Gamma Vibrational Bands and Chiral Doublet Bands in A≈100 Neutron-rich Nuclei

The level structures of neutron-rich ~(105)Mo,~(106)Mo,~(108)Mo and 110Ru nuclei in A≈100 region have been carefully investigated by coincidence measurements of the prompt γ-rays populated in the spontaneous fission of ~(252)Cf with the Gammasphere detector array.In 105Mo,one-phonon K =9/2 and two-phonon K=13/2 γ-vibrational bands have been identified.In ~(108)Mo,one-phonon γ-vibrational band is expanded and two-phonon γ-vibrational band has been identified.Two similar sets of bands in ~(106)Mo and ~(110)Ru are observed to high spins,which have been proposed as the soft chiral γ-vibrational bands.The characteristics for these γ-vibrational bands and chiral doublet bands have been discussed.     

Identification of Rotational Band in Doubly Odd 170Ta

High spin states in ¹⁷⁰Ta have been studied via ¹⁵⁹Tb(¹⁶O,5nγ)¹⁷⁰Ta reaction through excitation functions, K X-γ and γ-γ-t coincidence measurements. Three rotational bands have been identified among which one coupled band and an unfavored δI=2 E2 transition sequence are newly found in this work. The possible quasiparticle configurations of these bands are discussed. Received: 7 November 1997     

Observation of superdeformed states in 88Mo

High-spin states in ⁸⁸Mo were studied using the Gammasphere germanium detector array in conjunction with the Microball CsI(Tl) charged-particle detector system. Three γ-ray cascades with dynamic moments of inertia showing similar characteristics to superdeformed rotational bands observed in the neighbouring A= 80 region have been identified and assigned to the nucleus ⁸⁸Mo. The quadrupole moment of the strongest band, deduced by the Residual Doppler Shift Method, corresponds to a quadrupole deformation of β₂≈ 0.6. This confirms the superdeformed nature of this band. The experimental data are interpreted in the framework of total routhian surface calculations. All three bands are assigned to two-quasi-particle proton configurations at superdeformed shape. Received: 20 May 1999 / Revised version: 25 August 1999     

Search for decoupled bands in Odd-Odd 174Re

An effort has been made to search for the decoupled bands in odd-odd ¹⁷⁴Re via the ¹⁵⁹Tb(²⁰Ne,5n γ)¹⁷⁴Re reaction through excitation functions, K x-γ and γ-γ coincidence measurements. The doubly decoupled band and the semidecoupled one have been found in this work. Their quasiparticle configurations are proposed to be π1/2⁻[541] ⊗ν1/2⁻[521] and π1/2⁻[541] ⊗νi _13/2, respectively, according to the characteristics of band structures in this mass region. Another band with strongly coupled character is also reported. Received: 23 November 1999 / Revised version: 5 January 2000     

Count fluctuation analysis in the triaxial strongly deformed potential well of 163Lu

High-spin properties of the triaxial strongly deformed potential well in ¹⁶³Lu at excitation energies above resolvable bands have been investigated. Gated E _γ-E _γ spectra display clear ridges with moments of inertia corresponding to those observed for the discrete strongly deformed bands. A fluctuation analysis of the ridges yields a number of two-step paths of ≈ 40 and ≈ 20, when gating on triaxial strongly or normally deformed bands, respectively. These results show that a potential well at large deformation coexists with the normally deformed well, and indicate a mixing between states in the two wells at higher excitation energy. Received: 4 January 2002 / Accepted: 6 May 2002     

Observation of intermediate bands feeding the positive-parity yrast band in 155Gd

High-spin states of ¹⁵⁵Gd were populated by using the ¹⁵⁴Sm(α,3nγ)¹⁵⁵Gd reaction at E _α= 33 MeV. γ-γ coincidence, E _γ singles, excitation function, and the DCO ratios were measured. we have identified three intermediate bands with ΔI= 2 feeding the positive yrast band. The bands are interpreted as such candidate bands that are mixed with the negative-parity ground state band. This observation can provide a plausible explanation for unusually large population of the positive-parity yrast band observed in a recent Coulomb excitation. Received: 2 November 1999     

New insights into neutron-rich nuclei at high spin

With new high statistic data, new isotopes and new high-spin structures are observed in neutron-rich nuclei populated in the spontaneous fission of ²⁵²Cf. The ¹³⁵Te levels are extended, and many new levels in ^139,141Ba observed. The coexistence of collective and single particle-hole states is found in ¹³⁵Te. The N = 83 ¹³⁵Te and ¹³⁹Ba show marked differences associated with differences in their particle and hole states. New levels in ¹⁴¹Ba complete evidence for two opposite-parity doublets characteristic of stable octupole deformation. In ^114,116Pd a second backbend is observed for the first time in this mass region and the backbend in ¹¹⁸Pd occurs earlier than in ^112-116Pd because of a reduction in pairing. Gamma-type vibrational bands are seen up to 13⁺ to 15⁺ in ^104,106Mo, ^108-112Ru, and ^112-116Pd. Their behavior reflects prolate to triaxial shapes in these nuclei. The levels of ^162,164Gd are observed for the first time. As N increases toward mid-shell at 104, the moments of inertia in N = 100 ¹⁶⁴Gd show an unexpected decrease compared to N = 98 ¹⁶²Gd. The levels in ^162,164Gd form remarkable shifted identical bands with nuclei separated by 2n, 2p, α, and 2α. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: j.h.hamilton@vanderbilt.edu     

New multi-phonon gamma vibrational bands in A∼110 neutron-rich nuclei

The high spin states of neutron-rich ¹⁰³Nb, ¹⁰⁷Tc and ¹⁰⁹Tc nuclei in A～110 region have been investigated by measuring prompt γ-γ-γ coincident measurements populated with the spontaneous fission of ²⁵²Cf with the Gammasphere detector array. In ¹⁰³Nb, one-phonon K = 9/2 and two-phonon K = 13/2 γ-vibrational bands have been identified. In ¹⁰⁷Tc and ¹⁰⁹Tc, one-phonon K = 11/2 and two-phonon K = 15/2 γ-vibrational bands, in which the zero-phonon bands are based on K=7/2 excited states, have also been identified. The two-phonon bands are first observed in odd-Z nuclei. The characteristics for these band structures have been discussed.     

Signature inversion in the yrare band of <Superscript>119</Superscript>Xe

Excited states of the ¹¹⁹Xe nucleus have been studied by using in-beam γ-ray spectroscopy with the ¹⁰⁷Ag ( ¹⁶O, p3n) ¹¹⁹Xe fusion-evaporation reaction at a beam energy of 85 MeV. The level scheme of ¹¹⁹Xe has been derived from γ-γ coincidence and γ-γ angular correlation analyses. We have, for the first time, established the second negative-parity favored and unfavored states built on the 11/2^- state, namely the yrare rotational bands in ¹¹⁹Xe. In contrast to the behavior of the yrast bands where the favored states are lying lower in energy, the yrare favored states were observed to lie above the unfavored band. Such a signature inversion in ¹¹⁹Xe is changed to be normal at I = 12?. Received: 8 January 2002 / Accepted: 18 April 2002     

Shape co-existence and structural evolution of thevi13/2 band in157Yb

The high-spin states of¹⁵⁷Yb have been studied via the reaction of¹⁴⁴Sm(¹⁶O, 3n) at¹⁶O energy of 90 MeV using techniques of in-beam γ-ray spectroscopy. Measurement of γ-γ-t coincidences was performed with 11 BGO(AC)HPGe detectors. Based on the γ-γ coincidence relationships and the measured results of γ-ray anisotropies and DCO ratios, the level scheme for¹⁵⁷Yb was established. The shape co-existence and structural evolution of thevi_13/2 band with increasing angular momentum in¹⁵⁷Yb have been discussed. The systematics of thevi_13/2 bands in theN=87 odd-A isotones have been compared.     

Shape co-existence and structural evolution of the yrast band in 157Yb

The high-spin states of ¹⁵⁷Yb have been studied via the ¹⁴⁴Sm(¹⁶O, 3n) reaction at ¹⁶O energy of 90 MeV using techniques of in-beam γ-ray spectroscopy. Measurement of γ-γ-t coincidences was performed with 11 BGO(AC)HPGe detectors. Based on the measured results of γ-γ coincidences, γ-ray anisotropies and DCO ratios, the level scheme for ¹⁵⁷Yb was established. The shape co-existence and structural evolution of the νi _13/2 band with increasing angular momentum in ¹⁵⁷Yb have been discussed. The systematics of the νi _13/2 bands in the N = 87 odd-A isotones have been compared. Received: 7 January 2002 / Accepted: 10 April 2002     

Observation of signature inversion in odd-odd 178Ir

High spin states in ¹⁷⁸Ir have been studied via the ¹⁵²Sm(³¹P, 5nγ)¹⁷⁸Ir reaction through excitation functions, X-γ and γ-γ-t coincidence measurements. According to the band structure characteristics and the measured intraband B(M1)/B(E2) ratios, two rotational bands are identified and assigned to be associated with the πh _9/2⊗νi _13/2 and πh _11/2⊗νi _13/2 configurations, respectively. Both bands show the signature inversion feature. Received: 23 March 2000 / Accepted: 31 July 2000     

Studies of quadrupole collectivity in the γ -soft 106Ru

Various alternative models were used to describe the structure of ¹⁰⁶Ru . For example, the General Collective Model (GCM) predicts shape-coexistence for ¹⁰⁶Ru with a spherical and a triaxial minimum and strongly mixed structures, while in the IBA-2 calculations, where ¹⁰⁶Ru was considered as transitional from vibrational U(5) to γ -soft O(6) , no need was found to include the shape-coexisting configurations. In order to provide additional constraints on the model interpretations, we have applied the Advanced Time-Delayed (ATD) βγγ(t) method to measure the level lifetimes of the excited levels in ¹⁰⁶Ru . The new results include the half-lives of T _1/2 = 183(3) ps and 7.5(30)ps for the 2⁺ ₁ and 2⁺ ₂ states, respectively.     

Coulomb energy differences between isobaric analogue states in 70Br and 70Se

Gamma ray transitions de-exciting states in the N = Z nucleus ⁷⁰Br have been identified for the first time using the GASP and EUROBALL arrays coupled with ancillary detectors. The level scheme of ⁷⁰Br has been established by means of particle-gated γ-γ and γ-γ-γ coincidences. The Coulomb energy differences between isobaric analogue states in ⁷⁰Br and ⁷⁰Se show a deviation from the expected behaviour which could be related to dripline effects. Received: 18 July 2001 / Accepted: 25 September 2001     

Experiment aimed at the study of 252Cf binary and ternary fission

A new experiment devoted to the fission of ²⁵²Cf is described. It continued a series of our experiments based on correlation measurements of γ rays emitted by fission fragment pairs. The measurements of γ-γ and γ-γ-γ coincidences were done at Gammasphere with closed ²⁵²Cf sources. The open source was used for the first time in the last experiment. Fission fragment detectors were arranged in the center hole of Gammasphere. Correlations between fission fragment masses, total kinetic energy, and γ rays were observed. The first, preliminary results of data analysis are discussed. The text was submitted by the authors in English.     

High-spin states in odd-odd 168Lu

High-spin states of odd-odd ¹⁶⁸Lu were observed up to 18ħ for the first time using the ¹⁵⁹Tb(¹³C, 4nγ)¹⁶⁸Lu reaction at E _13C= 58.5 MeV. γ-γ coincidence, E _γ singles, excitation function, and the DCO ratios were measured. Two rotational bands with signature splitting were populated, among which the lower-energy band is identified as π 7/2⁺[404] ⊗ν 5/2⁺[642] in view of alignment and energy systematics. Received: 27 October 1997     

In-beam γ -ray study of the neutron-rich nuclei 240U, 246Pu, and 250Cm produced by the (18O, 16O) reaction

We have measured deexcitation γ rays in the neutron-rich nuclei of ²⁴⁰U, ²⁴⁶Pu, and ²⁵⁰Cm produced by the (¹⁸O, ¹⁶O) two-neutron transfer reactions, in coincidence with the ¹⁶O particles using Si ΔE−E detectors. The γ rays in these nuclei were identified by selecting the kinetic energies of ¹⁶O particles, which correspond to the excitation energies in the residual nuclei below the neutron separation energies. The ground-state bands of ²⁴⁰U, ²⁴⁶Pu, and ²⁵⁰Cm were established up to 12⁺ states and the K ^π = 0⁻ octupole band of ²⁴⁰U was established up to 9⁻ states. The systematics of the moments of inertia of the ground-state bands for actinide nuclei shows that the deformed subshell closure at N = 152 is sustained for ₉₆Cm isotopes and that it disappears for ₉₄Pu isotopes. The text was submitted by the authors in English.     

Evolution of shapes in 59Cu

High-spin states in ⁵⁹Cu were populated using the fusion-evaporation reaction ²⁸Si + ⁴⁰Ca at a beam energy of 122 MeV. The Gammasphere Ge-detector array in conjunction with the 4π charged-particle detector array Microball allowed for the detection of γ-rays in coincidence with evaporated light particles. The resulting extensive high-spin decay scheme of ⁵⁹Cu is presented, which comprises more than 320 γ-ray transitions connecting about 150 excited states. Their spins and parities have been assigned via directional correlations of γ-rays emitted from oriented states. Average quadrupole moments of rotational bands have been determined from the analysis of residual Doppler shifts. Shell model calculations in the fp shell are invoked to study some of the low-spin states, while the experimental characteristics of the rotational bands are analyzed in the configuration-dependent cranked Nilsson-Strutinsky (CNS) approach. Received: 15 February 2002 / Accepted: 23 April 2002     

Shape coexistence near the double-midshell nucleus 111Rh

The decay of ¹¹¹Ru obtained from fast on-line chemical and mass separation has been investigated by β-γ-t and γ-γ coincidence techniques. Earlier spin and parity assignments of ¹¹¹Rh levels based on extrapolations of level systematics are confirmed. In particular, the K=1/2 intruder band is supported by the hindrance of E2 transitions between deformed and spherical states and enhancement of intraband E2 transitions. The excitation energies of intruder band members in Rh isotopes show a minimum at ¹⁰⁹Rh₆₄, with two neutrons less than ¹¹¹Rh at the N=66 midshell. This trend, which differs from the one in the higher-Z neighbouring elements Ag and Cd with minima at N=66, follows the evolution of deformation observed in the lower-Z elements Ru and Mo. Received: 25 June 1997 / Revised version: 10 September 1997