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Background: Neutron-rich nuclei in the mass region around A=160 have been and will continue to be of interest for the study of nuclear structure because of the rapid onset of deformation between 88 and 90 neutrons. The observation of detailed changes in nuclear structures within this mass region has provided and will continue to provide insight into the nuclear force. Purpose: Investigations of γ rays emitted following Eu163 β-decay to Gd163 have been performed for evaluation of the nuclear structure of Gd163. Method: Data were collected at the LeRIBSS station of the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory with an array of four Clover HPGe detectors for γ-rays and two plastic scintillators for β detection. The γ rays were identified as belonging to Gd163 via mass selection and γ-γ-β, x-ray-γ, or γ-γ coincidences. Results: In total 107 new γ-ray transitions were observed in Gd163 from 53 newly identified levels. Conclusions: The structure of Gd163 has been identified for the first time. This structure has been evaluated in comparison to projected shell model, and potential energy surface calculations with good agreement.

We report the results of a β-decay study of fission products ^{86}Br, ^{89}Kr, ^{89}Rb, ^{90gs}Rb, ^{90m}Rb, ^{90}Kr, ^{92}Rb, ^{139}Xe, and ^{142}Cs performed with the Modular Total Absorption Spectrometer (MTAS) and on-line mass-separated ion beams. These radioactivities were assessed by the Nuclear Energy Agency as having high priority for decay heat analysis during a nuclear fuel cycle. We observe a substantial increase in β feeding to high excited states in all daughter isotopes in comparison to earlier data. This increases the average γ-ray energy emitted by the decay of fission fragments during the first 10 000 s after fission of ^{235}U and ^{239}Pu by approximately 2% and 1%, respectively, improving agreement between results of calculations and direct observations. New MTAS results reduce the reference reactor ν[over ¯]_{e} flux used to analyze reactor ν[over ¯]_{e} interaction with detector matter. The reduction determined by the ab initio method for the four nuclear fuel components, ^{235}U, ^{238}U, ^{239}Pu, and ^{241}Pu, amounts to 0.976, 0.986, 0.983, and 0.984, respectively.

Radioactive ^{136}Te has two valence protons and two valence neutrons outside of the ^{132}Sn double shell closure, providing a simple laboratory for exploring the emergence of collectivity and nucleon-nucleon interactions. Coulomb excitation of ^{136}Te on a titanium target was utilized to determine an extensive set of electromagnetic moments for the three lowest-lying states, including B(E2;0_{1}^{+}→2_{1}^{+}), Q(2_{1}^{+}), and g(2_{1}^{+}). The results indicate that the first-excited state, 2_{1}^{+}, composed of the simple 2p⊕2n system, is prolate deformed, and its wave function is dominated by excited valence neutron configurations, but not to the extent previously suggested. It is demonstrated that extreme sensitivity of g(2_{1}^{+}) to the proton and neutron contributions to the wave function provides unique insight into the nature of emerging collectivity, and g(2_{1}^{+}) was used to differentiate among several state-of-the-art theoretical calculations. Our results are best described by the most recent shell model calculations.

We report total absorption spectroscopy measurements of ^{92}Rb, ^{96gs}Y, and ^{142}Cs β decays, which are the most important contributors to the high energy ν[over ¯]_{e} spectral shape in nuclear reactors. These three β decays contribute 43% of the ν[over ¯]_{e} flux near 5.5 MeV emitted by nuclear reactors. This ν[over ¯]_{e} energy is particularly interesting due to spectral features recently observed in several experiments including the Daya Bay, Double Chooz, and RENO Collaborations. Measurements were conducted at Oak Ridge National Laboratory by means of proton-induced fission of ^{238}U with on-line mass separation of fission fragments and the Modular Total Absorption Spectrometer. We observe a β-decay pattern that is similar to recent measurements of ^{92}Rb, with a ground-state to ground-state β feeding of 91(3)%. We verify the ^{96gs}Y ground-state to ground-state β feeding of 95.5(20)%. Our measurements substantially modify the β-decay feedings of ^{142}Cs, reducing the β feeding to ^{142}Ba states below 2 MeV by 32% when compared with the latest evaluations. Our results increase the discrepancy between the observed and the expected reactor ν[over ¯]_{e} flux between 5 and 7 MeV, the maximum excess increases from ∼10% to ∼12%.

The β-delayed neutron emission of ^{83,84}Ga isotopes was studied using the neutron time-of-flight technique. The measured neutron energy spectra showed emission from states at excitation energies high above the neutron separation energy and previously not observed in the β decay of midmass nuclei. The large decay strength deduced from the observed intense neutron emission is a signature of Gamow-Teller transformation. This observation was interpreted as evidence for allowed β decay to ^{78}Ni core-excited states in ^{83,84}Ge favored by shell effects. We developed shell model calculations in the proton fpg_{9/2} and neutron extended fpg_{9/2}+d_{5/2} valence space using realistic interactions that were used to understand measured β-decay lifetimes. We conclude that enhanced, concentrated β-decay strength for neutron-unbound states may be common for very neutron-rich nuclei. This leads to intense β-delayed high-energy neutron and strong multineutron emission probabilities that in turn affect astrophysical nucleosynthesis models.

Produced in the reaction 48Ca+243Am, thirty correlated α-decay chains were observed in an experiment conducted at the GSI Helmholzzentrum fur Schwerionenforschung, Darmstadt, Germany. The decay chains are basically consistent with previous findings and are considered to originate from isotopes of element 115 with mass numbers 287, 288, and 289. A set-up aiming specifically for high-resolution charged particle and photon coincidence spectroscopy was placed behind the gas-filled separator TASCA. For the first time, γ rays as well as X-ray candidates were observed in prompt coincidence with the α-decay chains of element 115.

High-spin states in {sup 70}As were populated using the {sup 58}Ni ({sup 16}O, 3pn) reaction at 70 MeV energy. Lifetime measurements of the 11{sup (+)}{r_arrow}9{sup (+)} 980.7keV and (13{sup +}){r_arrow}11{sup (+)}1342.7keV transitions using the Doppler-shift attenuation method determined that both are enhanced E2 transitions. This measurement indicates the onset of deformation with increasing spin in {sup 70}As, as has been seen in neighboring nuclei. {copyright} {ital 1997} {ital The American Physical Society}

High-spin states in Kr-77 were identified via analysis of prompt gamma-gamma coincidences in the Ni-58(Si-29,2 alpha 2p) reaction at 128 MeV at the Gammasphere facility. Evaporation channel selecti ...

The high-spin structure of odd-odd {sup 86}Nb was studied with the early implementation of GAMMASPHERE using 36 Compton-suppressed Ge detectors and the MICROBALL, a 95-element full-sphere charged-particle detector array. High-spin states were populated using the {sup 58}Ni({sup 32}S,3pn) reaction at 135 MeV with beams from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. A number of bands were observed up to spins as high as (31{sup {minus}}). The yrast positive-parity band shares many characteristics with the {pi}g{sub 9/2}{circle_times}{nu}g{sub 9/2} bands in other odd-odd f-p-g shell nuclei. These similarities include the behavior of the moments of inertia, the magnitude and phase of the signature splitting and its phase reversal near the 10{sup +} state, and the strong alternations in the B(M1)/B(E2) strengths. The moments of inertia in the lowest pair of negative-parity bands start out with a sharp upbend and then gradually fall back to the rigid-body value. New positive- and negative-parity bands appear in the quasiparticle alignment region and eventually become yrast. Some additional positive-parity states around spin 17{h_bar} are candidates for fully or nearly fully aligned shell-model-like configurations. {copyright} {ital 1997} {ital The American Physical Society}

Beta decay of 86Ga was studied by means of β-neutron-γ spectroscopy. An isotopically pure ^{86}Ga beam was produced at the Holifield Radioactive Ion Beam Facility using a resonance ionization laser ion source and high-resolution electromagnetic separation. The decay of 86Ga revealed a half-life of 43(-15)(+21) ms and large β-delayed one-neutron and two-neutron branching ratios of P1n=60(10)% and P2n=20(10)%. The βγ decay of 86Ga populated a 527 keV transition that is interpreted as the deexcitation of the first 2+ state in the N=54 isotone 86Ge and suggests a quick onset of deformation in Ge isotopes beyond N=50.

A high-resolution , X-ray and gamma-ray coincidence spectroscopy experiment was conducted at the GSI Helmholtzzentrum für Schwerionenforschung. Thirty correlated alpha-decay chains were detected following the fusion-evaporation reaction 48Ca + 243Am. The observations are consistent with previous assignments of similar decay chains to originate from element Z = 115. For the first time, precise spectroscopy allows the derivation of excitation schemes of isotopes along the decay chains starting with elements Z > 112. Comprehensive Monte-Carlo simulations accompany the data analysis. Nuclear structure models provide a first level interpretation.

A search was undertaken to look for evidence of hyperdeformation in Gd-146,Gd-147. Three experiments employing Gammasphere for gamma-ray detection coupled with the Microball for channel selection via charged particle detection were carried out with increasing detection sensitivity and statistics. No definitive evidence for band structures that could be assigned to hyperdeformation could be found. Candidates previously reported are shown not to have properties consistent with a band structure.

The lifetimes of the 21 2 + and 17 2 − yrast states in 89Nb, 89Mo and 91Tc and of the 334 keV 5 2 − state in 87Nb were determined via the electronic timing method. The nuclei were populated by means of the VICKSI pulsed 36Ar beam impinging on a 58Ni target. The lifetimes measured for the first time are: 89Mo: 2271 keV 17 2 − τ = 1.6(4) ns ; 2584 keV 21 2 + τ = 13.7(3) ns ; 87Nb: 334 keV 5 2 − τ = 41(3) ns . The resulting B(E2) values of stretched E2-decay γ-rays largely confirm predictions of multiparticle shell model calculations carried out within the ( g 9 2 ,p 1 2 ) single-particle space and with the Gross-Frenkel residual interaction. The long lifetime of the 334 keV state in 87Nb arises from an f 5 2 excitation.

A high-spin rotational band of ten transitions between 1305 and 2641 keV with a nearly constant moment of inertia [ital J][sup (2)]=27[ital ];sh[sup 2]MeV[sup [minus]1] has been observed and is tentatively assigned to [sup 83]Sr. The properties of this band are in excellent agreement with theoretical calculations which predict the onset of superdeformation in and around [sup 83]Sr at spins [ital I][approx]35[ital ];sh. These results establish a new region of superdeformation in medium-mass nuclei with particle numbers [ital N],[ital Z][approx]40.

High spin states in81Y up to the probable 33/2+ and 29/2− yrast states have been measured via the reaction58Ni(28Si,αp, using the Cologne tandem accelerator. Directional correlations of oriented nuclei were determined and the level scheme was extended with lifetimes measured by means of DSA and recoil distance techniques. Reduced transition strengths inferred from lifetimes were consistent with the results of Hartree-Fock-Bogolyubov calculations, which predict a nearly axially symmetric deformation ofβ 2≈0.37 for theπ=+ band. Predictions reveal an alignment driving the system to a smaller and triaxial deformation withβ 2=0.23 andγ=−30°, although the data show this crossing to be somewhat delayed. Particle-rotor calculations for the one-quasiparticle bands corroborate the predictions of deformation parameters based on the cranking model.

The reactions (Ni(Ar, alpha2p)Mo)-Ni-58-Ar-36-Mo-88 and (Ni(Ar, 4p)Mo)-Ni-58-Ar-36-Mo-90 at 149 and 140 MeV beam energies have been used to carry out Doppler shift attenuation and recoil distance lifetime measurements and to determine DCO ratios in Mo-88 and Mo-90. A singles angular distribution experiment using the reaction (Ni(Cl, 3p)Mo)-Ni-59-Cl-35-Mo-90 has been performed at 120 MeV beam energy to measure spin assignments in Mo-90 and to calibrate the DCO analyses for both nuclei. Reduced transition probabilities for more than 60 transitions in Mo-88, Mo-90 have been deduced. The extensive data set is incorporated in a detailed comparison between the two nuclei and, for Mo-90, with predictions from shell model calculations within a restricted pi(p1/2), pi(99/2), nu(p1/2), and nu(g9/2) configuration space and with different residual interaction parameters. The electromagnetic properties of the negative parity states of Mo-90 are well reproduced by the calculations while the positive parity states show hints of seniority mixing at intermediate spins.

High spin states in the neutron deficient isotopes Ru90 and Ru91 have been studied in the 2p2n and 2pn exit channels of the reaction Ar-36 + Ni-58 using the 149 MeV Ar-36 beam at VICKSI and the OSIRIS array. By gating the gammagamma coincidences with evaporated neutrons and protons, excited states in Ru90 were identified for the first time and the level scheme of Ru-91 was extended. In both nuclei, the level energies and branching ratios follow the predictions of shell model calculations performed within the (1g9/2,2p1/2) single-particle space.

High spin states in the nucleus Tc-91 have been studied via the fusion evaporation reaction Ni-58(Ar-36,3p)Tc-91, at 149 MeV and 140 MeV beam energy. The OSWS spectrometer, an additional high purity Ge detector at 162 degrees, and several particle detectors were used to measure gamma gamma- and particle-gamma coincidences, directional correlations of oriented states, and line shapes due to Doppler-shift attenuation. Lifetimes in the 10(-12)-10(-9) S range were measured by the recoil distance Doppler-shift method. Some 110 transitions were placed into a level scheme comprising 51 levels reaching up to 12.2 MeV excitation energy and a possible spin I = 49/2 h ($) over bar. A large number of spin-parity assignments and E2/M1 mixing ratios and lifetimes were deduced. This extensive data set is used for a detailed test of shell model calculations. They were performed with different sets of parameters, within either a restricted pi(p(1/2)), pi(g(9/2)), nu(p(1/2)), and nu(g(9/2)) configuration space or an enlarged configuration space including the pi(p(3/2)) and pi(f(5/2)) orbits. (Less)

The fusion excitation functions for radioactive (132)Sn + (58)Ni and stable (130)Te + (58,64)Ni were measured at energies near the Coulomb barrier. The coupling of transfer channels in heavy-ion fusion was examined through a comparison of Sn + Ni and Te + Ni systems, which have large variations in the number of positive Q-value nucleon transfer channels. In contrast with previous experimental comparisons, where increased sub-barrier fusion cross sections were observed in systems with positive Q-value neutron transfer channels, the reduced excitation functions were equivalent for the different Sn + Ni and Te + Ni systems. The present results suggest a dramatically different influence of positive Q-value transfer channels on the fusion process for the Sn + Ni and Te + Ni systems.

The Holifield Radioactive Ion Beam Facility (HRIBF) is a national user facility for research with radioactive ion beams (RIBs) that has been in routine operation since 1996. It is located at Oak Ridge National Laboratory (ORNL) and operated by the ORNL Physics Division. The principal mission of the HRIBF is the production of high quality beams of shortlived radioactive isotopes to support research in nuclear structure physics and nuclear astrophysics. HRIBF is currently unique worldwide in its ability to provide neutron‐rich fission fragment beams post‐accelerated to energies above the Coulomb barrier for nuclear reactions. HRIBF produces RIBs by the isotope separator on‐line (ISOL) technique using a particle accelerator system that consists of the Oak Ridge Isochronous Cyclotron (ORIC) driver accelerator, one of the two Injectors for Radioactive Ion Species (IRIS1 or IRIS2) production systems, and the 25‐MV tandem electrostatic accelerator that is used for RIB post‐acceleration. ORIC provides a light ion beam (proton, deuteron, or alpha) which is directed onto a thick target mounted in a target‐ion source (TIS) assembly located on IRIS1 or IRIS2. Radioactive atoms that diffuse from the target material are ionized, accelerated, mass selected, and transported to the tandem accelerator where they are further accelerated to energies suitable for nuclear physics research. RIBs are transported through a beam line system to various experimental end stations including the Recoil Mass Spectrometer (RMS) for nuclear structure research, and the Daresbury Recoil Separator (DRS) for nuclear astrophysics research. HRIBF also includes two off‐line ion source test facilities, one low‐power on‐line ISOL test facility (OLTF), and one high‐power on‐line ISOL test facility (HPTL). This paper provides an overview and status update of HRIBF, describes the recently completed $4.7M IRIS2 addition and incorporation of laser systems for beam production and purification, and discusses a proposed replacement of the ORIC driver accelerator.

The inverted $^{12}\mathrm{C}$${(}^{68}$Zn,2n) reaction was used to populate states in $^{78}\mathrm{Kr}$ in the region of the first alignment of the yrast band at \ensuremath{\Elzxh}\ensuremath{\omega}=0.56 MeV. The precessions of these states in the transient magnetic field of an $^{54}\mathrm{Fe}$ foil were measured to determine the nature of the alignment. The relatively small precession angle of the ${8}^{+}$ state indicates that the aligning nucleons at the first crossing are neutrons. This is contrary to expectations based on the systematics in this region, and it suggests this shape-coexistent nucleus has an oblate shape throughout the ground state band.

Gamma-rays emitted following the 34S+104Ru reaction have been studied using a preliminary version of the European array, containing 29 Ge detectors, in conjunction with the Daresbury recoil mass separator. Highly deformed bands in 133Nd (5n reaction channel), 135Nd (3n), and 131Ce ( alpha 3n) have been observed. No evidence has been found in mass-selected data for the band previously assigned to 134Nd (the dominant 4n channel in this reaction) which has 'identical' energies to the 131Ce band.

The reaction58Ni(36Ar,α qρ)88Mo has been studied at 145 MeV beam energy. A detector array consisting of the OSIRIS spectrometer, four charged-particleΔE detectors and seven NE213 neutron detectors has been used to meaure the gamma radiation inγγ- and particle-γγ-coincidence mode. The level scheme of88Mo has been extended up to 11.6 MeV excitation energy and probable spin 23ħ; some 70 transitions and 40 levels have been identified. Spin assignments have been proposed on the basis of measured DCO ratios. Hartree Fock cranking calculations of the Total Routhians and shell model calculations of the high spin states are presented which imply near-sphericity of the yrast line up to the highest spins found. A classification of the high spin states according to their leading seniority is proposed.

The reaction58Ni(36Ar, 4p)90Mo has been studied at a beam energy of 149 MeV. A detector array consisting of the OSIRIS spectrometer (12 Compton-suppressed Ge detectors), four charged-particleδE detectors and seven NE-213 neutron detectors at the VICKSI accelerator in Berlin has been used to measure the gamma radiation inγγ- and particle-γγ-coincidence mode. An additional Ge detector was placed at 162‡ to the beam direction to provide information on DCO ratios. The level scheme of90Mo has been extended up to an excitation energy of about 12 MeV and probable spin 23ħ. Some 70 transitions and 40 levels have been newly identified. Spin assignments have been proposed on the basis of measured DCO ratios. Shell model calculations in a model space consisting of the proton 1f 5/2, 2p 3/2, 2p 1/2, and 1g 9/2 orbits and the neutron 2p 1/2, 1g 9/2, 1g 7/2, 2d 5/2, 2d 3/2, and 3s 1/2 orbits with some truncation were made for states above 9ħ and the predicted structure of these states is discussed.

The beta-delayed neutron branching ratios (P{betan}) for nuclei near doubly magic 78Ni have been directly measured using a new method combining high-resolution mass separation, reacceleration, and digital beta-gamma spectroscopy of 238U fission products. The P{betan} values for the very neutron-rich isotopes ;{76-78}Cu and 83Ga were found to be much higher than previously reported and predicted. Revised calculations of the betan process, accounting for new mass measurements and an inversion of the pi2p{3/2} and pi1f{5/2} orbitals, are in better agreement with these new experimental results.

As hindrance sets in for the fusion of heavier systems, the effect of large neutron excess in the colliding nuclei on their probability to fuse is still an open question. The detection of evaporation residues (ERs), however, provides indisputable evidence for the fusion (complete and incomplete) in the reaction. We therefore devised a system with which we could measure ERs using low intensity neutron‐rich radioactive ion beams with an efficiency close to 100%. We report on measurements of the production of ERs in collisions of 132,134Sn, 134Te and 134Sb ion beams with medium mass, neutron‐rich targets. The data taken with 132,134Sn bombarding a 64Ni target are compared to available data (ERs and fusion) taken with stable Sn isotopes. Preliminary data on the fusion of 132Sn with 96Zr target are also presented.

The level scheme of 87Mo was investigated with the reaction Ni-58(S-32, 2pn) at projectile energies of 110-130 MeV and the reaction Ca-40(Cr-50, 2pn) at 170 MeV. Extensive spectroscopic information was collected including gamma-gamma and neutron-gated gamma-gamma-coincidences as well as coincidences with identified 87Mo residues. Level lifetimes were studied with the RDDS and DSA methods. The angular distributions of low-lying transitions were measured. The results indicate a sudden transition from single-particle-like to collective behavior which is caused by g9/2 particle alignment. The level scheme was constructed up to 12 MeV excitation energy and a probable spin of 49/2 HBAR.