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Level lifetimes for the candidate chiral doublet bands of $^{80}$Br were extracted by means of the Doppler-shift attenuation method. The absolute transition probabilities derived from the lifetimes agree well with the M1 and E2 chiral electromagnetic selection rules, and are well reproduced by the triaxial particle rotor model calculations. Such good agreements among the experimental data, selection rules of chiral doublet bands and theoretical calculations are rare and outstanding in researches of nuclear chirality. Besides odd-odd Cs isotopes, odd-odd Br isotopes in the A≈ 80 mass region represent another territory that exhibits the ideal selection rules expected for chiral doublet bands.

The neutron-deficient nucleus 75Kr has been studied in two EUROBALL experiments. The analysis yielded a considerably extended level scheme including two newly observed excited high spin bands. The results are interpreted in the framework of the cranked Nilsson-Strutinsky approach. The calculations compare well to the experimentally established level scheme and predict the nucleus to be mainly prolate or triaxially deformed at high spin. Evidence for an oblate-prolate shape coexistence could not be found at high spin.

High-spin states in 73Kr were studied at the XTU tandem accelerator of the Laboratori Nazionali di Legnaro using the reaction 40Ca(40 Ca,2pn) at a beam energy of 185 MeV. Gamma rays were detected with the EUROBALL spectrometer. Particle detection enabled us to identify the reaction channel leading to 73Kr. The yrast bands of positive and negative parity were established up to probable spins of 61/2+ and 63/2- , respectively. The energies of the two identified bands agree well with the predictions of the configuration-dependent Cranked Nilsson-Strutinsky approximation and indicate that the nucleus remains collective up to the maximum spins of the respective high-spin configurations. The positive-parity band represents one of the rare cases where band non-termination at the highest spin of the configuration could be established experimentally.

High-spin states in Xe126 have been populated in the Se82(Ca48,4n)Xe126 reaction in two experiments, one at the VIVITRON accelerator in Strasbourg using the Euroball detector array, and a subsequent one with ATLAS at Argonne using the Gammasphere Ge-detector array. Levels and assignments made previously for Xe126 up to I=20 have been confirmed and extended. Four regular bands extending to a spin of almost I=60, which are interpreted as two pairs of signature partners with opposite parity, are identified for the first time. The α = 0 partner of each pair is connected to the lower-lying levels, whereas the two α = 1 partners remain floating. A fractional Doppler shift analysis of transitions in the strongest populated (Ï€,α)=(-,0) band provides a value of 5.20.50.4 b for the transition quadrupole moment, which can be related to a minimum in the potential-energy surface calculated by the ULTIMATE CRANKER cranked shell-model code at Îμâ‰0.35 and Îâ‰5°. The four lowest bands calculated for this minimum compare well with the two signature pairs experimentally observed over a wide spin range. A sharp upbend at â.,ω~1170 keV is interpreted as a crossing with a band involving the j15/2 neutron orbital, for which pairing correlations are expected to be totally quenched. The four long bands extend to within â5 spin units of a crossing with an yrast line defined by calculated hyperdeformed transitions and will serve as important stepping stones into the spin region beyond 60ħ for future experiments.

To be published in Physical Review C; High spin states in $^{126}$Xe have been populated in the $^{82}$Se($^{48}$Ca,4n)$^{126}$Xe reaction in two experiments, one at the VIVITRON accelerator in Strasbourg using the Euroball detector array, and a subsequent one with ATLAS at Argonne using the Gammasphere Ge-detector array. Levels and assignments made previously for $^{126}$Xe up to I = 20 have been confirmed and extended. Four regular bands extending to a spin of almost I = 60, which are interpreted as two pairs of signature partners with opposite parity, are identified for the first time. The α = 0 partner of each pair is connected to the lower-lying levels, while the two α = 1 partners remain floating. A peak-shape analysis of transitions in the strongest populated (π, α) = (-,0) band provides a value of $5.2 ^0.4 _0.5$ b for the transition quadrupole moment, which can be related to a minimum in the potential-energy surface calculated by the Ultimate Cranker cranked shell model code at ε ≈ 0.35 and γ ≈ 5 0. The four lowest bands calculated for this minimum compare well with the two signature pairs experimentally observed over a wide spin range. A sharp upbend at $\hbar$ω ∼1170 keV is interpreted as a crossing with a band involving the $j_15/2$ neutron orbital, for which pairing correlations are expected to be totally quenched. The four long bands extend to within ∼5 spin units of a crossing with an yrast line defined by calculated hyperdeformed transitions and will serve as important stepping stones into the spin region beyond 60 $\hbar$ for future experiments.

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient $\gamma$ -spectrometer MINIBALL. Using $^{30}$Mg ions accelerated to an energy of 2.25MeV/u together with a thin $^{nat}$Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative de-excitation $\gamma$ -ray yields the B(E2; 0$^{+}_{gs} \rightarrow 2^{+}_{1}$) value of $^{30}$Mg was determined to be 241(31)$e^{2}$fm$^{4}$. Our result is lower than values obtained at projectile fragmenttion facilities using the intermediate-energy Coulomb excitation method and confirms that the theoretical conjecture that the neutron-rich magnesium isotope $^{30}$Mg lies still outside the "island of inversion". We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin nat-Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative de-excitation gamma ray yields the B(E2: 0+ -> 2+) value of 30Mg was determined to be 241(31) e2fm4. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope 30Mg lies still outside the ``island of inversion''.

A survey of parity-changing E1 transitions has been pursued in Ngreater than or similar toZ nuclei near Ni-56 using data from an EUROBALL experiment. Linear polarization measurements have been combined with angular correlations of coincident gamma rays to determine the electromagnetic character of numerous transitions. The impact of the results on shell-model parametrizations is briefly discussed.

15 págs.; 9 figs.; 5 tabs. ; PACS number(s): 23.20.Lv, 21.10.Re, 27.50.1e, 21.60.2n, Excited levels in 69Se have been studied using the 40Ca(32S,2pn)69Se reaction at 95- and 105-MeV beam energy, γ rays have been detected with the EUROBALL spectrometer operated in conjunction with the neutron wall and the charged-particle detector array EUCLIDES. New level sequences with positive and negative parities have been identified from n-γγ and n-γγγ coincidences. Spins have been assigned to many of the levels on the basis of angular distribution and directional correlation measurements. Excitation energies of the positive-parity yrast band and the branching ratios of its decay are compared with the predictions of the rigid triaxial rotor plus particle model. ©2004 The American Physical Society, A. J. acknowledges financial support from the Deutsche Forschungsgemeinshaft (DPG) within the Heisenberg program. This work was supported by BMBF under Contract Nos. 06 OK 958 and 06 GÖ 951 and the EUROVIV Contract No. HPRI-CT-1999-000783.

Excited levels in 69As were studied using the 40Ca(32S, 3p)69 As reaction at 95 and 105 MeV beam energy. Gamma rays were detected with the EUROBALL spectrometer operated in conjunction with the Neutron Wall and the charged-particle detector array EUCLIDES. New level sequences with positive and negative paity were identified from 3p-γγ and γγγ coincidences. Spins were assigned to many of the levels on the basis of directional correlations of oriented states measurements. The previously observed positive-parity band was extended to spin 45/2+ and it was found to exhibit a crossing with another configuration identified up to spin 53/2+. Three negative-parity bands were observed for the first time up to spins (43/2-), 39/2-, and (41/2-). The previously known band was extended to spin (49/2-). No evidence for superdeformation was found, despite very high counting statistics. Configurations were assigned to each of the observed bands through comparisons to the cranked Nilsson-Strutunsky calculations.

High-angular-momentum states in 64Zn were populated in the 40Ca( 28Si,4p) reaction at a beam energy of 122 MeV. Evaporated, light, charged particles were identified by the Microball, while γ rays were detected using the Gammasphere array. The main focus of this paper is on two strongly coupled, collective bands. The yrast band, which was previously known, has been linked to lower-lying states establishing the excitation energies and angular momenta of in-band states for the first time. The newly identified excited band decays to the yrast band but firm angular-momentum assignments could not be made. In order to interpret these structures cranked-Nilsson-Strutinsky calculations have been performed. The calculations have been extended to account for the distribution of nucleons within a configuration. The yrast collective band is interpreted as based on the π(f 7/2) -1(p 3/2f 5/2) 2(g 9/2) 1 ν(p 3/2f 5/2) 4(g 9/2) 2 configuration. There are several possible interpretations of the second band but it is difficult to distinguish between the different possibilities.

PACS number(s): 23.20.Lv,21.10.Re, 27.50.+e,21.60.-n

Excited levels in 69Se have been studied using the 40Ca(32S,2pn)69Se reaction at 95- and 105-MeV beam energy. γ-rays have been detected with the EUROBALL spectrometer operated in conjunction with the wall and the charged-particle detector array EUCLIDES. New level sequences with positive and negative parities have been identified from n-γγ and n-γγγ coincidences. Spins have been assigned to many of the levels on the basis of angular distribution and directional correlation measurements. Excitation energies of the positive-parity yrast band and the branching ratios of its decay are compared with the predictions of the rigid triaxial rotor plus particle model.

The decay-out process of the yrast superdeformed band in 59Cu was studied. The spin, parity, excitation energy and the configuration of states were determined. A theoretical model was introdued that included a residual interaction and tunneling matrix element between the bands. The decay was found to occur through a small number of observed doorway states.

The nucleus 70Se was studied using the 40Ca(40Ca, 2α) reaction at a beam energy of 185 MeV. Gamma rays were measured with the EUROBALL III spectrometer. The known positive-parity bands have been extended and one new band of positive parity and two of negative parity have been identified. These bands are interpreted in terms of the cranked Nilsson-Strutinsky approach. Calculations suggest that the two negative-parity bands, which have the same signature, are both based on a configuration with two protons and three neutrons lifted from the fp shell to the g9/2 orbital, but at different nuclear shapes. This represents a shape coexistence at high spin.

Excited states of the nucleus 79Br were investigated via the reaction 76Ge(7Li,4n) at 35 MeV. Coincidence data of emitted gamma rays were measured with an arrangement of six EUROBALL CLUSTER detectors. The E2 bands built on the 9/2+ and 3/2- states were extended up to J = 37/2 at E \approx 8.8 MeV. The M1 band starting with a 15/2- state at 2.6 MeV was extended to J = (29/2) at E = 6.4 MeV. Cross-over E2 transitions within this band were observed for the first time. Mean lifetimes of 17 levels were deduced using the Doppler-shift-attenuation method. The M1 band can be described within theTilted-Axis-Cranking model on the basis of the tilted three-quasiparticle configuration pi(g9/2) nu (g9/2) nu (fp) which has a triaxial shape. This band appears as a mixed case including contributions of both, magnetic and collective rotation.

Gamma rays from the N = Z-2 nucleus (50)Fe have been observed, establishing the rotational ground state band up to the state J(pi) = 11+ at 6.994 MeV excitation energy. The experimental Coulomb energy differences, obtained by comparison with the isobaric analog states in its mirror (50)Cr, confirm the qualitative interpretation of the backbending patterns in terms of successive alignments of proton and neutron pairs. A quantitative agreement with experiment has been achieved by exact shell model calculations, incorporating the differences in radii along the yrast bands, and properly renormalizing the Coulomb matrix elements in the pf model space.

High-spin states in 90Y were populated in the 82Se(11B,3n) reaction at a beam energy of 37 MeV. Gamma rays were detected with the spectrometer GASP. The level scheme of 90Y was extended up to J^\pi = (18^+) at 9.6 MeV. Mean lifetimes of four levels were determined using the Doppler-shift-attenuation method. The structure of 90Y was interpreted in terms of the shell model. The calculations were performed in the model space \pi(0f_{5/2},1p_{3/2},1p_{1/2},0g_{9/2}) \nu(1p_{1/2},0g_{9/2},1d_{5/2}) and in an extended space including the \nu(0g_{7/2})$ orbital as well. The calculations in the extended model space reveal a correspondence between states in ^{90}Y and ^{89}Y. Moreover, a combination of the predicted states with J^\pi \geq 14^(+) can be found that reproduces the large B(M1) values of up to about 1 W.u.

The nucleus 68Ge has been studied by gamma-ray spectroscopy following its population at high spin in the reaction 40Ca(32S,4p) 68Ge. The reaction channel was selected with the Microball array and gamma rays were detected with the Gammasphere array. The level scheme is very complex, reflecting the many different, and presumably mixed, excitation modes in this nucleus. Nevertheless, there appear to be some simplifications in the spin range above 18ℏ where we have identified a superdeformed band and several terminating bands. The results are compared with a cranked Nilsson-Strutinsky model without pairing.

High-spin states in 72Br were studied with the EUROBALL III spectrometer . The negative-parity band observed in this experiment displays a signature inversion around spin I = 16. The interpretation within cranked Nilsson-Strutinsky approach shows that this signature pattern is a signal of a substantial triaxial shape change with increasing spin where the nucleus evolves from a triaxial shape with rotation around the intermediate axis at low spin through an axially symmetric prolate one to a triaxial shape but with rotation around the shortest principal axis at high spin. This is the first case in which signature inversion was clearly related to varying gamma deformation.

High-spin states in 72Br and 73Br were studied with the EUROBALL III spectrometer. The negative-parity band observed in 72Br displays a signature inversion around spin I = 16. Fully consistent cranked Nilsson-Strutinsky calculations show that this signature inversion can only be understood if the nuclear shape evolves from a triaxial shape with rotation around the intermediate axis at low spin through a collective prolate shape to a triaxial shape with rotation around the shortest principal axis at high spin. Rotational bands in 73Br were observed up to spins of I = 65/2 at excitation energies of 26 MeV. Cranked Nilsson-Strutinsky calculations indicate that one of the negative-parity bands terminates at spin 63/2. Since this spin coincides with the maximum spin observed in the corresponding experimental band, we conclude that we observed this band up to its termination.

The doubly magic superdeformed band in the N = Z nucleus 60Zn has been identified. Linking transitions connecting this band to the yrast line provide the first spin, parity, and excitation energy measurements for superdeformed states in the A˜60 region. The stretched- E2 character and relatively large B(E2) values of these transitions suggest a nonstatistical decay-out process.

The doubly magic superdeformed band in the N = Z nucleus Zn-60 has been identified. Linking transitions connecting this band to the yrast line provide the first spin, parity, and excitation energy measurements for superdeformed states in the A similar to 60 region. The stretched-E2 character and relatively large B(E2) values of these transitions suggest a nonstatistical decay-out process.