Shape changes at high spin in 78Kr

High-spin states in ⁷⁸Kr have been studied via the ⁶³Cu (¹⁹F, 2p2n)⁷⁸Kr reaction at a beam energy of 60 MeV using the Indian National Gamma Array (INGA). In this nucleus, lifetimes have been measured upto the I^π=22⁺ level in the yrast positive-parity band and upto the I^π=15^- level in the negative-parity band using the Doppler Shift Attenuation Method (DSAM). The deduced transition quadrupole moments Q_t's are found to decrease with rotational frequency for both the bands.     

A simple model for doublet bands in doubly odd nuclei

Nuclear structure of doublet bands in doubly odd nuclei with mass A ∼ 130 is investigated within the framework of a simple model where the even-even core couples with a neutron and a proton in intruder orbitals through a quadrupole-quadrupole interaction. The model reproduces quite well the energy levels of doublet bands and electromagnetic transitions. The staggering of the ratios B(M1;I → I - 1)/B(E2;I → I - 2) of the yrast bands turns out to be described by the chopsticks-like motion of two angular momenta of the unpaired neutron and the unpaired proton when they are weakly coupled with the core.-1 An erratum to this article is available at .     

Application of a quadrupole-coupling model to doublet bands in doubly-odd nuclei

A simple model is applied to the yrast and yrare states based on the νh _11/2 ⊗ πh _11/2 configuration in the doubly-odd nuclei around the mass 130. In the model, the basis state is constructed by one neutron and one proton both in the 0h _11/2 orbital, and by the collective core which couples with the two particles through a quadrupole interaction. The model reproduces quite well the overall energy levels and the electromagnetic transitions. The analysis of the yrast and yrare states reveals that the angular-momentum configuration of the neutron and the proton in the yrast states is different from that in the yrare states, when the two particles are weakly coupled with the quadrupole collective excitations of the core. The strong even-odd staggering of the ratios B(M1;I → I - 1)/B(E2;I → I - 2) for the yrast states is described by the chopsticks-like motion of two angular momenta of the neutron and the proton.     

Rotational alignment of the h11/2 band in 157Dy

High-spin states of ¹⁵⁷Dy were investigated using in-beam γ-ray spectroscopy techniques with a ¹⁵⁰Nd(¹²C, 5n) reaction. Three rotational bands with the h _9/2, i _13/2 and h _11/2 configurations were observed up to (43/2^-), 53/2⁺ and 45/2^-, respectively. Interband M1 transitions in the h _11/2 (high-Ω) band were also measured up to the highest-spin state. The high-spin states were well reproduced by calculations using the tilted-axis-cranking model (TAC). The B(M1)/B(E2) ratio, Routhian and the tilted angle of the angular-momentum vector are found to be in good agreement with the result of the TAC calculation. Received: 22 August 2002 / Accepted: 26 September 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: hayakawa@jball4.tokai.jaeri.go.jp Communicated by W. Henning     

Observation of a πh9/2 ⊗ νi13/2 oblate band in 188Tl

Excited states in ¹⁸⁸Tl have been studied experimentally using the ¹⁵⁷Gd (³⁵Cl, 4n) reaction at a beam energy of 170MeV. A rotational band built on the πh _9/2 ⊗ νi _13/2 configuration with oblate deformation has been established for ¹⁸⁸Tl. Based on the structure systematics of the oblate πh _9/2 ⊗ νi _13/2 bands in the heavier odd-odd Tl nuclei, we have tentatively proposed spin values for the new band in ¹⁸⁸Tl. The πh _9/2 ⊗ νi _13/2 oblate band in ¹⁸⁸Tl shows low-spin signature inversion, and it can be interpreted qualitatively by the two-quasiparticle plus rotor model including a J-dependent p-n residual interaction.     

E2 transitions between positive- and negative-parity states of the ground-state alternating-parity bands

Experimental transition probabilities between states of the ground-state alternating-parity bands of ¹⁴⁴Ba and their theoretical analysis are presented. Lifetimes of states in ¹⁴⁴Ba have been measured using the recoil distance method following spontaneous fission of ²⁵²Cf. The experiment was performed at the Lawrence Berkeley National Laboratory employing the Gammasphere array and the New Yale Plunger Device. The experimental data show a significantly larger value of the E2 transition probability between the negative-parity states compared to the positive-parity ones. It is shown that this effect can be explained by a higher weight of the deformed component in the wave functions of the odd-I states. In the framework of the cluster approach it is explained by a higher weight of the alpha-cluster component in the wave function of the negative-parity states compared to the positive-parity ones. In the framework of the traditional collective model with the quadrupole and octupole degrees of freedom the same effect is explained by a higher value of the quadrupole deformation at the minima of the potential energy as a function of β₂₀ and β₃₀ compared to its value at the top of the barrier separating two physically equivalent minima, having opposite signs of the octupole deformation. Additionally, the dependence on parity of the E2 transition probability is analyzed qualitatively in nuclei with a minimum at β₃₀ = 0 in the collective potential energy and compared to experimental data for ¹⁴⁸Nd.     

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.     

Half-lives of platinum isotopes from photoactivation

The half-lives of the platinum isotopes ^189,191,197Pt and of the 4-day isomer in ^195mPt have been measured with high precision using the photoactivation technique. The γ-ray activity was counted over several half-lives with a high-purity germanium detector. The measured half-lives are: T_1/2(¹⁸⁹Pt) = 12.1 ± 1.5 h; T_1/2(¹⁹¹Pt) = 2.862 ± 0.007 d; T_1/2(¹⁹⁷Pt) = 19.96 ± 0.05 h; T_1/2(^195mPt) = 4.0104 ± 0.0047 d. Received: 13 September 1999     

Spin-parity measurements in the neutron-rich N ∼ 20 34P and 36S nuclei

Yrast and near-yrast energy levels in the neutron-rich N ∼ 20 nuclei ³⁴P, ³⁶S were populated using transfer/deep-inelastic processes following the ³⁴S + ¹¹⁵In reaction at an incident energy of 140MeV. The use of a multi-clover array has facilitated polarization measurements of the observed γ-rays and necessitated some changes in the previously known level scheme. The observation of the negative-parity levels in these nuclei on the periphery of the “island of inversion” is indicative of the influence of the intruder orbitals on the level structure at low spins. Shell-model calculations indicate that the inclusion of the orbitals from the upper pf shell is important even for the low-lying positive-parity states.     

Rotational structures in the 125Cs nucleus

The collective band structures of the ¹²⁵Cs nucleus have been investigated by in-beam γ-ray spectroscopic techniques following the ¹¹⁰Pd ( ¹⁹F, 4n) reaction at 75MeV. The previously known level scheme, with rotational bands built on πg_7/2, πg_9/2 and πh_11/2 orbitals, has been extended and evolves into bands involving rotationally aligned ν(h_11/2)² and π(h_11/2)² quasiparticles. A strongly coupled band has been reassigned a high-K πh_11/2 ⊗ νg_7/2 ⊗ νh_11/2 three-quasiparticle configuration and a new side band likely to be its chiral partner has been identified. Configurations assigned to various bands are discussed in the framework of Principal/Tilted Axis Cranking (PAC/TAC) model calculations.     

Investigations into the alpha-decay of <Superscript>195</Superscript>At

The low-energy nuclear structure and decay properties of the neutron-deficient isotopes ¹⁹⁵At and ¹⁹¹Bi have been studied. ¹⁹⁵At was produced in the reaction ¹⁴²Nd(⁵⁶Fe,p2n)¹⁹⁵At and ¹⁹¹Bi as the daughter activity of ¹⁹⁵At. The activities were implanted in a position-sensitive silicon detector after being separated from the primary beam by a gas-filled recoil separator. The 1/2⁺ intruder state was determined to be the ground state in ¹⁹⁵At with an alpha-decay energy of E _α = 6953(3) keV and a half-life T _1/2 = 328(20) ms. Another state with an alpha-decay energy E _α = 7075(4) keV and a half-life T _1/2 = 147(5) ms was found to decay to a 148.7(5) keV excited state in ¹⁹¹Bi for which a spin and parity of 7/2^- were deduced. Consequently, the same 7/2^- character was assigned to the initial state at 32(7) keV in ¹⁹⁵At on the basis of unhindered alpha-decay. The 9/2^- state, being the ground state in heavier odd-mass astatine isotopes, was not observed. Received: 16 September 2002 / Accepted: 19 December 2002 / Published online: 25 March 2003 RID="a" ID="a"e-mail: heikki.kettunen@phys.jyu.fi RID="b" ID="b"Present address: Laboratory of Radiochemistry, P.O. Box 55, FIN-00014, University of Helsinki, Finland. RID="c" ID="c"Present address: Radiation and Nuclear Safety Authority, P.O. Box 14, FIN-00881 Helsinki, Finland. Communicated by W. Henning     

Rotational structures in 123Cs

High-spin states in ¹²³Cs, populated via the ¹⁰⁰Mo ( ²⁸Si, p4n) fusion-evaporation reaction at E _lab = 130 MeV, have been investigated employing in-beam γ-ray spectroscopic techniques. Rotational bands, built on πg _7/2, πg _9/2 and the unique-parity πh _11/2 orbitals, have been extended and evolve into bands involving rotationally aligned ν(h _11/2)² and π(h _11/2)² quasiparticles. A three-quasiparticle band based on the high-K πh _11/2 ⊗ νg _7/2 ⊗ νh _11/2 configuration has also been observed. Total Routhian Surface (TRS) calculations have been used to predict the nuclear shape parameters ( β₂, β₄, γ) for the various assigned configurations. The assigned configurations have been discussed in the framework of a microscopic theory based on the deformed Hartree-Fock (HF) and angular-momentum projection techniques.     

Loss of collectivity in 79Rb

High-spin states in ⁷⁹Rb were populated in the reaction at E(beam) = 60 MeV. The lifetimes of the excited states of the positive-parity yrast band and of the negative-parity band in ⁷⁹Rb were measured by the Doppler Shift Attenuation Method. The deduced transition quadrupole moments Q_t are found to have a decreasing trend with rotational frequency for both the bands, consistent with those found experimentally in neighbouring nuclei. An erratum to this article is available at .     

Maximally aligned states in 99Ag

Excited states of ⁹⁹Ag were populated via the ⁵⁰Cr + ⁵⁸Ni (261 MeV) reaction using the NORDBALL detector array equipped with charged-particle and neutron detector systems for reaction channel separation. On the basis of the measured γγ-coincidence relations and angular distribution ratios a significantly extended level scheme has been constructed up to E _x ∼ 7.8 MeV and I = 35/2. The experimental results were described within the framework of the shell model. Candidates for states fully aligned in the πg _9/2 ^-3ν(d _5/2, g _7/2)² valence configuration space were found at 4109 and 6265 keV. Received: 18 June 2002 / Accepted: 11 October 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: sohler@atomki.hu Communicated by J. ?yst?     

Lifetime measurements in 148Gd

Lifetimes of excited states in ¹⁴⁸Gd were measured using the recoil distance method with a plunger device coupled to the EUROBALL Ge detector array. The differential decay curve method in coincidence mode allowed the unambiguous determination of lifetimes of more than 20 excited states. The obtained transition strengths are in good agreement with the shell model calculations. The effect of the Z = 64 subshell closure on the B(E2 : 2⁺ → 0⁺) reduced strengths above the N = 82 magic number is discussed.-1 Received: 14 November 2002 / Accepted: 4 February 2003 / Published online: 29 April 2003     

On the unusual properties of the 282 keV state in <Superscript>135</Superscript>Sb

Recently the first excited state in ¹³⁵Sb has been observed at the unexpectedly low excitation energy of only 282keV and interpreted as mainly d _5/2 proton coupled to the ¹³⁴Sn core. Based on theoretical considerations it was suggested that its low excitation energy is related to a relative shift of the proton d _5/2 and g _7/2 orbits induced by the neutron excess. We have measured the lifetime of the 282keV state by the advanced time-delayed βγγ(t) method. The measured half-life, T _1/2 = 6.1(4)ns, yields exceptionally low limits of B(M1;5/2₁ ⁺→7/2₁ ⁺)≤3.0×10^-4 μ ² _N and B(E2;5/2₁ ⁺→7/2₁ ⁺)≤54e ² fm ⁴. These strongly hindered M1 and slow E2 transition rates are similar to those for the transition de-populating the first excited state at 405keV in ²¹¹Bi. Results of shell model calculations with realistic interactions are presented. The M1 decay rate was found to be extremely sensistive both to the wave function and to the M1 effective operator.     

Perturbative role in the inelastic electron scattering from <Superscript>29</Superscript>Si

Inelastic electron scattering form factors away from the closed sd -shell are investigated. This investigation covers the low-lying states in ²⁹Si , which is considered according to the many-particle configuration mixing shell model as the ¹⁶O core, plus thirteen nucleons distributed over the entire sd -shell orbits. The investigation concentrates on the perturbative role of the core, which is called core polarization effects, on the inelastic electron scattering form factors. Core polarization effects are taken into consideration through the excitation of nucleons from 1s and 1p core orbits, and also from the 2s -1d valence orbits into higher shells, with 2ℏω excitations. Core polarization matrix elements are calculated with the M3Y effective interaction. For the sd -shell model space, a new Hamiltonian, based on a renormalized G -matrix, USDB, is used. All calculations are performed without adjusting any parameters.