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     

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     

High-spin states in <Superscript>36</Superscript>Ar and their underlying shell model configurations

Eight high-spin states in ³⁶Ar below 10MeV excitation energy, among them a prospective J ^π = 8^- state at 9408keV and the J? 8 levels of the recently discovered superdeformed rotational band, have been observed by n-γ coincidence measurements with the ³³S(α, nγ) reaction at E _α = 14.4 and 13.4MeV. High-spin assignments of, respectively, J ^π = 6⁺ and 5^- were obtained for the E _p = 1209 and 1462keV (E _x = 9682 and 9927keV) resonances of the ³⁵Cl (p,γ) reaction by a measurement of γ-ray angular distributions. The spectrum of the high-spin and of the E _x? 7.4MeV levels is decomposed according to the underlying shell model configurations with n = 0, 1, 2, 4 particles excited from the N = 2 into the N = 3 major shell. The role of four-particle excitations, all connected with large prolate distortions, is elucidated for the entire A = 36-40 mass region. Received: 21 December 2001 / Accepted: 25 March 2002     

Magnetic properties of the υj15/2 intruder orbital in the superdeformed minimum of 197Pb

This work has established eight cross-talk transitions between the two signature partner superdeformed (SD) bands in ¹⁹⁷Pb with the EUROBALL IV spectrometer. Directional correlations from oriented states measurements confirm the ΔI = 1 character of these transitions. The flat behaviour of the dynamical moment of inertia and the agreement between the experimental and microscopic HF+BCS values of (g _K - g _R)K/Q ₀ suggest that the configuration of the SD bands is based upon the υ[752]5/2^- neutron intruder orbital. The derived effective spin gyromagnetic factor g _s ^eff is found to be not quenched, and is close to the theoretical g _s ^free value. Received: 14 June 2000 / Accepted: 14 September 2000     

Signature inversion in the yrare band of <Superscript>119</Superscript>Xe

Excited states of the ¹¹⁹Xe nucleus have been studied by using in-beam γ-ray spectroscopy with the ¹⁰⁷Ag ( ¹⁶O, p3n) ¹¹⁹Xe fusion-evaporation reaction at a beam energy of 85 MeV. The level scheme of ¹¹⁹Xe has been derived from γ-γ coincidence and γ-γ angular correlation analyses. We have, for the first time, established the second negative-parity favored and unfavored states built on the 11/2^- state, namely the yrare rotational bands in ¹¹⁹Xe. In contrast to the behavior of the yrast bands where the favored states are lying lower in energy, the yrare favored states were observed to lie above the unfavored band. Such a signature inversion in ¹¹⁹Xe is changed to be normal at I = 12?. Received: 8 January 2002 / Accepted: 18 April 2002     

Investigation of the magnetic rotation in 196Pb

Lifetimes for low-lying members of the irregular ΔI = 1 band in ¹⁹⁶Pb have been measured by means of the EUROBALL spectrometer using the coincidence recoil distance technique. The ¹⁶⁴Dy(³⁶S,4n)¹⁹⁶Pb reaction at a beam energy of 168 MeV was used. The data were analyzed with the differential decay-curve method (DDCM). A characteristic drop in the reduced transition probabilities with increasing angular momentum was observed in agreement with predictions by the semiclassical model for the shears mechanism. Received: 3 May 2001 / Accepted: 20 June 2001     

Microscopic study of Gamow-Teller and spin-dipole states in 208Bi

Gamow-Teller (GT) and spin-dipole (SD) states in ²⁰⁸Bi are studied by using self-consistent Hartree-Fock + Tamm-Dancoff approximation taking into account the coupling to the continuum. Most of SD strength is found at the excitation energy E _x≈ 25MeV with a very broad width, which agrees with recent experimental observations. It is shown that Landau damping effect is responsible for the large width of SD peak, while the escape width is found to be at most 1MeV. We study also electric dipole (E1) transitions between GT and SD states in ²⁰⁸Bi. Main E1 transitions for 0^- and 1^- states are found near excitation energy expected from Brink's hypothesis in which SD states are considered as E1 resonances built on the GT state. Calculated E1 transition strengths between GT and SD states are compared with the analytic sum rules within one-particle one-hole (1p-1h) configuration space and within both 1p-1h and 2p-2h model space. Received: 11 April 2000 / Accepted: 30 August 2000     

Signature inversion in πi13/2⊗νi13/2 structure in 178Ir

High-spin states in ¹⁷⁸Ir were investigated by means of in-beam γ-ray spectroscopy techniques using the multidetector array GASP. Excited states of ¹⁷⁸Ir were populated through the ¹⁵⁹Tb(²⁴Mg, 5n) fusion-evaporation reaction at E(²⁴ Mg) = 131-141 MeV. Several rotational bands were observed. Among them, the πi _13/2⊗νi _13/2 structure has been identified up to spin 36 ?. This band exhibits an anomalous signature splitting and a signature inversion around spin 25 ?. Received: 13 December 2000 / Accepted: 11 April 2001     

Observation of signature inversion in odd-odd 178Ir

High spin states in ¹⁷⁸Ir have been studied via the ¹⁵²Sm(³¹P, 5nγ)¹⁷⁸Ir reaction through excitation functions, X-γ and γ-γ-t coincidence measurements. According to the band structure characteristics and the measured intraband B(M1)/B(E2) ratios, two rotational bands are identified and assigned to be associated with the πh _9/2⊗νi _13/2 and πh _11/2⊗νi _13/2 configurations, respectively. Both bands show the signature inversion feature. Received: 23 March 2000 / Accepted: 31 July 2000     

Inelastic scattering on 12Be and disappearance of the N = 8 magic number

Experimental studies on in-beam γ-ray spectroscopy using a ¹²Be radioactive beam are presented. Inelastic scattering of the neutron-rich ¹²Be nucleus on ²⁰⁸Pb, ¹²C, and (CH₂)_n targets has been studied by measuring de-excitation γ-rays in coincidence with scattered particles. The level schemes and transition probabilities are determined for low-lying excited states in ¹²Be. The present paper presents a brief review of the spectroscopic results, which may be associated with the N = 8 shell quenching near the drip line. Received: 1 May 2001 / Accepted: 4 December 2001     

The directed flow maximum near cs = 0

We investigate the excitation function of quark-gluon plasma formation and of directed in-plane flow of nucleons in the energy range of the BNL-AGS and for the E ^kin _Lab = 40A GeV Pb + Pb collisions performed recently at the CERN-SPS. We employ the three-fluid model with dynamical unification of kinetically equilibrated fluid elements. Within our model with first-order phase transition at high density, droplets of QGP coexisting with hadronic matter are produced already at BNL-AGS energies, E ^kin _Lab≃ 10A GeV. A substantial decrease of the isentropic velocity of sound, however, requires higher energies, E ^kin _Lab≃ 40A GeV. We show the effect on the flow of nucleons in the reaction plane. According to our model calculations, kinematic requirements and EoS effects work hand-in-hand at E ^kin _Lab = 40A GeV to allow the observation of the dropping velocity of sound via an increase of the directed flow around midrapidity as compared to top BNL-AGS energy. Received: 7 May 2000 / Accepted: 2 August 2000     

Rotational bands in the doubly odd 130Cs nucleus

The band structures built on the 5^- isomeric state ( T _1/2 = 3.46 m) in the doubly odd ¹³⁰Cs nucleus have been established up to I = 24? via the ¹²⁴Sn(¹¹B, 5n)¹³⁰Cs reaction. The previously observed bands based on the πh _11/2⊗νh _11/2, πg _7/2⊗νh _11/2 and πd _5/2⊗νh _11/2 configurations and a positive-parity side band with multiple connections to the α = 0 signature partner of the yrast πh _11/2⊗νh _11/2 band have been extended to higher spins. A new band based on the πh _11/2⊗νg _7/2 configuration is observed. The yrast πh _11/2⊗νh _11/2 band exhibits anomalous signature splitting whose magnitude decreases up to spin 15 and then increases without restoring the normal signature splitting. Received: 20 February 2001 / Accepted: 9 May 2001     

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     

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     

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     

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     

Count fluctuation analysis in the triaxial strongly deformed potential well of 163Lu

High-spin properties of the triaxial strongly deformed potential well in ¹⁶³Lu at excitation energies above resolvable bands have been investigated. Gated E _γ-E _γ spectra display clear ridges with moments of inertia corresponding to those observed for the discrete strongly deformed bands. A fluctuation analysis of the ridges yields a number of two-step paths of ≈ 40 and ≈ 20, when gating on triaxial strongly or normally deformed bands, respectively. These results show that a potential well at large deformation coexists with the normally deformed well, and indicate a mixing between states in the two wells at higher excitation energy. Received: 4 January 2002 / Accepted: 6 May 2002     

Candidates for chiral doublet bands in 136Nd

The even-even nucleus ¹³⁶Nd was studied via in-beam γ-ray spectroscopy using the ¹⁶O + ¹²⁵Te reaction at 100 MeV and the EUROBALL array. One new dipole band was observed. Together with a previously identified dipole band, whose position in the level scheme is revised, the new band forms a doublet structure similar to the recently observed chiral bands in the odd-odd neighboring nuclei. This would be the first case of a chiral doublet in an even-even nucleus. Received: 23 May 2002 / Accepted: 30 September 2002 / Published online: 27 November 2002 RID="a" ID="a"e-mail: costel.petrache@unicam.it RID="b" ID="b"Present address: Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. RID="c" ID="c"Present address: Gesellschaft für Schwerionenforschung D-64291 Darmstadt, Germany. Communicated by C. Signorini     

Quadrupole moments of superdeformed bands in 193Tl

Lifetimes of states in the two strongest superdeformed (SD) bands in ¹⁹³Tl were measured using the Doppler-shift attenuation method. The reaction ¹⁷⁶Yb(²³Na,6n)¹⁹³Tl at a beam energy of 129 MeV was used and γ-rays were detected by the Gammasphere array. Quadrupole moments of 18.3(10) eb and 17.4(10) eb were extracted for SD bands 1 and 2, respectively, using the fractional Doppler-shifts of the SD transitions. The previously reported linking transitions of these SD bands to normal deformed near yrast levels could not be confirmed. No other candidates for linking transitions could be established. Received: 27 April 1999     

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