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     

Proton-neutron structure of collective excitations in 94Mo

Due to the recent development of radioactive beam production, various direct reaction studies in reversed kinematics have been made to investigate the behavior of the N = 20 shell closure in the neutron-rich region. Coulomb excitation, proton inelastic scattering, and fragmentation of unstable nuclei have been studied with γ-ray detection. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: motobaya@rikkyo.ne.jp     

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     

New region of deformation in the neutron-rich 60 24Cr36 and 62 24Cr38

The neutron-rich nuclei ₂₃ ^60-63V have been produced at GANIL via interactions of a 61.8A·MeV ⁷⁶Ge beam with a ⁵⁸Ni target. Beta-decay to ₂₄ ^60-63Cr has been investigated using combined β- and γ-ray spectroscopy. Half-lives of the ^60-63V nuclei have been determined, and the existence of a beta-decay isomer in the ⁶⁰V nucleus is strongly supported. The observation of low-energy 2⁺ states in ⁶⁰Cr (646keV) and ⁶²Cr (446keV) suggests that these isotopes are strongly deformed with β₂ ∼ 0.3. This is confirmed by shell model calculations which show the dominant influence of the intruder g and d orbitals to obtain low 2⁺ energies in the neutron-rich Cr isotopes. Received: 13 June 2002 / Accepted: 27 August 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: sorlin@ipno.in2p3.fr Communicated by D. Guerreau     

Intense radioactive-ion beams produced with the ISOL method

For fifty years the isotope separation on-line (ISOL) technique has been used for the production of radioactive-ion beams (RIBs). Thick-target ISOL facilities can provide very intense RIBs for a wide range of applications. The important design parameters for an ISOL facility are efficiency, rapidity and selectivity of all steps of the separation process. To achieve the anticipated beam intensities with the next-generation RIB facilities, the production rate in the ISOL target has to be increased by orders of magnitude. This is only possible by adapting the projectile beam for optimum production cross-sections and simultaneously minimizing the target heating due to the electronic stopping power of charged-particle projectiles. ISOL beams of 75 different elements have been produced up to now and further beam development is under way to produce a still greater variety of isotopes and to improve existing beams in intensity and purity. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: Ulli.Koster@cern.ch     

Rotational bands and signature inversion phenomena in πh9/2⊗νi13/2 and πi13/2⊗νi13/2 structures in odd-odd 176Ir

The ^109,111,113Rh nuclei have been produced as fission fragments in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85 MeV. Their level schemes have been built from gamma-rays detected using the Euroball IV array. High-spin states of the neutron-rich ^111,113Rh nuclei have been identified for the first time. Several rotational bands with the odd proton occupying the πg _9/2, πp _1/2 and π(g _7/2/d _5/2) sub-shells have been observed. A band of low-energy transitions has been identified at excitation energy around 2 MeV in ^109,111Rh, which can be interpreted in terms of three-quasiparticle excitation, πg _9/2νh _11/2νg _7/2/d _5/2. In addition another structure built on states located at low excitation energy (608 keV in ¹¹¹Rh, 570 keV in ¹¹³Rh) points out that, as already observed in the lighter isotopes ^107,109Rh, triaxial deformation plays a role in the neutron-rich Rh isotopes well beyond the mid-shell. Received: 15 July 2002 / Accepted: 9 October 2002 / Published online: 3 December 2002 RID="a" ID="a"e-mail: porquet@csnsm.in2p3.fr RID="b" ID="b"Present address: CSNSM IN2P3-CNRS and Université Paris-Sud 91405 Orsay, France. RID="c" ID="c"Present address: CEA/Saclay, DSM/DAPNIA/SPhN, 91191 Gif-sur-Yvette Cedex, France. Communicated by D. Schwalm     

Large-scale shell model calculations for exotic nuclei

Recent shell model calculations for the neutron-rich nuclei around the magic numbers N = 20, N = 28 and N = 40 are reviewed. We stress two points: i) The crucial role played by the monopole part of the effective interaction that determines the evolution of the spherical mean field. In particular, the reduction in the quasiparticle gaps at the magic numbers can erode or even erase the shell closures. ii) The rich variety of structures that can be found in these situations, with coexisting deformed and spherical states, rapid changes of behaviour with N or Z, and the massive occurrence of intruder states as ground states. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: alfredo.poves@uam.es     

Haloes and clustering in light, neutron-rich nuclei

Clustering is a relatively widespread phenomenom which takes on many guises across the nuclear landscape. Selected topics concerning the study of halo systems and clustering in light, neutron-rich nuclei are discussed here through illustrative examples taken from the Be isotopic chain. The production and detection of multineutron clusters is also briefly presented. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: orr@caelav.in2p3.fr     

High-spin states following multi-nucleon transfer

High-spin states in neutron-rich nuclei, populated following deep-inelastic multi-nucleon transfer, have been studied using the GAMMASPHERE array at the LBNL, USA. A ⁶⁴Ni beam at an energy ∼ 15% above the Coulomb barrier was incident upon a thick ²⁰⁸Pb target, leading to the population of more than 130 different nuclei. The strongest channels correspond to nuclei close to the projectile and target, although transfer of up to 50 nucleons has been observed. New high-spin states in neutron-rich ^60,62Fe and ^68,70,72Zn nuclei have been observed. Some limitations of this method of high-spin spectroscopy are discussed, including the apparent difficulty of populating odd-odd and odd-even isotopes via this type of reaction. The data have been searched for superdeformed (SD) states in the A = 190-200 region, but no evidence for their presence has been found. Received: 14 April 2000 / Accepted: 12 September 2000     

Isomeric decay of 67Fe --Evidence for deformation

Decay-spectroscopy study of the ^67mFe isomer has been performed at GANIL. This isomer is found to have an energy of 387 keV and a half-life of 75(21) μs. An intermediate excited state is introduced at 367 keV. The results are interpreted in terms of various nuclear models, and a deformed shape is inferred for ⁶⁷Fe. Received: 12 August 2002 / Accepted: 12 September 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: sawicka@ganil.fr and sawicka@zsjlin.igf.fuw. edu.pl Communicated by W. Henning     

In-beam spectroscopy of 253, 254No

In-beam conversion electron spectroscopy experiments have been performed on the transfermium nuclei ^{253, 254}No using the conversion electron spectrometer SACRED in nearly collinear geometry in conjunction with the gas-filled separator RITU at the University of Jyv?skyl?. The experimental setup is discussed and the spectra are compared to Monte Carlo simulations. The implications for the ground-state configuration of ²⁵³No are discussed. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: rdh@ns.ph.liv.ac.uk RID="b" ID="b"Present address: GANIL, F-14021 Caen, France. RID="c" ID="c"Permanent address: IReS Strasbourg, IN2P3-CNRS, F-67037-Strasbourg, France. RID="d" ID="d"Present address: CEA/DIF DCRE/SDE/LDN F-91680 Bruyeres-le-Chatel. RID="e" ID="e"Present address: Daresbury Laboratory, Daresbury WA4 4AD, UK. RID="f" ID="f"Permanent address: IPN Lyon, IN2P3-CNRS, F-69037 Lyon, France.     

Isomeric states in proton-unbound 187, 189Bi isotopes

Prompt and delayed gamma-ray spectroscopy of very neutron-deficient bismuth isotopes ^{187, 189}Bi has been performed using the Recoil Decay Tagging (RTD) method. The isomeric i _13/2 states have been identified and their lifetimes have been measured. The systematics of these long-lived M2 isomers has been extended to the proton-unbound isotopes. The general behaviour of single-proton states is discussed within the systematics and interpreted within the shell-model framework. Received: 2 May 2002 / Accepted: 3 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: wkorten@cea.fr RID="b" ID="b"Present address: GANIL, Caen, France. RID="c" ID="c"Present address: Radiation and Nuclear Safety Authority, P.O. Box 14, FIN-00881 Helsinki. Communicated by D. Schwalm     

First evidence for triaxial superdeformation in 161Lu and 162Lu

High-spin states in ¹⁶¹Lu and ¹⁶²Lu have been investigated using the GASP γ-ray spectrometer array. Excited states in these nuclei have been populated through the ¹⁰⁰Mo(⁶⁵Cu, xn) reaction at a beam energy of 260 MeV. Four presumably triaxial superdeformed bands, three in ¹⁶²Lu and one in ¹⁶¹Lu, have been observed. This is the first evidence for triaxial superdeformation in the two isotopes. Received: 18 October 2002 / Accepted: 21 November 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: bringel@iskp.uni-bonn.de RID="b" ID="b"Present address: Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06520, USA. RID="c" ID="c"Present address: Physik Department, Technische Universit?t München, James Franck Str., D-85748 Garching, Germany. Communicated by D. Schwalm     

On the difference between proton and neutron spin-orbit splittings in nuclei

The latest experimental data on nuclei at ¹³²Sn permit us for the first time to determine the spin-orbit splittings of neutrons and protons in identical orbits in this neutron-rich doubly magic region and compare the case to that of ²⁰⁸Pb. Using the new results, which are now consistent for the two neutron-rich doubly magic regions, a theoretical analysis defines the isotopic dependence of the mean-field spin-orbit potential and leads to a simple explicit expression for the difference between the spin-orbit splittings of neutrons and protons. The isotopic dependence is explained in the framework of different theoretical approaches. Received: 13 February 2002 / Accepted: 20 February 2002     

Conversion electron measurement in the β<Superscript>-</Superscript>-decay of <Superscript>151</Superscript>Pr

The β^--decay of ¹⁵¹Pr produced by the thermal neutron-induced fission of ²³⁵U has been studied using an on-line isotope separator. From an internal-conversion electron measurement with a Si(Li) detector, K-conversion coefficients were obtained for 20 γ-transitions. Spins and parities of 6 excited levels in ¹⁵¹Nd were newly determined from the deduced multipolarities: even parities for the 543 and 627 keV levels, odd parities for 250 and 599 keV, (3/2, 5/2)⁺ for 685 keV, and (1/2, 3/2)⁺ for 880 keV. The level structure was compared with the rotation-vibration coupling Nilsson model. Received: 28 June 2002 / Accepted: 31 October 2002 / Published online: 25 February 2003 RID="a" ID="a"e-mail: ykojima@hiroshima-u.ac.jp Communicated by J. ?yst?     

In-beam studies of very neutron-deficient heavy nuclei

The JYFL gas-filled recoil separator RITU, combined with Ge detector arrays and a SACRED magnetic solenoid spectrometer, has been successfully employed in recoil-decay-tagging (RDT) experiments in order to probe structures of very neutron-deficient heavy nuclei. The present contribution focuses on the light Pb region where the new data extend the systematics of shape-coexisting yrast states towards the proton dripline. Similarities between band structures and their relation to possible multi-particle multi-hole intruder excitations will be discussed. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: rauno.julin@phys.jyu.fi     

Ion traps --Precision measurements and more

Today ion traps are an important experimental tool. Applications range from high-precision measurements of masses and moments, realization of atomic clocks, to the study of ion chemical reactions. Ion traps have gained particular importance in the field of nuclear physics where they are used for the precise determination of nuclear binding energies, decay studies, and radioactive ion beam manipulation. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: bollen@nscl.msu.edu     

Nuclear-matter distributions of halo nuclei from elastic proton scattering in inverse kinematics

Proton-nucleus elastic scattering at intermediate energies, a well-established method for probing nuclear-matter density distributions of stable nuclei, was applied for the first time to exotic nuclei. This method is demonstrated to be an effective means for obtaining accurate and detailed information on the size and radial shape of halo nuclei. Absolute differential cross-sections for small-angle scattering were measured at energies near 700 MeV/u for the neutron-rich helium isotopes ⁶He and ⁸He, and more recently for the lithium isotopes ⁶Li, ⁸Li, ⁹Li and ¹¹Li, using He and Li beams provided by the fragment separator FRS at GSI Darmstadt. Experiments were performed in inverse kinematics using the hydrogen-filled ionization chamber IKAR which served simultaneously as target and recoil-proton detector. For deducing nuclear-matter distributions, differential cross-sections calculated with the aid of the Glauber multiple-scattering theory, using various parametrizations for the nucleon density distributions as input, were fitted to the experimental cross-sections. The results on nuclear-matter radii and matter distributions are presented, and the significance of the data for a halo structure is discussed. Nuclear-matter distributions obtained for ⁶He and ⁸He conform with the concept that both nuclei compose of α-particle like cores and significant neutron halos. The matter distribution in ¹¹Li exhibits, as expected from previous reaction cross-section studies with nuclear targets, the by far most extended halo component of all nuclei being investigated. In addition the present data allow a quantitative comparison of the structure of the He and Li isobares of either the mass number A = 6 or A = 8. The measured differential cross-sections have also been used for probing density distributions as predicted from various microscopic calculations. A few examples are presented. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: p.egelhof@gsi.de     

First identification of excited states in 59Zn

Excited states in ⁵⁹Zn were observed for the first time following the fusion-evaporation reaction ²⁴Mg + ⁴⁰Ca at a beam energy of 60 MeV. The GASP array in conjunction with the ISIS Silicon ball and the NeutronRing allowed for the detection of γ-rays in coincidence with evaporated light particles. The mirror symmetry of ⁵⁹Zn and ⁵⁹Cu is discussed. Received: 28 May 2002 / Accepted: 10 July 2002 / Published online: 3 December 2002 RID="a" ID="a"Present address: Oliver Lodge Laboratory, University of Liverpool, P.O. Box 147, Liverpool, L69 7ZE, United Kingdom; e-mail: ca@ns.ph.liv.ac.uk. RID="b" ID="b"Present address: Physik-Departament, Technische Universit?t München, D-85748 Garching, Germany. Communicated by W. Henning     

High-spin study of odd- A 49In isotopes beyond the neutron mid-shell

The ^{115, 117, 119, 121}In nuclei have been produced as fission fragments in three reactions induced by heavy ions: ¹²C + ²³⁸U at 90MeV bombarding energy, ¹⁸O + ²⁰⁸Pb reaction at 85MeV, and ³¹P + ¹⁷⁶Yb at 152MeV. Their level schemes have been built from gamma-rays detected using the EUROBALL III and IV arrays. High-spin states of ^{117, 119, 121}In nuclei have been identified for the first time. Moreover, isomeric states lying around 2.5MeV have been established in ^{119, 121}In from the delayed coincidences between the fission fragment detector SAPhIR and the gamma array. Most of the observed states can be described by coupling a proton g _9/2 hole to a Sn core, while the intruder band based on an orbital from the π[g _7/2/d _5/2] sub-shells behaves as the ground-state band of neighbouring Cd isotopes. Received: 24 May 2002 / Accepted: 11 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: porquet@csnsm.in2p3.fr RID="b" ID="b"Present address: CEA-DIF, DAM/DCRE/SDE/LDN, 91680 Bruyères-le-Chatel, France. RID="c" ID="c"Present address: CSNSM IN2P3-CNRS and Université Paris-Sud 91405 Orsay, France. RID="d" ID="d"Present address: CEA/Saclay, DSM/DAPNIA/SPhN, 91191 Gif-sur-Yvette Cedex, France. Communicated by D. Schwalm