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Superheavy nuclei exist solely due to quantum shell effects, which create a pocket in the potential-energy surface of the nucleus, thus providing a barrier against spontaneous fission. Determining the height of the fission barrier and its angular-momentum dependence is important to quantify the role that microscopic shell corrections play in enhancing and extending the limits of nuclear stability. In this talk, the first measurement of a fission barrier in the very heavy nucleus 254No will be presented. [ABSTRACT FROM AUTHOR]

The heaviest nuclei where excitations above the ground state can be studied lie near Z ~ 100. These nuclear structure studies are important testing grounds for theoretical models that aim to describe superheavy nuclei. To study the highest neutron orbitals (150 ≤ N ≤ 154), we have populated high angular momentum states in a series of Pu (Z = 94), Cm (Z = 96) and Cf (Z = 98) nuclei, via inelastic and transfer reactions, with heavy beams on long-lived radioactive actinide targets. Multiple collective excitation modes and structures were identified, and their configurations deduced. Quasiparticle alignments are mapped, with odd-A band structures helping identify specific orbital contributions via blocking arguments. Higher-order multipole shapes are observed to play a significant role in disentangling competing neutron and proton alignments. The N > 152 data provide new perspectives on physics beyond the N = 152 sub-shell gap. [ABSTRACT FROM AUTHOR]

In-beam and decay-spectroscopy studies of neutron-deficient nuclei near the Z=82 shell closure were carried out using the Fragment Mass Analyzer (FMA) and the Gammasphere array, in conjunction with symmetric fusion reactions and the Recoil Decay Tagging (RDT) technique. The primary motivation was to study properties of 179Tl and 180Tl, and their daughter, and grand-daughter isotopes. For the first time, in-beam structures associated with 179Tl and 180Tl were observed, as well as γ rays associated with the 180Tl α decay. No long-lived isomer was identified in 180Tl, in contrast with the known systematics for the heavier odd-odd Tl isotopes. [ABSTRACT FROM AUTHOR]

Nuclei far from the line of stability are a focal point of contemporary nuclear physics. Nuclear structure studies along the proton dripline remain an important component of the scientific program at the ATLAS facility. The Gammasphere gamma-ray array and the Fragment Mass Analyzer offer the unprecedented sensitivity and selectivity required to study nuclei at and beyond the proton dripline. Recent results in proton decay studies, in-beam γ-ray spectroscopy around 100Sn and studies of proton resonances in light nuclei involved in the astrophysical hydrogen burning are presented. Future prospects in the context of planned and on-going upgrades of ATLAS and experimental apparatus are also briefly discussed. [ABSTRACT FROM AUTHOR]

The structure of 246Fm has been investigated using in-beam γ-ray spectroscopy. The experiment was performed at the University of Jyväskylä using JUROGAM 2 coupled to RITU and GREAT. The 246Fm nuclei were produced using a 186 MeV beam of 40Ar impinging on a target of 208Pb. JUROGAM 2 was fully instrumented with TNT2D digital acquisition cards. The use of digital acquisition cards and a rotating target allowed for unprecedented beam intensities up to 71 particle-nanoamperes for prompt γ-ray spectroscopy. With all these major advances for spectroscopy a rotational band is observed for the first time in 246Fm and discussed here. [ABSTRACT FROM AUTHOR]

The isotope 257Rf was produced in the fusion-evaporation reaction 208Pb (50Ti, n) 257Rf. Reaction products were separated by the Argonne Fragment Mass Analyzer. Radioactive decay and spontaneous fission of 257Rf and its decay products were investigated. An isomeric state in 257Rf, with a half-life of 160-3142 μS, was discovered by detecting internal conversion electrons followed by alpha decays. It is interpreted as a three-quasiparticle high-K isomer. A second group of internal-conversion electrons which were succeeded by alpha decay, with a half-life of 4.1-1.3+2.4 s, was observed. These events might originate from the decay of excited states in 257Lr, populated by electron-capture decay of 257Rf, or from another isomer in 257Rf. [ABSTRACT FROM AUTHOR]

A search for γ-ray transitions in 101Sn, which contains only one neutron outside the 100Sn core, was carried out at the Argonne Tandem-Linac Accelerator System. 101Sn nuclei were produced using the 46Ti(58Ni,3n)101Sn reaction. In-beam γ rays were detected in the Gammasphere Ge-detector array and were correlated with 101Sn β-delayed protons using the Recoil-Decay Tagging method. A γ-ray line at 172 keV was assigned to 101Sn and was interpreted as the transition between the single-neutron g7/2 and d5/2 states. The measured νg7/2-ν d5/2 energy splitting is compared with predictions of various mean-field potentials. It is also used to calculate multi-neutron configurations in light Sn isotopes. Prospects of using β-delayed protons for tagging other exotic nuclei in the 100Sn region are discussed. [ABSTRACT FROM AUTHOR]

Fusion-fission reactions were induced by bombarding a thick 27Al target with 178Hf projectiles at a laboratory energy of 1150 MeV using the ATLAS accelerator at Argonne National Laboratory. The subsequent γ-ray decays were measured using the GAMMASPHERE germanium detector array. The beam was pulsed at two different ON/OFF cycles of 82.5/825 ns and 25/75 μs in order to observe the γ rays from the decay of isomeric states. In 121Sb 2721+Δ keV, Iπ=(25/2+) and 2434 keV, Iπ=19/2- states have measured half-lives of T1/2=200(30) μs and 8.2(2) ns respectively. The 2614+Δ keV, Iπ=(27/2+) and 2486 keV, Iπ=19/2+ states in 123Sb have measured half-lives of T1/2=52(3) μs and 7.9(4) ns respectively. The positive parity isomers in these nuclei correspond to a πd5/2 or πg7/2 configuration, in 121Sb and 123Sb respectively, coupled to aligned (h11/2)2 neutrons. The Iπ=19/2- isomeric state in 121Sb is proposed to have a νh11/2 x νd3/2 x πd5/2 configuration. A previously unobserved isomer has been identified in 99Mo at an energy of 3010 keV, decaying with T1/2=18(5) ns. This state is interpreted as an energetically favoured 3 quasi-particle alignment of ν (d5/2 g7/2) x π(g9/2)2 configuration which is observed systematically in the even-Z N=57 isotones. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The quasi-continuum of γ rays from the feeding and decay of superdeformed and normal bands in the nucleus 152Dy have been extracted in 1 - and 2 - dimensional spectra. The Eγ-Eγ correlations in the latter reveals strong ridges associated with superdeformed and normal states in this nucleus. The entry distributions for normal and superdeformed bands were also extracted. A Monte Carlo model was developed that simultaneously describes all the quasi-continuum and ridge spectra as well as the feeding intensity of the superdeformed bands. Through the calculation of the continuum of γ rays at finite temperature, the rotational damping widths in the normal and superdeformed wells were extracted. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Both the quasi-continuum of γ rays as well as the ridges from the feeding (and decay) of superdeformed and normal bands have been extracted in the nucleus 152Dy. A model has been developed that simultaneously describes all these spectra as well as the feeding intensity of the superdeformed bands. Through the calculation of the continuum of γ rays at finite temperature, the rotational damping width in the normal and superdeformed wells are extracted. This is the first time the rotational damping width in a superdeformed well has been measured in the A∼150 mass region. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Different treatments of the decay-out of superdeformed bands are reviewed, focusing on the role of the normally deformed doorway states to which the superdeformed band primarily couples at decay-out. A schematic description based on sparse GOE matrices is formulated for the normally deformed states including one doorway state. This allows for a gradual change of the normally deformed spectrum from an ordered to a chaotic situation by means of the effective dimensionality parameter. It is found that the distribution of the small normally deformed admixtures into the superdeformed band, through which it decays, depends sensitively on the effective dimensionality parameter. Analyzing the measured distribution of transition strength of decay-out γ-ray lines in 194Hg, it is concluded that the spectrum of normally deformed states is closer to the chaotic than the ordered situation at the energy and angular momentum of the superdeformed band. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Gamma-ray spectroscopy of high-spin states of the neutron-deficient nucleus 189Pb has been conducted with the 158Gd(36Ar,5n) and 164Er(29Si,4n) reactions. With the first of these, detection of evaporation residues and mass gating were used to unambiguously assign a number of prompt γ ray transitions to 189Pb. With the second reaction and a pulsed beams, an isomer with a mean life of 32 μsec was found and the decay scheme below it established. Although inconclusive, the available evidence favours identification of the isomer with the 33/2+ state of the v(i13/2)-3 configuration. The levels observed below the isomer can be identified with states involving three different structures: the neutron (i13/2)-3 multiplet in the spherical well; a prolate-deformed band involving mixed i13/2 neutron orbitals; and a state with the oblate π(2p-2h)0+ x v(i13/2)-1 configuration. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The low-lying levels in 59,60Cr have been populated with 13,14C(48Ca, 2p) reactions using a beam energy of 130 MeV. Prompt electromagnetic radiation was detected using the Gammasphere array, in coincidence with the detection of recoiling ions performed with the Fragment Mass Analyzer and a gas-filled ion chamber. The residues of interest were selected and identified on the basis of charge-to-mass ratio, energy-loss and time-of-flight measurements. These methods have isolated γ rays from the 2p evaporation channel for the first time. The low-lying structure of 59Cr is clearly inconsistent with the results of shell-model calculations within the full fp shell model and requires the inclusion of the g9/2 orbital. The sequence of states can be understood within the Nilsson model assuming a moderate oblate ground-state deformation. Preliminary results for 60Cr are discussed in terms of the development of deformation in the region. None of the low-lying structures observed so far exhibit a clear indication for a well-developed oblate shape. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have detected the electrons accompanying the decay of a long-lived isomer in 254No, which was produced with the 208Pb(48Ca,2n) reaction. Time and spatial correlations of A = 254 evaporation residues, isomeric electrons and ground-state α-particles confirmed the existence of the isomer. The measured electron sum-energy spectrum favors a Kπ = 7- assignment for the isomer. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The heavy-ion radiative capture reaction, 12C(12C,γ), has been investigated at beam energies around 16 MeV. Three different experiments were performed. Capture cross-sections were obtained by measuring fused 24Mg residues using the FMA at ANL. These were found to significantly exceed values reported earlier. Subsequently, the decay pathways associated with radiative capture were studied in two separate measurements: one with the high-resolution Gammasphere array and a second with a high efficiency BGO array, where gamma rays were recorded in coincidence with 24Mg residues detected at the focal plane of the DRAGON recoil separator at TRIUMF. Both measurements indicate that a substantial fraction of the decay is mediated through high-lying doorway states, possibly associated with the long-predicted shape-isomeric band in 24Mg. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Based on GOE sparse matrices, a model for decay-out of superdeformed bands is formulated, with focus on the degree of chaoticity of the spectrum of normally deformed states, to which the superdeformed band couples at decay-out. By means of the effective dimensionality parameter, the spectrum may be varied between the two limiting situations of complete order and complete chaos. The model is applied to the measured distribution of transition strength of decay-out γ-ray lines in 194Hg, and it is found that the normally deformed states should be closer to the chaotic than the ordered situation, with GOE-type spectral correlations extending over at least 10 levels. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Properties of thermally excited rotating nuclei are discussed by means of the cranked shell model. Focus is put on the violation of the K-quantum number in the rare-earth deformed nucleus 163Er and on the peculiar features of rotational motion in superdeformed Hg nuclei, which appear to be precursory of ergodic rotational bands. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The excitation energy, spin and parity of the yrast superdeformed (SD) band in 152Dy have been established. The excitation energy of the lowest observed level in this band is found to be 10,644 keV with an assigned spin and parity of 24+. In addition, nine transitions of dipole character have been identified, which connect the excited superdeformed band 6 in 152Dy to the yrast SD band. The excitation energy of the lowest level in the excited SD band 6 is 14,239 keV. The spin and parity of this state has been determined to be either 29- or 31-. The measured properties of SD band 6 are consistent with an interpretation in terms of a rotational band built on an octupole vibration. A comparison with an RPA calculation by Nakatsukasa et al. suggests that the spin of the lowest SD band 6 level is 31-. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The heavy ion radiative capture reaction 12C(12C,γ) has been investigated around a beam energy of 16 MeV. The total cross-section has been measured with the Fragment Mass Analyser at Argonne National Laboratory and found to be somewhat larger than has previously been measured. A subsequent measurement with the Gammasphere array has shown that a considerable proportion of this extra cross-section relates to a highly non-statistical decay through high-lying states in 24Mg. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The excitation energy, spin and parity of the yrast superdeformed band 1 in [sup 152]Dy have been established. The evidence comes mainly from the measured properties of a 4011 keV single-step transition connecting the yrast superdeformed level fed by the 693 keV transition to the 27[sup -] yrast state. Four additional, weaker, linking γ rays have been placed as well. The excitation energy of the lowest superdeformed band 1 member is 10,644 keV and its spin and parity are determined to be 24[sup +]. A further nine transitions of dipole character have been identified, which connect the excited superdeformed band 6 in [sup 152]Dy to the yrast SD band 1. The excitation energy of the lowest level in the excited SD band 1 is 14,239 keV. The lowest state in band 6 has been determined to be of negative parity and odd spin (29[sup -] or 31[sup -]). The measured properties of band 6 are consistent with an interpretation in terms of an octupole vibrational band. A comparison with an RPA calculation by Nakatsukasa et al. suggests that the spin of the lowest SD band 6 level is 31[sup -]. [ABSTRACT FROM AUTHOR]

The systematic study of entry distributions populating proton-rich Pt and Au isotopes indicates a limit in the excitation energy and angular momentum in the population of [sup 173]Au, a nucleus beyond the proton drip line. [ABSTRACT FROM AUTHOR]

High-energy γ rays emitted in the symmetric reaction [sup 90]Zr + [sup 89]Y have been measured at the Argonne National Laboratory using the LEPPEX array coupled to the Fragment Mass Analyzer (FMA). The high-energy γ-ray spectra emitted by the giant dipole resonance built on excited states and associated with the evaporation of 0, 1 and 2 nucleons have been measured. The statistical model analysis indicates that the radiative-fusion process, namely the 0 particle emission channel, can be well described as a compound nucleus decay channel. In addition, the analysis of the highenergy γ-ray spectra associated with the different evaporation channels provides the first evidence that shell effects still play a role in determining the nuclear shapes of [sup 179]Au in the temperature interval 0.5-1 MeV. [ABSTRACT FROM AUTHOR]

Three, possibly four, regularly spaced rotational bands with large dynamical moments of inertia, which are consistent with known superdeformed bands in the Lu/Hf region, have been identified in [sup 174]Hf. The states were populated in the [sup 130]Te([sup 48]Ca,4n) reaction at a beam energy of 194 MeV. The Gammasphere array detected the emitted gamma radiation. Ultimate cranker calculations predict substantial triaxial deformation (γ ≈ 17°) for highly deformed [sup 174]Hf structures. However, [sup 174]Hf is eight neutrons away from the previously established N = 94 triaxial superdeformed shell gap. Shell gaps at N = 100 and 106 with γ ≥ 15° are observed when ε[sub 2] ≈ 0.45, which may be responsible for the predicted TSD minima in [sup 174]Hf. [ABSTRACT FROM AUTHOR]

The radiative capture reaction involving heavy ions is a relatively unexplored reaction mechanism which affords the possibility of gaining insights into molecular structure in nuclei and exotic, highly-deformed nuclear shapes. The [sup 12] C([sup 12]C,γ) reaction has been studied using an unconventional application of gamma-ray spectroscopy using the Gammasphere array. A resonance for this reaction has been located for E[sub beam]=15.8 MeV. Favored population of the K=2 rotational band in [sup 24]Mg as well as particular states around 10 MeV in excitation is observed. Using a Monte Carlo simulation of the previously uncharacterised high energy efficiency of Gammasphere, we set a lower limit on the cross-section for the species of decays observed of lµb. [ABSTRACT FROM AUTHOR]

The cranked relativistic Hartree+Bogoliubov theory has been applied for a systematic study of the nuclei around [sup 254]No, the heaviest nuclei for which detailed spectroscopic data are available. The deformation, rotational response, pairing correlations, quasi-particle and other properties of these nuclei have been studied with different relativistic mean field (RMF) parametrizations. For the first time, the quasi-particle spectra of odd deformed nuclei have been calculated in a fully selfconsistent way within the framework of the RMF theory. The energies of the spherical subshells, from which active deformed states of these nuclei emerge, are described with an accuracy better than 0.5 MeV for most of the subshells with the NL1 and NL3 parametrizations. However, for a few subshells the discrepancy reach 0.7-1.0 MeV. The implications of these results for the study of superheavy nuclei are discussed. [ABSTRACT FROM AUTHOR]

Four regularly spaced rotational bands with large dynamical moments of inertia, which are consistent with known superdeformed bands in the Lu/Hf region, have been identified in [SUP174]Hf. The states were populated in the [SUP130]Te([SUP48]Ca,4n) reaction ar a beam energy of 194 MeV. The Gammasphere array detected the emitted gamma radiation. Ultimate cranker calculations predict substantial triaxial deformation (&b.gamma; ± 17°) for highly deformed [SUP174]Hf structures. However, [SUP174]Hf is eight neutrons away from the previously established N = 94 triaxial superdeformed shell gap. Shell gaps at N = 100 and 106 with &b.gamma; ≤ 15° are observed when &b.epsi;2 ≈ 0.45, which may be responsible for the predicted TSD minima in [SUP174]Hf. [ABSTRACT FROM AUTHOR]

The Argonne Fragment Mass Analyzer (FMA) is designed to separate and identify evaporation residues according to their mass-to-charge ratio. The FMA in combination with GAMMASPHERE — an array of 100 compton-suppressed germanium detectors — allowed for a number of very interesting in-beam gamma-spectroscopy studies, as the FMA provided a very clean trigger on evaporation residues to obtain extremely background-free gamma-spectra. This setup was used to measure the total gamma-energy and multiplicity after particle evaporation — the so-called entry distribution — by exploiting the calorimetric properties of GAMMASPHERE, using its germanium detectors as well as its BGO shields for a maximum gamma efficiency. The entry distribution can be used to estimate the height of the fission barrier as a function of angular momentum. This method is especially favorable for unstable nuclei, for which the fission barrier is otherwise very difficult to measure. Here, the entry distributions of [sup 220]Th at beam energies of 206 MeV and 219.5 MeV in the [sup 176]Yb([sup 48]Ca,4n) reaction are presented. The results are compared to a previous measurement. [ABSTRACT FROM AUTHOR]

The common feature to all superdeformed bands is the way they abrupty terminate when the nucleus suddenly changes shape. In most cases, it is extremely hard to tell how the nucleus then decays to its normally deformed groundstate, and this is why only very scarce information is available for superdeformed states. After a brief introduction on superdeformation, I will show, with a few examples, what is the status of our knowledge about the decay from superdeformed states. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The last five years have witnessed a rapid increase in the volume of data on proton decaying nuclei. The path was led by decay studies with recoil mass separators equipped with double-sided Si strip detectors. The properties of many proton decaying states were deduced, which triggered renewed theoretical interest in the process of proton decay. The decay experiments were closely followed by in-beam 7-ray studies which extended our knowledge of high-spin states of proton emitters. The unparalleled selectivity of the Recoil-Decay Tagging method combined with the high efficiency of large arrays of Ge detectors allowed, despite small cross sections and overwhelming background from strong reaction channels, the observation of excited states in several proton emitters. Recently, in-beam studies of the deformed proton emitters [sup141]Ho and [sup131]Eu have been performed with the GAMMASPHERE array of Ge detectors and the Fragment Mass Analyzer at ATLAS. Evidence was found for rotational bands in [sup141]Ho and [sup131]Eu. The deformations and the single-particle configurations proposed for the proton emitting states from the earlier proton-decay studies were confronted with the assignments deduced based on the in-beam data. It should be noted that the cross section for populating [sup131]Eu is only about 50 nb, and it represents the weakest channel ever studied in an in-beam experiment [ABSTRACT FROM AUTHOR]