Lifetimes and configuration mixing in 110Cd

Lifetimes of excited states in ¹¹⁰Cd have been measured by the Doppler shift attenuation method in the reaction (α,2nγ) at E _α= 25 MeV. Lifetime values for 8 states and lifetime limits for 3 states were obtained. The band structures of ¹¹⁰Cd have been interpreted in terms of a modified version of the interacting boson model (IBM + 2 q.p.). The calculations explain well the excitation energies and electromagnetic transition probabilities up to J ^π= 16⁺, except for the 10⁺ ₁ state. The structural features are discussed in terms of collective and two quasiparticle excitations. Received: 20 March 1999 / Revised version: 28 May 1999     

The Z = 82 shell closure in neutron-deficient Pb isotopes

Recent mass measurements show a substantial weakening of the binding-energy difference δ_2p(Z, N) = E(Z - 2, N) - 2E(Z, N) + E(Z + 2, N) in the neutron-deficient Pb isotopes. As δ_2p is often attributed to the size of the proton magic gap, it might be speculated that reduction in δ_2p is related to a weakening of the spherical Z = 82 shell. We demonstrate that the observed trend is described quantitatively by self-consistent mean-field models in terms of deformed ground states of Hg and Po isotopes. Received: 25 October 2001 / Accepted: 28 February 2002     

Physics with heavy neutron-rich RIBs at the HRIBF

The Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory has recently produced the world's first post-accelerated beams of heavy neutron-rich nuclei. The first experiments with these beam are described, and the results discussed. B(E2;0⁺ → 2⁺) values for neutron-rich ^126,128Sn and ^132,134,136Te isotopes have been measured by Coulomb excitation in inverse kinematics. The results for ¹³²Te and ¹³⁴Te (N = 80, 82) show excellent agreement with systematics of lighter Te isotopes, but the B(E2) value for ¹³⁶Te (N = 84) is unexpectedly small. Single-neutron transfer reactions with a ¹³⁴Te beam on ^natBe and ¹³C targets at energies just above the Coulomb barrier have also been studied. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: radford@mail.phy.ornl.gov     

Shapes of the neutron-rich 88-94Kr nuclei

Neutron-rich, ^88-94Kr nuclei, populated in spontaneous fission of ²⁴⁸Cm, have been studied with EUROGAM 2, by measuring prompt γ-rays. Many new excited states in even-even Kr isotopes have been identified. For the first time spins and parities were determined experimentally in these nuclei. Our results indicate that the quadrupole deformation of Kr isotopes will appear only above N = 58, as observed in Sr and Zr nuclei. The newly found 3^- level at 1506.4 keV in ⁹⁰Kr suggests the exsistence of a new region of increased octupole correlations, probably associated with the ν(d _5/2 h _11/2) pair of Δl = Δj = 3 orbitals. Received: 18 September 2000 / Accepted: 18 October 2000     

Spectroscopy of 112Pd using heavy-ion-induced fission

High-spin states in ¹¹²Pd were studied using prompt γ-ray spectroscopy with Gammasphere following heavy-ion-induced fission in the reaction ¹⁸O + ²⁰⁸Pb at 91 MeV. A new 8⁺ level at 2638 keV was discovered with transitions connecting it to the yrast band and the quasi-gamma band. The three, now established, closely spaced 8⁺ states indicate a mixing between the ground-state band, s-band, and quasi-gamma band. Several high-spin structures with likely negative parity have been extended to higher spin and it is proposed that they are based on the νh _11/2(g _7/2 d _5/2) and νh _11/2(s _1/2 d _3/2) configurations. Received: 28 November 2000 / Accepted: 8 February 2001     

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     

Coulomb excitation of 72 30Zn42

The reduced transition probability B(E2: 0₁ ⁺→ 2⁺) of ⁷²Zn has been measured for the first time by Coulomb excitation at intermediate energy. The result B(E2: 0₁ ⁺→ 2⁺) = 1740±210 e²fm⁴, corresponds to the deformation parameter β₂ of 0.23, in close agreement with expectations derived from the neighboring nucleus ⁷³Zn. A discussion of the evolution of the N = 40 sub-shell closure as a function of Z is presented. Received: 19 December 2001 / Accepted: 14 March 2002     

On the nature of the 17 μs isomer of the <Superscript>133</Superscript>Sb valence nucleus

The decay of the 17 μs isomer of ¹³³Sb was re-investigated experimentally. It was produced by thermal neutron induced fission of ²⁴¹Pu. Its detection is based on time correlation between fission fragments selected by the LOHENGRIN spectrometer at ILL (Grenoble), and the γ-rays, and conversion electrons from the isomer. The interpretation of the level scheme is based on shell model calculations, where empirical two-body matrix elements were employed. The good agreement between theory and experiment suggests that the isomer is the 21/2⁺ member of the 2p-1h πg _7/2ν(f _7/2 h _11/2 ⁻¹) configuration. Received: 20 January 2000 / Revised version: 28 February 2000     

Measurement of the lifetime of the first 2+ state in 124Ba

The lifetime of the first 2⁺ state in the nucleus ¹²⁴Ba has been measured by γ-γ coincidence recoil-distance Doppler shift method. The ¹⁰⁹Ag(¹⁹F, 4n)¹²⁴Ba reaction at a beam energy of 75 MeV was employed to populate excited states in ¹²⁴Ba. The mean lifetime was determined to be 275(12) ps using the differential decay curve method. The value of B(E2;0₁ ⁺→ 2₁ ⁺) for ¹²⁴Ba deduced from the lifetime is in reasonable agreement with the prediction of the proton-neutron interacting boson model(IBM-2) without Pauli blocking effect. Received: 23 February 1998     

New high-spin microsecond isomers in 131Sb

In this work we have identified and studied the decay of two new microsecond isomers 19/2^- and 23/2⁺ in ¹³¹Sb. This neutron-rich nucleus was produced by thermal neutron-induced fission of ²⁴¹Pu. The detection is based on time correlation between fission fragments selected by the LOHENGRIN spectrometer at ILL (Grenoble) and the γ-rays or conversion electrons from isomers. These new data on high-spin states complete the level scheme previously obtained from β-decay. A large fraction of the members of the πg _7/2ν(h _11/2 ^-1 d _3/2 ^-1) and πg _7/2νh _11/2 ^-2 multiplets are now known and were compared with a multi-particle shell model calculation. Received: 7 June 2000 / Accepted: 19 September 2000     

Recoil distance half-life measurements of the excited states in 145Sm

A recoil distance method was used to measure half-lives of the excited states of ₁₄₅Sm. The reaction used was ₁₃₉La(₁₀B, 4n)₁₄₅Sm. A plunger system was used. Half-lives were determined for two excited states for the first time. The yrast 27/2₊ state was found to have a half-life of 1.1 ± 0.2 ns corresponding to the retardation of 3.1 × 10₋₄ comparing with the single particle estimate of M1. The excitation energy of this state was well reproduced by the shell model calculation having a mixed configuration of [π{h^11/2(g^7/2)₋₂ (d^5/2)₋₁}¹⁰⁻, νf^7/2] + [π{h^11/2(g^7/2)₋₁}⁹⁻,νh^9/2]. Another retarded E1 transition was also found in a decay of a 21/2₊ state. Its retardation was 1.6 × 10₋₄ comparing with the single particle value. Received: 9 September 1997 / Revised version: 12 June 1998     

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     

(EC+β +) decay of 130Nd and 140Tb

The available astrophysical S(E) factor data for the reaction ⁷Li (p,α)α at 10 < E < 1000keV exhibit an exponential increase at low energies due to the effects of electron screening. A parametrisation of the data using a non-resonant, direct process and two subthreshold resonances reproduces the data at energies E≥ 100keV, while at lower energies this calculated S _b(E) factor curve for bare nuclides drops below the data, which in turn represent the case of electron-shielded nuclides, i.e. the electron-shielded S _s(E) factor. The comparison between S _b(E) and S _s(E) leads to an electron-screening potential energy U _e = 350eV, which is much higher than the adiabatic limit of 175eV and not understood at present. The deduced value of S _b(0) is considerably smaller than the previously adopted value of 59keV b, significantly increasing the calculated abundance of ⁷Li in big-bang nucleosynthesis. The Trojan-horse method was applied to the reaction ⁷Li (p,α)α to determine the energy dependence of the S _b(E) factor for 10 < E < 370keV, free from the effects of the Coulomb barrier and electron screening. The THM results are close to the calculated S _b(E) curve and suggest that the THM may become a powerful way to obtain improved information on low-energy cross-sections and associated electron-screening effects in a model-independent way. Received: 9 September 2000 / Accepted: 5 December 2000     

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.     

Evidence for deformation in <Superscript>113-116</Superscript>Cd isotopes

High-spin states in ^113-116Cd have been investigated with the fusion-fission reaction ²⁸Si +¹⁷⁶Yb at 145 > MeV. The experiment has been performed with the Eurogam2 array. New rotational bands based on two quasi-particle states have been observed for even-even cadmium isotopes. A new level scheme based on the 11/2⁻ isomeric state is proposed for ¹¹⁵Cd and the one of ¹¹³Cd has been extended to spin (31/2⁻). The decoupled bands identified in both odd-A nuclei are interpreted as being built upon a low-Ωh_11/2 quasi-neutron configuration. Microscopic Hartree-Fock + BCS calculations confirm the prolate deformation in this mass region especially for the odd cadmium isotopes. Received: 21 December 1999 / Revised version: 16 February 2000     

High-spin isomers in 183Os

High-spin states in ¹⁸³Os have been studied by means of in-beam γ-ray spectroscopy. Two isomers with I ^π= ((43/2)⁻) and (43/2)⁺ have been identified at excitation energies of E _x = 5.068 and 5.168 MeV. The half-lives are determined as T _1/2= 27 ± 3 and 24 ± 2 ns. Received: 22 December 1997     

Evolution of the N = 20 and N = 28 shell closures in neutron-rich nuclei

The isospin dependence of shell closure phenomena is studied for light neutron-rich nuclei within a microscopic self-consistent approach using the Gogny force. Introducing configuration mixing, ³²Mg is found to be dynamically deformed, although the N = 20 spherical shell closure persists at the mean-field level for all N = 20 isotones. In contrast, the N = 28 spherical shell closure is found to disappear for N - Z≥ 10 whereas deformed shell closures are preserved and lead to shape coexistence in ⁴⁴ S. Configuration mixing shows that the ground state of this nucleus is triaxially deformed. The first 2⁺ excitation energy E_x = 1.46 MeV and the reduced transition probability B(E2;0⁺ _gs→ 2⁺ ₁)= 420 e ² fm ⁴ obtained with our approach are in good agreement with experimental data. Received: 26 July 2000 / Accepted: 30 August 2000     

Microscopic study of low-lying yrast spectra in <Superscript>100–108</Superscript>Mo isotopes

Variation-after-projection (VAP) calculations in conjunction with Hartree-Bogoliubov (HB) ansatz have been carried out for A=100−108 molybdenum (Mo) isotopes. In this framework, the yrast spectra with J _max ^π ≥10⁺, B(E2) transition probabilities, quadrupole (β₂) and hexadecapole (β₄) deformation parameters, moment of inertia (I) and square of cranking frequency (ω²) for even-even Mo isotopes have been obtained. The results of the calculation give an indication that it is important to include the hexadecapole-hexadecapole component of the two-body interaction for obtaining various nuclear structure quantities in these Mo isotopes.     

Simplex s = ±i excitations in 141Xe

Excited levels in ¹⁴¹Xe, populated in spontaneous fission of ²⁴⁸Cm, were studied by means of prompt -ray spectroscopy, using the EUROGAMM2 array. Level scheme of ¹⁴¹Xe obtained in this work shows patterns characteristic of simplex symmetry with s=+i and s=-i bands present but low value of D₀ moment indicates that octupole correlations in Xe isotopes are systematically lower than in Ba nuclei. Received: 9 March 2000 / Accepted: 4 May 2000