Gamow-Teller transitions from 58Ni to discrete states of 58Cu

Under the assumption that isospin is a good quantum number, symmetry is expected for the transitions from the ground states of T = 1, T _z = ±1 nuclei to the common excited states of the T _z = 0 nucleus situated between the two nuclei. The symmetry can be studied by comparing the strengths of Gamow-Teller (GT) transitions obtained from a (p, n)-type charge-exchange reaction on a target nucleus with T _z = 1 with those from the β-decay of the T _z = - 1 nucleus. The A = 58 system is the heaviest for which such a comparison is possible. As a part of the symmetry study, we measured the GT transitions from ⁵⁸Ni (T _z = 1) to ⁵⁸Cu (T _z = 0) by using the zero-degree (³ He, t) reaction at 150 MeV/nucleon. With the achieved resolution of 50 keV, many hitherto unresolved GT states have been identified. The GT transition strengths were obtained for states up to 8 MeV excitation, i.e., near to the Q window limitation ( Q _EC = 9.37 MeV) of the β-decay from ⁵⁸Zn (T _z = - 1) to ⁵⁸Cu. The strength distribution is compared with that from shell-model calculations. Received: 24 November 2001 / Accepted: 30 January 2002     

Decay of proton-rich nuclei between 39Ti and 49Ni

Decay studies of very neutron-deficient nuclei ranging from ³⁹Ti to ⁴⁹Ni have been performed during a projectile fragmentation experiment at the GANIL/LISE3 separator. For all nuclei studied in this work, ^{39, 40}Ti, ^{42, 43}Cr, ⁴⁶Mn, ^{45, 46, 47}Fe and ⁴⁹Ni, half-lives and decay spectra have been measured. In a few cases, γ coincidence measurements helped to successfully identify the initial and final states of transitions. In these cases, partial decay scheme are proposed. For the most exotic isotopes, ³⁹Ti, ⁴²Cr, ⁴⁵Fe and ⁴⁹Ni, which are candidates for two-proton radioactivity from the ground state, no clear evidence of this process is seen in our spectra and we conclude rather on a delayed particle decay. Received: 26 July 2000 / Accepted: 20 December 2000     

High-spin proton and alpha-particle emission as probes for nuclear structure

During recent years the nuclear decay modes of discrete prompt proton and α-particle emission from (super)deformed high-spin states have been discovered in nuclei in the vicinity of ⁵⁶Ni. The latest news from experiments performed at EUROBALL and GAMMASPHERE regarding these decays are presented and discussed. Published online: 31 October 2002 RID="a" ID="a"e-mail: dirk.rudolph@kosufy.lu.se     

Evidence for an isomeric 3/2- state in 53Co

The fragmentation of a 550MeV/u primary beam of ⁵⁸Ni on a ⁹Be target has been used to measure time- and energy-correlated γ decays following the implantation of event-by-event discriminated secondary fragments into a ⁹Be stopper plate. A new isomeric γ decay with T _1/2 = 14( ) ns and E _γ = 646.2(2) keV is observed and attributed to the decay of the yrast 3/2^- state in ⁵³ ₂₇Co₂₆ . This short-lived isomeric state has been populated by means of nuclear reactions during the stopping process of the secondary fragments. The experimental findings are discussed in the framework of large-scale spherical shell model calculations in conjunction with isospin symmetry-breaking residual interactions for the A = 53 , T _z = ±1/2 mirror nuclei ⁵³Co and ⁵³Fe .     

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     

Breaking of the SU(4) limit for the Gamow-Teller strength in N ∼ Z nuclei

We have performed large-scale shell model calculations of the Gamow-Teller strength distributions in N ∼ Z pf -shell nuclei. These calculations were motivated by the experimental attempts to measure the low-lying GT strength for the even-even N = Z + 2 or N = Z - 2 nuclei ⁴⁶Ti , ⁵⁰Cr , ⁵⁴Fe and ⁶²Ge , where a sizable low-energy GT strength could be interpreted as reminiscence of SU(4) symmetry; in the limit of exact SU(4) symmetry the GT_- strength would be concentrated in a single transition to the lowest T = 0, J = 1⁺ state in the daughter. We confirm that the SU(4) symmetry is strongly broken by the spin-orbit interaction and by increasing neutron excess.     

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     

Decay studies of N ≈ Z nuclei from 75Sr to 102Sn

Neutron deficient nuclei near ¹⁰⁰Sn have been produced by fragmentation of a 1 ^. AGeV ¹¹²Sn beam. The fragments were separated, identified and stopped in a highly segmented silicon strip detector stack. This detector measured the total energy of emitted β⁺-particles. γ-radiation was measured with surrounding detectors. The half-lives for many nuclides have been determined for the first time and give important information for the following topics: For the heaviest particle-stable odd-odd nuclei ⁹⁰Rh, ⁹⁴Ag and ⁹⁸In we observed for the first time fast β-decays, compatible with superallowed Fermi transitions and confirmed such decays for ⁷⁸Y, ⁸²Nb and ⁸⁶Tc. We have also observed long-lived T = 0 states in some of these nuclei. We measured the half-lives of all rp-process waiting-point nuclei from ⁸⁰Zr up to ^{92, 93}Pd. In addition we find the proton drip line nucleus ⁷⁷Y to decay dominantly via β-decay. To study the Gamov-Teller strength in the β-decay near the doubly magic ¹⁰⁰Sn we measured the half-life, β- and γ-spectrum of ¹⁰²Sn. We propose a level scheme for the daughter nuclide ¹⁰²In and deduce the Gamov-Teller strength (B _GT = 4.0±0.6). This is one of the largest values known. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: thomas.faestermann@ph.tum.de     

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     

First observation of the ν9/2[404] orbital in the A ∼ 100 mass region

A new band, populated by the spontaneous fission of ²⁴⁸Cm and studied by means of prompt γ-ray spectroscopy using the EUROGAM2 array, was observed in ⁹⁹Zr. The 1038.8 keV band head with a half-life T _1/2 = 54(10) ns is interpreted as a K-isomer, corresponding to the 9/2[404] neutron-hole excitation. It is the first observation of this orbital in the mass A ∼ 100 region. The quadrupole moment, Q ₀ = 3.9(3) eb deduced for the new band indicates a large deformation of β = 0.41, which is produced by a specific shape-coexistence mechanism, known in other regions and now found in the A ∼ 100 nuclei. Received: 11 September 2002 / Accepted: 31 October 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: urban@fuw.edu.pl Communicated by D. Schwalm     

Band termination in the Z = N odd-odd nuclei <Superscript>46</Superscript>V and <Superscript>50</Superscript>Mn

The configuration-dependent cranked Nilsson-Strutinsky approach has been employed to study the properties and band structures at high spin in the Z = N odd-odd nuclei ⁴⁶V and ⁵⁰Mn. The observed bands are explained and terminating states are confirmed by the calculations. The calculated and observed bands are in good agreement without normalization, especially for terminating states. Possible bands with rotation around the intermediate axis and the effect of γ-deformation on the total energy of several interesting configurations are discussed. Received: 2 April 2002 / Accepted: 27 January 2003 / Published online: 15 April 2003     

Single-particle quadrupole electromagnetic transitions in p-shell and sd-shell nuclei

Electron scattering Coulomb form factors for the single-particle quadrupole transitions in p-shell and sd-shell nuclei have been studied. Core polarization effects are included through a microscopic theory that includes excitations from the core orbits up to higher orbits with 2ω excitations. The modified surface delta interaction is adopted as a residual interaction. The results are discussed for the ( 1p _1/2 ^-1↦1p _3/2 ^-1) proton transition in ¹⁵N, ( 1d _5/2↦2s _1/2) neutron transition in ¹⁷O and ( 1d _3/2 ^-1↦2s _1/2 ^-1) proton transition in ³⁹K. The inclusion of core polarization effects modifies the form factors markedly and describes the experimental data very well in both the absolute strength and the momentum transfer dependence. Received: 18 April 2002 / Accepted: 1 July 2002 / Published online: 6 March 2003 RID="a" ID="a"e-mail: baguniv@uruklink.net Communicated by A. Molinari     

Rotational bands in 169Re

Based on the systematic investigation of the data available for nuclei with A≥ 40, a Z ^1/3-dependence for the nuclear charge radii is shown to be superior to the generally accepted A ^1/3 law. A delicate scattering of data around R _c/Z ^1/3 is inferred as owing to the isospin effect and a linear dependence of R _c/Z ^1/3 on N/Z (or (N - Z)/2) is found. This inference is well supported by the microscopic Relativistic Continuum Hartree-Bogoliubov (RCHB) calculation conducted for the proton magic Ca, Ni, Zr, Sn and Pb isotopes including the exotic nuclei close to the neutron drip line. With the linear isospin dependence provided by the data and RCHB theory, a new isospin-dependent Z ^1/3 formula for the nuclear charge radii is proposed. Received: 23 September 2001 / Accepted: 21 January 2002     

The A = 51 mirror nuclei 51Fe and 51Mn

Eighteen previously unknown γ-ray transitions were identified in the T _z = - 1/2 nucleus ⁵¹Fe following the fusion-evaporation reaction ³²S(²⁸Si,2α1n)⁵¹Fe. The level scheme reaches the fully aligned I ^π = 27/2^- terminating state of the five holes in the f _7/2 shell. The 17/2^- state was found to be isomeric, and the lifetime was measured to be 2.87 ns. The mirror symmetry of ⁵¹Fe and ⁵¹Mn is discussed, and the level scheme of ⁵¹Fe is compared to shell-model calculations. Received: 20 July 2000 / Accepted: 1 August 2000     

Evidence for excited states in 95Ag

The first evidence for excited states in ⁹⁵Ag is presented. ⁹⁵Ag is the heaviest T _z = 1/2 nucleus for which gamma-rays have been identified. The reaction ⁴⁰Ca(⁵⁸Ni, 1p2n)⁹⁵Ag was used in the experiment, which resulted in the assignment of three gamma-rays to ⁹⁵Ag. A detector system consisting of the detector arrays Euroball, Neutron Wall and Euclides was used to detect gamma-rays, neutrons and charged particles, respectively. Received: 31 May 2002 / Accepted: 8 July 2002     

Beta-decay of the N = Z nucleus 72Kr

The beta-decay of the N = Z, even-even nucleus ⁷²Kr has been studied at the ISOLDE PSB facility at CERN. Measurements of βγ and βγγ coincidences have enriched the decay scheme of the daughter nucleus ⁷²Br with 27 new low spin levels. A more precise half-life of T _1/2 = 17.1(2) s has been determined. Strong feeding to the ⁷²Br ground state is established yielding an unambiguous J ^π = 1⁺ assignment for this state. Candidates for the ⁷²Br g.s. wave function are discussed in the framework of a self-consistent deformed mean-field calculation with SG2 Skyrme force and pairing correlations. A search for beta-delayed particle emission was made and an upper limit of 10^-6 for this decay branch obtained. The cumulated experimental level density of 1⁺ states has been fitted with the constant temperature formula. The comparison indicates that most likely all 1⁺ levels up to 1.2 MeV have been observed in this investigation. The corresponding nearest-neighbour level spacing does not follow a Poisson distribution. The Gamow-Teller strength distribution is compared, in terms of nuclear deformation, with different calculations made in the framework of the quasiparticle random phase approximation. Received: 7 February 2002 / Accepted: 31 October 2002 / Published online: 6 March 2003 RID="a" ID="a"e-mail: borge@pinar2.csic.es RID="b" ID="b"Present address: Centre d' Etudes Nucléaires de Bordeaux-Gradignan, Le Haut Vigneau, F-33175 Gradignan Cedex, France. RID="c" ID="c"Present address: University Mentouri, 25000 Constantine, Algeria. Communicated by J. ?yst?     

Synthesis of superheavy nuclei in the reactions of 244Pu and 248Cm with 48Ca

This paper presents results of the experiments aimed at producing long-lived superheavy elements located near the spherical-shell closures with Z ⩾ 114 and N ⩾ 172 in the ²⁴⁴Pu + ⁴⁸Ca and ²⁴⁸Cm + ⁴⁸Ca reactions. The large measured α-particle energies of the newly observed nuclei, together with the long decay times and spontaneous fission terminating the chains, offer evidence of the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^{288, 289}114 and ²⁹²116, produced in the 3n and 4n evaporation channels with cross-sections of about a picobarn. The present observations can be considered as experimental evidence of the existence of the “island of stability” of superheavy elements. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: utyonkov@sungns.jinr.ru     

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?     

First evidence for the two-proton decay of 45Fe

Decays of five ⁴⁵Fe atoms have been observed at the fragment separator of GSI. Whereas one event is consistent with the β-decay of ⁴⁵Fe accompanied by the emission of a 10 MeV proton, four decays are consistent with the emission of particle(s) of total energy 1.1±0.1 MeV. This observation represents the first evidence for two-proton ground-state radioactivity, a decay mode predicted for ⁴⁵Fe. The time distribution of the observed decay events corresponds to a half-life of 3.2^+2.6 _-1.0 ms. Received: 17 May 2002 / Accepted: 28 May 2002     

New insights into neutron-rich nuclei at high spin

With new high statistic data, new isotopes and new high-spin structures are observed in neutron-rich nuclei populated in the spontaneous fission of ²⁵²Cf. The ¹³⁵Te levels are extended, and many new levels in ^139,141Ba observed. The coexistence of collective and single particle-hole states is found in ¹³⁵Te. The N = 83 ¹³⁵Te and ¹³⁹Ba show marked differences associated with differences in their particle and hole states. New levels in ¹⁴¹Ba complete evidence for two opposite-parity doublets characteristic of stable octupole deformation. In ^114,116Pd a second backbend is observed for the first time in this mass region and the backbend in ¹¹⁸Pd occurs earlier than in ^112-116Pd because of a reduction in pairing. Gamma-type vibrational bands are seen up to 13⁺ to 15⁺ in ^104,106Mo, ^108-112Ru, and ^112-116Pd. Their behavior reflects prolate to triaxial shapes in these nuclei. The levels of ^162,164Gd are observed for the first time. As N increases toward mid-shell at 104, the moments of inertia in N = 100 ¹⁶⁴Gd show an unexpected decrease compared to N = 98 ¹⁶²Gd. The levels in ^162,164Gd form remarkable shifted identical bands with nuclei separated by 2n, 2p, α, and 2α. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: j.h.hamilton@vanderbilt.edu