Nuclear structure in the vicinity of the N = Z line in the A = 90-100 region

Neutron-deficient nuclei in the mass region A≈ 90-100 exhibit a large variety of phenomena. In this region the heaviest N = Z nuclei are identified and enhanced neutron-proton correlations are expected when protons and neutrons occupy identical orbitals. A variety of nuclear shapes are predicted and observed for A? 91, including superdeformed shapes. The nucleus ¹⁰⁰Sn is the heaviest N = Z doubly magic nucleus believed to be bound. Knowledge of the shell structure around ¹⁰⁰Sn is of utmost importance for understanding the nuclear shell model. New results on both the N = Z nucleus ⁸⁸Ru, superdeformed structures in A≈ 90 nuclei as well as the first result on the level structure in ¹⁰³Sn, and an extended level structure in ¹⁰²In are presented. The limitations of using stable beams and targets and the possibilities with new radioactive beams are briefly outlined. Received: 1 May 2001 / Accepted: 4 December 2001     

High-spin states in neutron-rich nuclei explored with deep-inelastic HI reactions

The use of deep-inelastic heavy-ion reactions for the spectroscopy of neutron-rich nuclei is discussed. Conclusions from the N/Z equilibration process studies are outlined and examples of spectroscopic results obtained for the neutron-rich spdf shell nuclei, N = 82 isotones and nuclei from the ²⁰⁸Pb region are reviewed. Received: 1 May 2001 / Accepted: 4 December 2001     

Spectroscopy of nuclei far from stability at GANIL: Recent experiments

The structure of neutron-rich light nuclei around N = 20, 28 has been investigated at GANIL by means of in-beam gamma spectroscopy using fragmentation reactions of ³⁶S and ⁴⁸Ca beams on a Be target. Gamma decay of relatively high-lying excited states have been measured for the first time in nuclei around ³²Mg and ⁴⁴S. Level schemes are proposed and discussed for a large number of these neutron-rich nuclei around N = 20 and N = 28. Received: 1 May 2001 / Accepted: 4 December 2001     

High-spin states in the deformed 122Ba nucleus

The even-even nucleus ¹²²Ba was studied via in-beam γ-ray spectroscopy using the ⁴⁰Ca + ⁹²Mo reaction at 190 MeV. Five bands were observed, four more that can be found in the literature. Configurations were assigned to the various bands from the comparison with the neighboring even-even nuclei. Received: 10 July 2001 / Accepted: 10 October 2001     

Synthesis and decay properties of superheavy atoms in nuclear reactions induced by stable and radioactive ion beams

This talk consists of two parts. The first one presents the results of investigations performed in 1998-2000 in Dubna on the synthesis of superheavy nuclei in reactions induced by ⁴⁸Ca ions. The radioactive decay properties of the nuclei, indicating a considerable increase in the α-decay and spontaneous fission half-lives of the isotopes of elements 110-116 when approaching the closed neutron shell at N = 184, are given. In the second part the possible ways of advancing into the region of more neutron-rich nuclei, using stable and radioactive ion beams, are discussed. Since so far no intense radioactive ion beams are available, some experiments with stable beams are considered as a test for the suggested ideas. Received: 1 May 2001 / Accepted: 4 December 2001     

Rotational bands in the near-drip-line nucleus 128Nd

The even-even nucleus ¹²⁸Nd was studied via in-beam γ-ray spectroscopy using the ⁴⁰Ca + ⁸²Mo reaction at 190 MeV. Two new bands were observed besides the yrast one, that has been extended up to spin 34⁺. Configurations were assigned to the three bands by analysing their rotational properties and by comparison with the neighboring even-even nuclei. Received: 18 July 2001 / Accepted: 25 October 2001     

First identification of rotational bands in 103Tc: Evolution of intrinsic proton states of the 97-105 43Tc isotopes

High-spin states of ¹⁰³ ₄₃Tc have been identified for the first time, this nucleus being produced as fission fragment following the fusion reaction ³⁷Cl + ¹⁷⁶Yb at 170 MeV bombarding energy. The high-spin level scheme has been built from the prompt gamma rays detected using the Euroball III array. It exhibits similarities with those of the neighbouring isotopes and isotones. All the band head configurations observed in the ^97-105Tc isotopes are identified from the behaviour of the rotational bands built on them. The single-proton states located around the Fermi level are discussed as a function of deformation of these nuclei. Received: 19 January 2001 / Accepted: 23 February 2001     

High-spin spectroscopy of 63 31Ga32 and 65 31Ga34

High-spin states in the nuclei ⁶³Ga and ⁶⁵Ga were studied following the reaction ³²S + ⁴⁰Ca at a reaction energy of 125 MeV and using the GASP γ-ray spectrometer in combination with the ISIS charged-particle detector system. In addition to low-lying negative-parity states with single-particle character, rotational-like cascades built on the 9/2⁺ and 19/2^- states have been observed in both nuclei. Sidebands with negative parity in ⁶³Ga and positive parity in ⁶⁵Ga could be established. The experimental results are compared with Total Routhian Surface calculations which describe the nuclei as moderately deformed (β₂≈ 0.25) and γ soft at low rotational frequencies. The highest experimentally observed positive-parity state in ⁶⁵Ga (41/2⁺) is in good agreement with the calculated crossing of the collective band with a non-collective one terminating at this spin. Received: 20 December 2000 / Accepted: 2 May 2001     

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

As a tool for studying the structure of nuclei far off stability the technique of γ-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable ³⁶S beam on deuteron and ⁹Be targets. It is demonstrated that the Doppler broadening of γ lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 10⁶ particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the γ background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross-sections of neutron-rich light nuclei on ²H and ⁹Be targets at REX-ISOLDE energies of 2.2 MeV ^. A are confirmed. Received: 9 October 2000 / Accepted: 28 January 2001     

Muon capture on neutron-rich nuclei

Future facilities will allow the exploration of extremely neutron-rich nuclei far from the valley of stability. It is discussed that the strong-neutron excess results in changes in the collective excitations of such nuclei compared to conventional stable nuclei. We propose muon capture as an experimental tool to explore such changes. We will quantify our discussion by the calculation of the total and differential muon capture rates on selected calcium isotopes between ⁴⁰Ca and ⁶⁰Ca. Our calculations are based on the random phase approximation and agree nicely with the measured rates for ⁴⁰Ca and ⁴⁴Ca. Received: 6 April 2001 / Accepted: 14 May 2001     

Development of magnetic rotation in light Gd nuclei; study of 142Gd

High-spin states in ¹⁴²Gd have been populated by means of the ⁹⁹Ru(⁴⁸Ti, 2p3n) reaction at 240 MeV and investigated with the γ-spectrometer EUROBALL III and the charged-particle detector array ISIS. The features of four dipole bands have been determined and compared with tilted-axis cranking model calculations indicating that they are magnetic rotational bands. A transition from regular to irregular bands has been found approaching N = 82 demonstrating that this is a general phenomenon in nuclei near a double-shell closure. Received: 9 December 2001 / Accepted: 9 January 2002     

Evidence for a proton halo in 27P through measurements of reaction cross-sections at intermediate energies

Measurements of reaction cross-sections ( σ_R's) for some proton-rich nuclei ( N = 11–15 isotones) on carbon target at intermediate energies have been performed on RIBLL of HIRFL. A larger enhancement of the σ_R for ²⁷P has been observed than for its neighboring nuclei. A large difference between the proton and neutron density distributions (proton halo) is necessary to explain the enhanced cross-section for ²⁷P within the framework of the Glauber model. Density distributions with HO-type core plus Yukawa-square tail and rms radii for ²⁷P have been deduced from the measured σ_R data for the first time, which conform the long tail in its densities as predicted by RMF calculations. Received: 23 May 2001 / Accepted: 7 November 2001     

Low-spin states of doubly odd 182Au

Low-lying states in the odd-odd light gold isotope ¹⁸²Au have been populated by the β⁺/EC decay of the ¹⁸²Hg nuclei. Level energies, spin and parity values were determined using on-line γ-ray and e^- spectroscopy measurements. No isomeric state with a long half-life was found in the ¹⁸²Au nucleus. Spin and parity I ^π = 2⁺ can be very likely assigned to the ¹⁸²Au ground state. Received: 19 December 2000 / Accepted: 28 February 2001     

Analysis of decay data from neutron-rich nuclei

The β-decays of the neutron-rich nuclei ¹²Be and ²⁹Ne have been studied. The statistical correlations between the almost identical half-lives of ¹²Be and its daughter ¹²B are analysed for a large sample of ¹²Be decay data. Stringent mutual bounds are obtained on the parameter set, leading to a precise determination of the ¹²Be half-life of 21.50±0.04 ms. From a simultaneous detection of β-particles and neutrons from the decay of ²⁹Ne the neutron emission probability, P_n, is determined to 17±5%. No indication of two-neutron emission is seen from this nucleus. An upper limit of 2.2% (90% confidence level) is established for P_2n. Received: 3 May 2001 / Accepted: 6 August 2001     

Decoupling and coherence in E1 and E2 moments in drip line nuclei

I discuss first the effect of decoupling of extended wave functions and the coherence in the low-energy E1 strength in drip line nuclei ¹²Be and ¹³O, which are studied by large-scale shell model calculations including 3 ?ω configuration space. The calculated results are compared to recent experimental data of Coulomb excitations. The quenching of the core polarization charges in drip line nuclei is also discussed in relation to recent observations of quadrupole moments in B-isotopes. Received: 1 May 2001 / Accepted: 4 December 2001     

Quantum Monte Carlo calculations of light nuclei

Quantum Monte Carlo calculations using realistic two- and three-nucleon interactions are presented for nuclei with up to ten nucleons. Our Green's function Monte Carlo calculations are accurate to ∼1-2% for the binding energy. We have constructed Hamiltonians using the Argonne v₁₈ NN interaction and reasonable three-nucleon interactions that reproduce the energies of these nuclear states with only ∼500 keV rms error. Other predictions, such as form factors, decay rates, and spectroscopic factors also agree well with data. Some of these results are presented to show that ab initio calculations of light nuclei are now well in hand. Received: 1 May 2001 / Accepted: 4 December 2001     

The new isotope 270110 and its decay products 266Hs and 262Sg

The even-even nucleus ²⁷⁰110 was synthesized using the reaction ⁶⁴Ni + ²⁰⁷Pb. A total of eight α-decay chains was measured during an irradiation time of seven days. Decay data were obtained for the ground-state and a high-spin K isomer. The new nuclei ²⁶⁶Hs and ²⁶²Sg were identified as daughter products after α-decay. Spontaneous fission of ²⁶²Sg terminates the decay chain. The measured data are in agreement with calculations using the macroscopic-microscopic model and with self-consistent HFB calculations with Skyrme-Sly4 interaction. Received: 27 November 2000 / Accepted: 16 January 2001     

Probing shell structure in neutron-rich nuclei with in-beam γ-spectroscopy

The structure of neutron-rich light nuclei around N = 20 and 28 has been investigated at GANIL by means of in-beam gamma-spectroscopy using fragmentation reactions of ³⁶S and ⁴⁸Ca beams on a Be target. Gamma-decay of relatively high-lying excited states have been measured for the first time in nuclei around ³²Mg and ⁴⁴S. Level schemes are proposed and discussed for a large number of these neutron-rich nuclei around N = 20 and N = 28. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: azaiez@ipno.in2p3.fr     

Spectroscopy on very neutron-rich nuclei beyond N = 20

Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator (RIPS) are introduced. Special emphasis is given to two selected experiments from recent programs that highlight studies on the magicity loss observed for very neutron-rich nuclei beyond N = 20 in the “island-of-inversion” region; the particle stability of ³¹F, and the low-lying excited states of ³⁴Mg. Received: 1 May 2001 / Accepted: 4 December 2001