Spectral property and its shape transition on 72—84Kr isotopes in microscopic core plus two—quasiparticle approach

By using a microscopic sdIBM-2+2q.p. approach, the levels of the ground-band, γ-band and partial two-quasi-particle bands for {}^{72-84}Kr isotopes are calculated. The data obtained are in good agreement with the recent experimental results, and successfully reproduce the nuclear shape phase transition of {}^{72-84}Kr isotopes at zero temperature. The ground-state band is described successfully up to J^π=18^+ and E_x=10.0MeV. Based on this model, the aligned requisite minimum energy has been deduced. The theoretical calculations indicate that no distinct change of nuclear states is caused by the abruptly broken pair of a boson, and predict that the first backbending of Kr isotopes may be the result of aligning of two quasi-neutrons in orbit g_{9/2}, which gains the new experimental support of the measurements of g factors in the {}^{78-86}Kr isotopes.     

In-beam spectroscopy of 120Xe

Excited states of ¹²⁰Xe have been studied by in-beam γ-ray spectroscopy using the ¹¹⁰Cd(¹³C, 3n) and ¹¹¹Cd(¹²C, 3n) reactions. The yrast band was observed up to the 14⁺ level, showing backbending at the 12⁺ state. A γ-band and two new negative-parity bands were found. The low-lying levels of ¹²⁰Xe could be described by the IBA-1 hamiltonian plus a triaxial degree of freedom.     

MICROSTRUCTURE ON COLLECTIVITY IN EVEN 116-122Xe ISOTOPES

Using microscopic interacting boson-fermion model formalism, the ground-band, β-band, γ-band and high spins states in even ^116-122Xe isotopes are successfully described. It can explain the recent experimental result that collective structures may coexist with the single-particle states and subshell feature. It predictes the energies of 1⁺ states.     

Reduced transition probabilities for high-spin states in 232Th

The lifetimes and reduced transition probabilities for spins 6⁺ ? 18⁺ in the ground-state rotational band of ²³²Th have been measured by recoil-distance, Doppler-broadened lineshape, and multiple Coulomb excitation methods using ⁸⁴Kr and ¹³⁶Xe projectiles. Within the limits of experimental uncertainty, transition probabilities from all of the methods are consistent with those of a good rotor for spins 6⁺ ? 18⁺. The descrepancy with the prediction of simple β-band mixing and centrifugal stretching models is discussed.     

Description of Even–Even 114–134Xe Isotopes in the Transitional Region of IBM

Properties of ^114–134Xe isotopes are studied in the U(5)?SO(6) transitional region of Interacting Boson Model (IBM-1). The energy levels and B(E2) transition rates are calculated via the affine SU(1,1) Lie Algebra. The agreement with the most recent experimental is acceptable. The evaluated Hamiltonian control parameters suggest a spherical to γ-soft shape transition and propose the ¹³⁰Xe nucleus as the best candidate for the E(5) symmetry.     

Connection between backbending and high-spin isomer decay in 179W

A new 710 ns isomer in ¹⁷⁹W is found to decay directly into backbending region of the $\text{7}\text{2}{}^{\mathrm{-}}\text{(514)}$ gorund-state band. The $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$ band is observed up to spin $\text{41}\text{2}$ and shows no evidence for backbending at core rotational frequencies, where the effect is observed in the ¹⁷⁹W and ¹⁸⁰W ground-state bands.     

Anomalous moments of inertia for high-spin levels in the beta vibrational band of 154Gd

The levels of ¹⁵⁴Gd populated by the ¹⁵⁴Sm(α,4n) reaction have been studied with γ-γ coincidence techniques. The energy spacings of the β-band levels of spin > 8 are found to differ from the rotational picture in a manner similar to that recently observed in the ground-state bands of some other rare-earth nuclei. The ground-state band of ¹⁵⁴Gd does not show the anomaly;     

New nuclides: Neptunium-243 and neptunium-244

We have observedγ-rays following the β^?-decay of²⁴³Np and²⁴⁴Np after chemical isolation of neptunium isotopes from the products of the reaction of 835 MeV¹³⁶Xe with²⁴⁴Pu. The ground-state of 1.85-min²⁴³Np hasJ=5/2. The decay of 2.29-min²⁴⁴Np (probableJ ^π=7^?) populates high-spin members of the ground state rotational band in²⁴⁴Pu.     

The strength of octupole correlations in neutron-rich Xe isotopes

Excited levels in ¹⁴⁰Xe and ¹⁴²Xe nuclei, populated in the spontaneous fission of ²⁴⁸Cm, were studied by means of prompt γ-ray spectroscopy, using EUROGAM2 array. We report the first observation of an octupole band in ¹⁴²Xe and extend the octupole band in ¹⁴⁰Xe. Level schemes of ¹⁴⁰Xe and ¹⁴²Xe obtained in this work show patterns characteristic of ocupole-vibrational bands. Properties of octupole bands in Xe isotopes indicate that octupole correlations in these nuclei are lower than in the corresponding Ba nuclei. The electric dipole moment of ¹⁴²Xe was found to be larger than in other Xe isotopes, contrary to theoretical predictions. This may be due to the special role of the N = 88 neutron number. Received: 18 November 2002 / Accepted: 15 December 2002 / Published online: 25 February 2003 RID="a" ID="a"e-mail: urban@fuw.edu.pl Communicated by D. Schwalm     

Multipolarities of prompt transitions in 156Dy and spin-parities of the upper band levels

The prompt γ-ray and conversion-electron spectra following the ¹⁵⁹Tb(p, 4n)¹⁵⁶Dy and ¹⁵⁶Gd(α, 4n)¹⁵⁶Dy reactions have been measured with, respectively, a Ge(Li)-NaI(Tl) Compton suppression device and a mini-orange spectrometer. On the basis of the deduced multipolarities, earlier spin-parity assignments for levels in ¹⁵⁶Dy have been confirmed. Furthermore, the recently reported upper band levels could be divided into a negative-parity band up to 11^? or maybe 13^?, and a positive-parity band with tentative K = 0 character and spins 4⁺, 6⁺, 8⁺, 10⁺ and maybe 2⁺. The negative-parity band is described as an aligned-octupole band; the positive-parity band may possibly represent the low-spin extension of the superband responsible for backbending in the β- and ground-state bands in this nucleus.     

Magnetic moments in the backbending region

Interpreting backbending as the crossing between the ground-state band and an aligned two-quasiparticle band, the change of the g-factor in the backbending region is related to the aligned angular momentum extracted from the experimental spectrum. Illustrative examples are discussed. The hybridization of the bands in the crossing region smooths the jump of the g-factor and causes strong M1 transitions between the yrare and yrast levels.     

含三体势的IBM1的O(6)极限对偶偶120～130Xe和124～130Ba核的Yrast带回弯研究

应用含三体势的ＩＢＭ１的Ｏ（６）极限［Ｏ（６）＋Ｖ３］对过渡区偶偶核１２３～１３０Ｘｅ、１２４～１３０Ｂａ的Ｙｒａｓｔ带能谱和回弯现象进行了分析研究．结果表明:Ｏ（６）＋Ｖ３能较好地描述回弯的主要特征.The Yrast band energy spectra of even even nuclei 120～130 Xe and 124～130 Ba are calculated by using the O (6) limit of IBM1 containing three body potential. It shows that the backbending characteristics in the Yrast bands can be described fairly by O (6)+ V 3 .     

Rotational band crossings in the actinides

In the actinides bothi _13/2 protons andj _15/2 neutrons are close to the Fermi surface. At rapid rotation these high-j particles will unpair and align their orbital angular momentum along the axis of rotation giving rise tos-bands that cross the ground-state band. Coulomb excitation of the odd nuclei ₂₃₇ ⁹³ Np (established up to the 45/2⁺ state) and ₂₃₅ ⁹² U (established up to the 51/2^? state) provides specific information about these band crossings: From the saturating alignment of the odd high-j particle in both nuclei at intermediate rotational frequencies we find the aligned angular momentum of thei _13/2 protons-band to be 6.6? while the corresponding value for thej _15/2 neutrons-band is 5.5?. At more rapid rotation above ?ω=0.18 MeV we observe additional alignment in²³⁵U. This is ascribed to the interaction of the protons-band. From the gradual onset of the additional alignment we deduce that forZ=92 the protons-band interacts strongly with the ground-state band and from a comparison of the actual amount of alignment with the full value of 6.6? we estimate the crossing to occur around ?gw _c ^p =0.25 MeV.     

High-spin states in 222Th

Levels to spin 16⁺ in the ground band and 17^? in the octupole band in ²²²Th have been identified using the reaction ²⁰⁸Pb(¹⁸O, 4n)²²²Th at 93 MeV. To suppress intense γ-ray background from fission a residue detector was built and operated in coincidence with the Ge(Li) and NaI(Tl) detectors. The apparent moment of inertia in the ground-state rotational band rises very rapidly with increasing spin, however, no indications of backbending were observed. The octupole band at low spin has an aligned angular momentum $\text{J ≈ 3}\text{h}\text{?}$ relative to the ground-state band. Strong cross-band E1 transitions competed with collective E2 transitions within each band. Analysis showed that the ratios $\text{B(}\text{E}\text{1)}\text{B(}\text{E}\text{2)}$ were within experimental uncertainty independent of both the spin and parity of the parent state.The average γ-ray multiplicity per cascade was measured for ²²²Th. The results were in reasonable agreement with the computer code ORNL-ALICE.     

Search for octupole deformation in neutron-rich Xe isotopes

A search for octupole deformation in neutron-rich Xe isotopes has been conducted through prompt gammaray spectroscopy of secondary fragments produced in the spontaneous fission of²⁴⁸Cm. The spectrometer consisted of the Eurogam 1 array and a set of 5 LEPS detectors. Level schemes were constructed for Xe isotopes with mass number ranging from 140 to 144 and excited states for^143,144Xe nuclei were observed for the first time. None of the level schemes exhibit an alternating parity quasimolecular band, a feature usually expected in nuclei in which octupole correlation effects are strong enough to produce stable octupole deformation. For several isotopes, structures observed in the level schemes are consistent with an octupole softness of the nuclei.     

The nature of the superband in backbending for the nucleus 156Dy

In two recent publications by Broglia et al., it has been shown that backbending can be understood by mixing the ground (g) and a K = 1 (super)band (s1). It has also been claimed that a K = 0 superband (s0) is not able to reproduce the data. Here we show that, in ¹⁵⁶Dy, and with the inclusion of the γ-band, a K = 0 or a K = 1 superband can explain the data equally well. The parameters of the band-mixing model may be reduced by using, for the g-, γ- and β-bands, a generalization of the rotational-vibrational model. The g, γ, β and sK band mixing fits are greatly improved if the moment of inertia of the g-, γ- and β-bands are described by the VMI model. Inclusion of the low spin members of the superband in the fit indicates a slight preference for an sO band.     

Experimental evidence for low-spin members of “upper” bands in156Dy

Study of the energy levels of¹⁵⁶Dy through the¹⁵⁹Tb(p, 4n)¹⁵⁶Dy reaction has revealed the existence of six states with excitation energies between 1.8 and 2.8 MeV and spins between 6 and 12. Some of them can tentatively be assigned as low-spin members of “upper” bands which are thought to be responsible for the backbending phenomenon experimentally observed in the ground-state andβ-vibrational bands of this nucleus. Others could be levels of a negative-parity octupole band.     

Nuclear deformation and the two-neutrino double- β decay in <Superscript>124, 126</Superscript>Xe , <Superscript>128, 130</Superscript>Te , <Superscript>130, 132</Superscript>Ba and <Superscript>150</Superscript>Nd isotopes

The two-neutrino double-beta decay of ^{124, 126}Xe , ^{128, 130}Te , ^{130, 132}Ba and ¹⁵⁰Nd isotopes is studied in the Projected Hartree-Fock-Bogoliubov (PHFB) model. Theoretical 2ν β^-β^- half-lives of ^{128, 130}Te , and ¹⁵⁰Nd isotopes, and 2ν β⁺β⁺ , 2ν β⁺ EC and 2ν ECEC for ^{124, 126}Xe and ^{130, 132}Ba nuclei are presented. Calculated quadrupolar transition probabilities B(E2 : 0⁺ → 2⁺) , static quadrupole moments and g -factors in the parent and daughter nuclei reproduce the experimental information, validating the reliability of the model wave functions. The anticorrelation between nuclear deformation and the nuclear transition matrix element M _2ν is confirmed.     

High-spin bands in117,119I and118Xe

Excited states in¹¹⁷I,¹¹⁸Xe and¹¹⁹I were populated in the reactions¹⁰⁶Pd+¹⁶O and^{108, 110}Cd+¹²C. The groundstate band in¹¹⁸Xe was observed to its (18⁺) member and the 11/2^? bands in¹¹⁷I and¹¹⁹I to their (35/2^?) and (43/2^?) members, respectively. The structure of the bands in the I isotopes is interpreted as aπ h _11/2 proton coupled to an even-even band structure in Xe, and the band crossings observed in¹¹⁸Xe and¹¹⁹I are interpreted in the Cranked Shell Model framework asAB _n ofv h _11/2.