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Properties of neutron-rich nuclei in the A˜160 region are important for achieving a better understanding of the nuclear structure in this region where little is known owing to diffculties in the production of these nuclei at the present nuclear physics facilities. These properties are essential ingredients in the interpretation of the rareearth peak at A˜160 in the r process abundance distribution, since theoretical models are sensitive to nuclear structure input. Predicated on these ideas, we have initiated a new experimental program at Argonne National Laboratory. During the first experiment, beams from the Californium Rare Isotope Breeder Upgrade radioactive beam facility were used in conjunction with the SATURN decay station and the X-array. We focused initially on several odd-odd nuclei, where β decays of both the ground state and an excited isomer were investigated. Because of the spin difference, a variety of structures in the daughter nuclei were selectively populated and characterized based on their decay properties. Mass measurements using the Canadian Penning Trap aimed at establishing the excitation energy of the β-decaying isomers were also carried out. Evidence was found for a change in the single-particle structure, which in turn results in the formation of a sizable N=98 sub-shell gap at large deformation. Results from the first experimental campaign using the newly-commissioned β-decay station at Gammasphere are also presented. [ABSTRACT FROM AUTHOR]

Analysis of high statistics triple coincidence fission γ data from 252Cf at Gammasphere including angular correlations yielded well-expanded high-spin level schemes with more complete and reliable spin/parity assignments for 82Ge, 118,120,122cd and 114,115Rh. Both the quasi-particle/hole couplings and quasi-rotational degrees of freedom are implied to play roles in these Cd isotopes. Evidence for triaxial shapes and octupole components in the Cd isotopes is presented. These Cd isotopes may have triaxial deformations. High-spin level schemes of 114,115Rh have been established for the first time. The existence of a relatively large signature splitting and an yrare band shows typical features of a triaxially deformed nucleus. Possible excited deformed rotational bands are observed, for the first time, in 82Ge. From the multipole mixing ratio measurement, the ground state configurations of 109,111Ru, as well as excited states in 103,107Mo and 111Ru were determined. [ABSTRACT FROM AUTHOR]

Lifetimes in the negative parity favored signature have been studied up to the Iπ = 53/2- state in the 8339Y nucleus. The reaction 58Ni(32S,α3p) at Elab = 135 MeV was utilized to populate highspin states in 83Y. Lifetimes were measured using the Doppler shift attenuation method with the Gammasphere and Microball arrays. The evolution of the transitional quadrupole values are in agreement with a possible band termination. [ABSTRACT FROM AUTHOR]

The coincident detection of particles and gamma rays allows the study of the structure of exotic nuclei via inverse kinematics reactions using radioactive ion beams and thick targets. We report on the status of the project to couple the highresolution charged-particle detector ORRUBA to Gammasphere, a high-efficiency, high-resolution gamma ray detector. [ABSTRACT FROM AUTHOR]

Nuclear properties of nuclei with a few valence nucleons outside the doubly-magic 132Sn core and located in the octupole correlation region have been investigated via γ-γ-γ coincidence measurements of prompt gamma-ray emitted in the spontaneous fission of 252Cf with Gammasphere. The high spin level scheme of 134I has been identified for the first time. Shell model calculations reproduce the level scheme quite well. The level schemes of 137I and 139Cs have been reinvestigated and extended. Their nuclear structure is well described by realistic shell model calculations. The g-factors of the 4+ state in 134Te, 15/2+ state in 135I, and 15/2- state in 137Xe were determined using a newly developed program for angular correlation analysis. The measured g-factors compared favorably with shell model calculations. Octupole correlations are proposed in 141Cs and 142Cs. The variations of D0 in the Cs isotopes exhibit a pronounced drop of dipole moment with increasing neutron number. [ABSTRACT FROM AUTHOR]

With our high statistics γ-γ-γ data set taken with Gammasphere, we have identified a variety of new phenomena in A = 100–112 nuclei. The one- and two- phonon γ bands in 104,106Mo have been extended and similar bands identified in 108Mo and for the first time in both an odd N nucleus, 105Mo and an odd Z nucleus, 103Nb. The one- and two- phonon γ band energy levels in 104,105,106,108Mo are remarkably similar with a trend toward lower energy as N increases. In 108,110,112Ru the one phonon γ bands show a completely different behavior from 104–108Mo with very large odd-even spin level energy staggering in 112Ru. Our data indicate a maximum triaxiality around 112Ru. In 100Nb which lies between N = 58 and 60 where a sudden shift from spherical to superdeformed structures occurs in Sr and Zr nuclei, we have discovered a Kπ = 1+ deformed ground state band. In 108Tc, we found the first pseudospin partner bands with nearly degenerate energies in this mass region. [ABSTRACT FROM AUTHOR]

During the last years, several Cm and Cf isotopes have been studied by our research group in the frame of a systematic investigation of gas emission characteristics in ternary fission. Here we report on new results on the energy distribution and the emission probability of 3H, 4He and He particles emitted in the spontaneous ternary fission of 250Cf (Eexc = 0 MeV) and in the neutron induced ternary fission of 249Cf (Eexc = 6.625 MeV). Both measurements were performed using suited and well-calibrated ΔE-E telescope detectors, at the IRMM (Geel, Belgium) for the spontaneous fission and at the very intense neutron beam PF1b at the Institute Laue-Langevin (Grenoble, France) for the neutron induced fission measurement. In this way, the existing database can be enlarged with new results for Z = 98 isotopes, which is important for the systematic investigation. Moreover, the investigation of the ‘isotope couple’ 249Cf(n,f)–250Cf(SF), together with corresponding data for other isotopes, will yield valuable information on the influence of the excitation energy on the particle emission probabilities. [ABSTRACT FROM AUTHOR]

In an experiment carried out at the 4π Ge detector facility Gammasphere the yields of ternary fission events of 252Cf accompanied by the emission of He, Be and C LCPs were measured. For the first time independent yields were measured for 59 pairs of secondary fragments of the He ternary fission and for many fission fragments detected together with the He, Be and C LCPs. With a reasonable assumption that the detected LCPs predominantly are 4He, 10Be and 14C their yields and the charge distributions of fission fragments were conjointly analyzed in terms of the “energy cost” concept first suggested by Halpern. [ABSTRACT FROM AUTHOR]

Over the last decade, we have performed in-beam experiments using Gammasphere+FMA to measure excited states in proton-rich Au, Hg, Tl and Pb isotopes. In these studies, the use of the FMA is essential in order to differentiate evaporation residues from the large fission background which dominates the reaction cross-section. In addition, we have found that using near-symmetric reactions at bombarding energies near the Coloumb barrier is beneficial in performing these studies. By keeping the bombarding energy low, fission is minimized and the reaction products are concentrated in only a few channels. New results have recently been obtained using the 90Zr+92Mo reaction to study shape co-existence in 181Tl via the lp evaporation channel. In addition, we have measured the total γ-ray energy and multiplicity associated with the surviving compund system, 179Au, following the fusion reaction, 90Zr+89Y. [ABSTRACT FROM AUTHOR]

Excited states in 124Xe have been studied via the 12C(124Xe,124Xe*) Coulomb excitation reaction. Relative cross-sections have been determined from the γ-ray yields observed with Gammasphere. The E2 strengths between the low-spin states in 124Xe have been determined. The results clearly show the realization of the O(5) symmetry even for the off-yrast states with high τ, while the O(6) symmetry seems to be already broken. [ABSTRACT FROM AUTHOR]

The structure of neutron-rich nuclei produced as secondary fission fragments was investigated using the EUROGAM and GAMMASPHERE ACS arrays, the LOHENGRIN fission-fragment mass separator and the FIFI fission-fragment identifier. Fission products were populated in spontaneous fission of 248Cm and 252Cf and in thermal neutron-induced fission of 233U, 235U and 241Pu at ILL Grenoble. Particularly useful in such studies are isomeric states, well populated in fission due to their yrast character, easy to detect due to their long half lives and easy to interpret because of their relatively simple composition. We discuss their role in studies of neutron-rich nuclei, giving examples of isomers found in our recent experiments. A special type of K-isomers, resulting from ‘crossing’ of extruder and intruder orbitals plays a role in the mechanism of a sudden onset of deformation in the A = 100 and A = 150 regions. We present evidence for these isomers in both regions. Possible further studies in this field are proposed. [ABSTRACT FROM AUTHOR]

The discoveries observed in the ongoing conflict between collective and single-particle nuclear behaviour with increasing angular momentum have driven the field of nuclear spectroscopy for many decades and have given rise to new nuclear phenomena. Recently a new frontier of γ spectroscopy at ultra-high spin has been opened in the rare-earth region with rotational bands that bypass the classic band-terminating states that appear at spin 45h in the N 90 Er nuclei. These weakly populated rotational structures have characteristics of triaxial strongly-deformed bands. Such structures have been observed in 157,158,160Er, following a series of experiments using the Gammasphere spectrometer. These observations herald a return to collective excitations at spins of about 50 to 65h. This talk reviews the status of the spectroscopy and understanding of the observed structures in these Er and neighbouring nuclei. [ABSTRACT FROM AUTHOR]

The experimental knowledge of the 2961Cu32 and 3061Zn31 nuclei has been largely extended via the joint results from three experiments. The fusion-evaporation reaction used a 36Ar beam and a 28Si target foil to produce the two nuclei via the evaporation of either three protons (61Cu) or two protons and a neutron (61Zn). The experimental set-ups comprised the Ge-array GAMMASPHERE as well as neutron and charged-particle detectors placed around the target position. The resulting level schemes include around ten rotational superdeformed structures in each isotope. Most of them are linked to normally deformed states and in many cases spins and parities of the low-lying states in each structure have been determined. The collective structures are compared with results from configuration dependent Cranked Nilsson-Strutinsky calculations. The different structures are in general well understood from the calculation but the results do also suggest modifications of the standard Nilsson parameters in the mass A∼60 region. [ABSTRACT FROM AUTHOR]

The talk will focus on spin-dependent competition between oblate and prolate shape minima in the potential energy landscape of 180Hf (the most neutron-rich stable isotope), mediated via the alignment of valence nucleons. Results of a prompt spectroscopic study, using deep inelastic reactions with Gammasphere and CHICO, bombarding a thin 232Th target with a 180Hf beam ∼25% above the Coulomb barrier, will be presented. Nucleon alignments in both prolate and oblate minima will be discussed, as well as the favoring of oblate collective rotation at high spins, observed through a mixing with gamma vibrations built on the prolate shape. [ABSTRACT FROM AUTHOR]

In-beam γ-ray studies of nuclei with a large proton excess are faced with ever decreasing cross sections and large backgrounds due to strong less exotic reaction channels. However, many of the nuclei along the proton drip line α decay, proton decay or emit β-delayed particles. This offers a very selective tag for prompt γ rays and has been known as the Recoil-Decay Tagging (RDT) method. RDT has been extensively used with the Gammasphere array of Compton suppressed Ge detectors coupled with the Argonne Fragment Mass Analyzer. Recently, despite a very small cross section of about 50 nb and a long half life of ∼2 s, protons emitted following the β decay of 101Sn were used to identify γ-ray transitions in 101Sn. As a result, the energy splitting between the neutron g7/2 and d5/2 orbitals outside the 100Sn core was deduced. The structure of light Sn isotopes is discussed in the framework of the shell model in light of the new results. Another state-of-the-art example is the observation of a rotational ground-state band in the fast proton emitter 145Tm, which decays with a half live of only ∼3 μs. In this experiment, coincidences between the proton decay to the 2+ excited state in the daughter nucleus and the 2+→0+ γ-ray transition were also measured, which constitutes the first direct proof of proton decay fine structure. Based on the data the possibility of a triaxial shape in 145Tm is considered. [ABSTRACT FROM AUTHOR]

In a recent experiment at Argonne National Laboratory, new spectroscopic information was obtained for the mirror pair 67As and 67Se via the measurement of the lifetime of analogue 9/2I+ excited states. The nuclei of interest were populated via the fusion evaporation reaction of 32S on 40Ca, and selected by the combination of Gammasphere, Microball and the Neutron Wall arrays. The mirror B(E1) strengths were measured via the centroid shift method in analogue E1 transitions in 67As and 67Se. These findings are significant in determining the extent of conservation of isospin symmetry. [ABSTRACT FROM AUTHOR]