the γ-ray entry regions in the energy versus angular-momentum plane are also evaluated as functions of beam energy and projectile. The γ-ray multiplicities and both coordinates of the entry region centroids increase initially with bombarding energy and then level off or even decrease at the highest energies. The observed increase is reproduced by a statistical-decay model, but at the higher bombarding energies strong deviations occur which are discussed in view of the reaction mechanisms, including incomplete fusion and non-equilibrium neutron emission, particularly absorptive breakup.