Alpha decay of the new isotopes 188,189Po

New neutron-deficient isotopes ^188,189Po have been produced in the complete fusion reaction of ⁵²Cr ions with a ¹⁴²Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of α-γ and α-conversion electron coincidence measurements and of α-α position and time correlations. In ¹⁸⁹Po a ground state to ground state α decay with E_α1= 7540(20) keV, T_1/2= 5(1) ms and two fine structure α-decays at E_α2= 7264(15) keV and E_α3= 7316(15) keV have been observed. In ¹⁸⁸Po (T_1/2= 400⁺²⁰⁰ ₋₁₅₀μs) a ground state to ground state α decay at E_α= 7915(25) keV and a fine structure α decay at E_α= 7350(40) keV have been found. Improved data on the α-decay of ¹⁸⁹Bi were obtained. Received: 15 October 1999     

Decay modes of 24Mg excited at 46.4 MeV

⁸Be_gs coincidences with ¹²C, ⁸Be_gs and α-particles produced in the ¹²C +¹²C interaction at 65 MeV have been measured in a wide in-plane angular range. The 3 −⁸Be_gs final state is found to be produced, even if poor statistics prevent any identification of the ¹⁶O states involved in the first stage of the process. The ⁸Be_gs−α and ⁸Be_gs−¹²C coincidence yields are found to be due to ¹²C and ¹⁶O excited states, decaying into the ⁸Be_gs+α and ¹²C +α systems, respectively. Received: 15 June 1998 / Revised version: 4 December 1998     

Microscopic analysis of the breathing mode in 40Ca and 58Ni

Recent experimental studies of the giant electric resonance region in ⁵⁸Ni and ⁴⁰Ca with inelastically scattered α-particles of energy E_α= 240 MeV are analyzed within a microscopic nuclear structure model. The model includes the continuum RPA and more complex 1p1h⊗phonon configurations. By superimposing the contributions of different multipoles up to L = 4 we obtain good agreement with the newest (reanalyzed) data for the isoscalar monopole strength and for the total (α,α′) cross section in ⁵⁸Ni. Agreement with experiment for the isoscalar monopole resonance in ⁴⁰Ca is obtained too. We emphasize the necessity of using microscopic transition densities and discuss consequences for the analyses of such experiments in light and medium mass nuclei. It is shown that the gross structure of the isoscalar monopole resonance in ⁴⁰Ca is caused by the 1p1h⊗phonon configurations. Received: 23 December 1999 / Revised version: 28 February 2000     

Dynamic polarization potential of 12C+12C system at molecular-resonance energies

The nuclear reaction dynamics leading to the formation of recently discovered resonance in the mutual-0₂ ⁺ channel of the ¹²C+¹²C inelastic scattering around E _c.m.≃ 32 MeV is studied in terms of the dynamic polarization potential (DPP) induced by the channel coupling among various excited states in ¹²C. The microscopic 3α cluster-model wave functions are used to generate the ¹²C−¹²C diagonal and coupling potentials in the double-folding model. It is found that DPP for the 0₂ ⁺+ 0₂ ⁺ channel is an unusually strong attractive potential which even exceeds the zeroth-order folding-model potential of this channel around the nuclear surface region and that the strong coupling between the 0₂ ⁺ and 2₂ ⁺ states is predominantly responsible for the unusual DPP in this channel. The effective potential, the sum of the original folding-model potential and the attractive DPP, is found to generates resonance states in the same energy region as that of the resonance states generated by the original folding-model potential but the former states are found to be higher-nodal states having four additional radial nodes. Similar but more moderate property of DPP is also found in the entrance (elastic) channel. These results suggest that the reaction dynamics of generating the resonance in the ¹²C(0₂ ⁺) +¹²C(0₂ ⁺) channel may rather differ from that of the simple crossing of the zeroth-order molecular band generated by the potentials in the entrance and exit channels suggested by the standard band-crossing model. Received: 17 December 1998 / Revised version: 1 March 1999     

New states in heavy Cd isotopes and evidence for weakening of the N = 82 shell structure

A chemically selective laser ion source has been used in a β-decay study of heavy Ag isotopes into even-even Cd nuclides. Gamma-spectroscopic techniques in time-resolving event-by-event and multiscaling modes have permitted the identification of the first 2⁺ and 4⁺ levels in ¹²⁶Cd₇₈, ¹²⁸Cd₈₀, and tentatively the 2⁺ state in ¹³⁰Cd₈₂. From a comparison of these new states in ₄₈Cd with the E(2⁺) and E(4⁺)/E(2⁺) level systematics of ₄₆Pd and ₅₂Te isotopes and several recent model predictions, possible evidence for a weakening of the spherical N = 82 neutron-shell below double-magic ¹³²Sn is obtained. Received: 13 July 2000 / Accepted: 12 October 2000     

The low-spin structure of <Superscript>115</Superscript>Xe

The low-spin band structure of neutron-deficient ¹¹⁵ ₅₄Xe has been studied following a low-energy (212.3 MeV) ⁶⁰Ni(⁵⁸Ni,2pnγ) reaction performed at IReS, Strasbourg, with the Garel⁺ spectrometer. The relative excitation energies of νh_11/2 (isomeric), νg_7/2, and νd_5/2 bandheads have been established. Received: 23 March 2000     

High-spin states in odd-odd 168Lu

High-spin states in the odd-odd ¹⁶⁸Lu nucleus, populated in the ¹⁵⁴Sm(¹⁹F,5n) reaction at a beam energy of 96 MeV, were investigated using in-beam γ-ray spectroscopy techniques. The BC neutron crossing in the yrast band, based on πg _7/2[404]7/2⁺⊗νi _13/2[642]5/2⁺ configuration, occurs at ħω= 0.31 MeV. The two side bands, based on πh _11/2[514]9/2⁻⊗νi _13/2[642]5/2⁺ and πh _9/2[541]1/2⁻⊗νi _13/2[642]5/2⁺ configurations, show anomalous signature-splitting and signature-inversion in the first one, to occur at ħω= 0.24 MeV. A moderately delayed BC-crossing is anticipated in the second one. Received: 15 June 1998 / Revised version: 11 January 1999     

Level structure of 99Rh at high spins

High spins states in ⁹⁹Rh were populated via the ⁶⁶Zn(³⁷Cl, 2p2n)⁹⁹Rh reaction at an incident beam energy of 130 MeV. Seventeen new transitions have been observed in the present study and the level scheme has now been extended up to a spin of J∼ 25ħ and an excitation energy of about E _x∼ 10 MeV. The observation of a positive parity E2 cascade based on the 9/2⁺ isomeric level is suggestive of collective behaviour in this nucleus up to high spins. Spherical shell model (within restricted model space) and Cranked shell model calculations were performed to obtain an insight into the observed level structure. The new collective band observed up to a spin of J∼ 25ħ is suggested to be based on (πg _9/2 ³) ⊗ (νg _7/2 ²) quasi-particle excitations. Received: 12 July 1999 / Revised version: 14 September 1999     

Study of the N = 50 major shell effect toward 78Ni at PARRNe⋆

The γ-ray de-excitations following the β-decay of ⁸³Ga and the β-n decay of ⁸⁴Ga have been studied. The radioactive species were produced using the PARRNe on-line mass-separator installed at the IPN Orsay Tandem accelerator. Two γ-lines were attributed to ⁸³Ge with the aid of β-γ and γ-γ coincidences. The Z identification of the γ-lines was provided by time analysis of a buid-up/decay cycle. The excited levels of ⁸³Ge can be explained by the coupling of the single neutron state ν2d _5/2 to the first 2⁺ excitation of the ⁸²Ge core.     

Isovector deformation and its link to the neutron shell closure

DWBA analysis of the inelastic ^30-40S(p,p') and ^18-22O(p,p') scattering data measured in the inverse kinematics has been performed to determine the isoscalar (δ₀) and isovector (δ₁) deformation lengths of the 2⁺₁ excitations in the Sulfur and Oxygen isotopes using a compact folding approach. A systematic N-dependence of δ₀ and δ₁ has been established which shows a link between δ₁ and the neutron-shell closure. Strong isovector deformations were found in several cases, e.g., the 2⁺₁ state in ²⁰O where δ₁ is nearly three times larger than δ₀. These results confirm the relation δ₁>δ₀ anticipated from the core polarization by the valence neutrons in the open-shell (neutron rich) nuclei. The effect of neutron shell closure at N=14 or 16 has been discussed based on the folding model analysis of the inelastic ²²O+p scattering data at 46.6 MeV/u measured recently at GANIL.     

Rotational bands in 181Ta

High-spin states in ¹⁸¹Ta have been studied via the ¹⁷⁶Yb(¹¹B,α2n) reaction at 52 MeV using the PEX array and at 57 MeV using the NORDBALL array, with α-particle detection. The previously known, K^π=(7/2)⁺ ground state band and K^π=(9/2)⁻ band have been extended to spins (29/2)⁺ and (31/2)⁻, respectively. Two new one-quasiparticle bands, the K^π=(5/2)⁺ band built on the known (5/2)⁺ isomer and a K^π=((1/2)⁻) band have been observed. Two other rotational bands with three-quasiparticle structure, K^π=(15/2)⁻ and ((19/2)⁺ with π(7/2)[404]ν²(1/2)[510](9/2)[624] and π(9/2)[514]ν²(1/2)[510](9/2)[624] configurations, respectively, have been newly observed. The half-life of the K^π=((19/2)⁺) bandhead which decays to the head of the (15/2)⁻ band has been measured to be 140(36) ns. However, transitions from the ((19/2)⁺) state to the (15/2)⁻ band have not been observed. Received: 26 August 1998     

Structure of 231Ac: Measurements of level half-lives

There is lack of knowledge of the structure of the ²³¹Ac nucleus located in the upper border of the island of octupole deformation centered around A = 225. We have populated the excited states in ²³¹Ac by beta-decay from the parent ²³¹Ra produced at ISOLDE, CERN. The low-energy structure of ²³¹Ac has been investigated by means of γ, conversion electrons, γ-γ and γ-e^- spectroscopy. We report here in the precise determination in ²³¹Ac of the half-life of the most intensively populated states by the Advanced Time Delayed ββγ(t) technique.     

Study of the 90Zr(p,α)87Y reaction at 22 MeV

The (p,α) reaction on ⁹⁰Zr has been studied in a high resolution experiment at an incident proton energy of 22 MeV. The cross section and asymmetry angular distributions for transitions to 36 levels of ⁸⁷Y with an excitation energy up to 3 MeV have been measured. DWBA analyses of experimental angular distributions, using either Woods-Saxon or Double Folded potentials for the exit channel, have been done, allowing either the confirmation of previous spin and parity values or the assignment of new spin and parity to a large number of states. The structure of low lying states of ⁸⁷Y has been studied in the framework of the shell model, using the OXBASH code. With the interaction PMM90 reasonable agreement is obtained for part of the negative parity spectrum. Received: 17 November 1997     

Shell-model structure of exotic nuclei beyond <Superscript>132</Superscript>Sn

We report on a study of exotic nuclei around doubly magic ¹³²Sn in terms of the shell model employing a realistic effective interaction derived from the CD-Bonn nucleon-nucleon potential. The short-range repulsion of the bare potential is renormalized by constructing a smooth low-momentum potential, V_low-k, that is used directly as input for the calculation of the effective interaction. In this paper we focus attention on the nuclei ¹³⁴Sn and ¹³⁵Sb which, with an N/Z ratio of 1.68 and 1.65, respectively, are at present the most exotic nuclei beyond ¹³²Sn for which information exists on excited states. Comparison shows that the calculated results for both nuclei are in very good agreement with the experimental data. We present our predictions of the hitherto unknown spectrum of ¹³⁶Sn.     

High spin structure of 117I

High-spin states in ¹¹⁷I have been studied via the ¹⁰³Rh(¹⁸O, 4n) reaction at a beam energy of 85 MeV. Many deformed rotational bands built on the proton h_11/2, g_7/2, and g_9/2 orbitals have been identified. Among them, an unfavoured rotational band and a quasi-gamma band based on the h_11/2 state have been newly observed. Moreover, positive-parity states above I^π= 35/2⁺ reported in the previous work have been rearranged. Several energetically favoured states have been assigned to noncollective oblate states based on various quasiparticle configurations with β₂≈ 0.18–0.19 and γ= 60°. Received: 1 March 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     

New states and γ-transitions in 209Bi observed in the (3He, d*γγ)-subcoulomb-reaction

High-spin states in ¹⁴³Pm have been investigated via the ¹²⁸Te(¹⁹F,4nγ)¹⁴³Pm reaction using techniques of in-beam γ-spectroscopy. γ-ray singles, γ-γ coincidences, γ-ray anisotropies and DCO ratios have been measured. Based on these measurements, the level scheme of ¹⁴³Pm has been extended up to an excitation energy of 10535.4 keV, including 48 new γ-rays deexciting 28 new levels. The yrast levels in ¹⁴³Pm can be understood qualitatively in the framework of a weak-coupling model. Received: 10 July 2000 / Accepted: 12 October 2000     

In-beam study of 145Tb

The high-spin states of ¹⁴⁵Tb have been studied in the ¹¹⁸Sn(³²S, 1p4n) reaction at ³²S energies from 161 to 175 MeV using techniques of in-beam γ-ray spectroscopy. Measurements of γ-ray excitation functions and γ-γ-t coincidences were performed with 12 BGO(AC)HPGe detectors. Based on the measured results of γ-γ coincidences, γ-ray anisotropies and DCO ratios, a level scheme for ¹⁴⁵Tb was established for the first time. The observed excited states show typical irregular pattern in a spherical nucleus, and the low-lying levels have been interpreted qualitatively with a particle-core coupling. Received: 21 September 2001 / Accepted: 9 November 2001     

High-spin states in 160Lu

High-spin states of ¹⁶⁰Lu have been studied through the ¹⁴⁴Sm( ¹⁹F, 3n) reaction. The previously known πh _11/2⊗υi _13/2 yrast band is extended from I ^π = 21^- to 25^- and a four quasiparticle band with configuration πh _11/2[523]7/2 ^-⊗υh _9/2[521]3/2 ^-⊗ (υi _13/2)² is reported. Received: 21 May 2001 / Accepted: 19 July 2001