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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]

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]

The chain of Barium isotopes enables us to study experimentally the evolution of nuclear quadrupole collectivity from the shell closure at N = 82 towards neutron-deficient or neutron-rich deformed nuclei. The TU Darmstadt group has investigated several nuclei from stable 130,132Ba up to radioactive 140,142Ba with the projectile-Coulomb excitation technique including the use of the Doppler-shift attenuation method (DSAM). Lifetimes of quadrupole-collective states of 132Ba and 140Ba were obtained for the first time as well as the static electric quadrupole moments Q(21+) for 130,132Ba and 140,142Ba. The results are compared to Monte Carlo shell model and Beyond-Mean-Field calculations. The phenomenon of shell stabilization in the N = 80 isotones is further investigated by measurements of the B(E2;21+ → 00+) values of 140Nd and 142Sm and comparison to the quasi-particle phonon model and shell-model calculations. [ABSTRACT FROM AUTHOR]

Excited states in neutron-rich tantalum isotopes have been studied with deep-inelastic reactions using 136Xe ions incident on a 186W target. New transitions observed below the τ=1.3 µs isomer in 183Ta have enabled the establishment of its energy and put limits on the spin and parity. On the basis of the reduced hindrances for the depopulating transitions, a 3-quasiparticle configuration of v1/2-[510]11/2+[615] ⊗ π9/2-[514] is suggested. [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]

Spectroscopic studies of nuclei in the A∼250, Z∼100 region provide critical input to theoretical models that attempt to describe the structure and stability of the heaviest elements. We report here on new spectroscopic studies in the N = 150,151 nuclei 244,245Pu. (Z = 94). Excitations in these nuclei on the neutron-rich side of the valley of stability, accessed via inelastic and transfer reactions, complement fusion-evaporation studies of Z≥100 nuclei. States in 244,245Pu were populated using 47Ti and 208Pb beams incident on a 244Pu target, with delayed and prompt gamma rays detected by the Gammasphere array. The new results are discussed in the context of emerging systematics of one- and two-quasiparticle excitations in N≥150 nuclei. [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]

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]

Classical novae are a site of explosive nucleosynthesis where hydrogen rich material from a companion giant star accretes onto the surface of a white warf. Critical to our understanding of nova explosions are proton-capture reaction rates involved in the nucleosynthesis. While, ideally, all of the relevant (p,γ) reactions would be measured directly, in practice, such measurements are very challenging and are only possible in a few cases. This provides considerable scope for indirect measurements including transfer reactions, mass measurements, beta-decay and gamma-ray spectroscopy. The latter technique, until recently largely neglected as an input in nuclear astrophysics analyses, has clear advantages in locating resonances with high energy precision and assisting in determining the spin and parity of resonances. Such information is very valuable in a complementary approach to indirect determinations of key reaction rates. [ABSTRACT FROM AUTHOR]

The identification of single-particle states in heavy actinide nuclei by means of studying their decay schemes plays a seminal role in understanding the structure of the heaviest elements and testing the predictive power of modern theoretical models. The heaviest odd-mass nuclides available in sufficient quantity for detailed decay spectroscopic studies are 20-h 255 Fm (for neutrons) and 20-d 253Es (for protons). Decay spectra of these isotopes, together with those for the odd-odd 276-d 254Es nuclide, were measured using a variety of α-particle and γ-ray spectroscopy techniques. Well-defined decay data are also essential pre-requisites for the detection and accurate characterization of fissile radionuclides. The parameters of greatest relevance include actinide half-lives, branching fractions, and α-particle and γ-ray energies and emission probabilities. Their quantification to good accuracy provides the means of monitoring their presence, behavior and transport in nuclear facilities as well as any clandestine movement and usage. As a consequence of recommendations made at recent IAEA research coordination meetings on “Updated Decay Data Library for Actinides,” measurements were undertaken to determine specific decay data of the more inadequately defined radionuclides. [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]

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]

Proton radioactivity of 117La, was measured with much improved statistics, but no evidence is observed for the isomeric proton decay. Prompt γ-rays were identified in proton emitters 117La, 151Lu, and beta-delayed proton precursor 117Ba using recoil decay tagging method by coupling the Gammasphere Ge array at the target position with the Argonne Fragment Mass Analyzer (FMA). [ABSTRACT FROM AUTHOR]

A new decay mode, the so called prompt proton decay, was discovered in 1998. It has since proven to be an important decay mechanism for several neutron deficient nuclei in the A∼60 region. To measure with high accuracy the energies and angular distributions of these protons, a state-of-the-art charged particle detector—LuWuSiA—was developed. It was first utilized during a fusion-evaporation reaction experiment performed at Argonne National Laboratory, U.S.A. In this contribution, the characteristics of the prompt proton decay are discussed along with the special features of LuWuSiA as well as a revisit to the prompt proton decay in 58Cu. [ABSTRACT FROM AUTHOR]

There are many new results in the study of the heavy-ion fusion hindrance phenomenon at extreme sub-barrier energies since Fusion03. Some of them have been published. Presented are some new results, including the first evidence in fusion hindrance for a system with small negative Q-value, 28Si + 64Ni, and the impact of fusion hindrance on reactions of interest to astrophysics. © 2006 American Institute of Physics [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]

Coulomb excitation of Kπ=6+(t1/2=77 ns), 8-(t1/2=4.0 s) and 16+(t1/2=31 y) 178Hf isomers has led to the measurement of a set of Eλ matrix elements, coupling the isomer bands to the γ- and ground state bands. The resulting matrix elements, derived using a coupled-channel semiclassical Coulomb excitation search code, have been used to probe the K-components in the wave functions and revealed the onset and saturation of K-mixing in low-K bands, whereas K-mixing is negligible in the high-K bands. The implications can be applied to other quadrupole-deformed nuclei. An upper limit on the Coulomb depopulation yield of the 16+ isomer was calculated based on the present set of matrix elements. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

α-decay is known in a few cases just above 100Sn. The lowest isospin nuclei in which it has been observed are the Tz=1 nuclei 114Ba, 110Xe, and 106Te. For the next lower isospin with Tz=1/2, N=Z+1, we used the reactions 58Ni + 58Ni and 58Ni + 54Fe to search for the 113Ba→109Xe→105Te→101Sn α-decay chain. We found no candidates in our study. We set upper limits on the production of 113Ba via the 58Ni + 58Ni reaction and upper limits on the production of 109Xe via the 58Ni + 54Fe reaction. © 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]

Gamma decays depopulating excited states in the odd-odd N=Z - 2 nucleus 48Mn have been observed for the first time. The yrast band has been built up from the 4+ ground state to 13+, just shy of the expected band termination at 15+. When compared with its mirror, 48V, the Coulomb energy differences are unlike any other previously measured which, until now, have been dominated by a sudden rotational alignment. Such an alignment is expected to be blocked in these odd-odd nuclei and calculations not only confirm this, but also infer that any Coulomb difference is mostly due to monopole effects. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

High-K states in the region of 177Lu have been studied using multi-nucleon transfer reactions with 136Xe beams and Gammasphere. Results include identification of the predicted 5-quasiparticle Kπ = 39/2 - isomer in 177Lu, a 7-quasiparticle Kπ = 49/2 + isomer in 179Ta with an anomalously fast decay, and numerous other examples in a range of Yb close to stability. The results are discussed in the context of the expectations for multi-quasiparticle states near Z = 72 and the factors which may both govern isomer formation and also give an insight into K-purity, specifically chance degeneracies, and statistical mixing above the yrast line. © 2005 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]

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]

Gamma rays from the transitional proton emitting nuclei 145,146,147Tm have been observed using the recoil-decay tagging technique. The ground state band of 147Tm was confirmed and extended and the unfavoured signature sequence was observed. A ground state rotational band with properties of a decoupled h11/2 band was observed in 145Tm. In addition coincidences between the proton fine structure line and the 2+→0+ γ-ray transition in 144Er were detected at the focal plane of the FMA. This is the first time that coincidences between proton radioactive decays and γ rays have been seen. The particle decay of 146Tm has been measured with improved statistics and a rotational band similar to 147Tm has been observed. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The level structure of 249Bk has been investigated by measuring the γ-ray spectra following the α decay of a chemically and isotopically pure 253Es sample. Alpha-gamma coincidence measurement was performed using a Si detector for α particles and a 25% Ge detector for γ rays. A gamma-gamma coincidence measurement was performed with the Gammasphere spectrometer. The Es sample was obtained by extracting the 253Es which grew in a 253Cf source material produced in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Additional information on the 249Bk levels was obtained from the study of γ rays produced in the β- decay of 249Cm. The 249Cm sample was produced by neutron irradiation of 248Cm. Using the results of the present study and the results of previous 248Cm(α,t) and 248Cm(3He,d) reaction spectra, the following single-particle states have been identified in 249Bk: 7/2+[633], 0.0 keV; 3/2-[521], 8.78 keV; 1/2+[400], 377.55 keV: 5/2+[642], 389.17 keV; 1/2-[530], 569.19 keV; 1/2-[521], 643.0 keV; 5/2-[523], 672.8 keV; 9/2+[624], 1075.1 keV. Four vibrational bands were identified at 767.9, 932.2, 1150.7 and 1223.0 keV with tentative assignments of {7/2+[633]x1-}9/2-, {7/2+[633]x0-}7/2-, {7/2+[633]x1-}5/2- and {7/2+[633]x0+}7/2+, respectively. © 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 structure and decay of neutron-rich r-process nuclides has been studied by a variety of means that take advantage of enhanced selectivity to permit identification of exotic nuclides. New level structures are presented for 134,135Sb along with data for Ag isomers and Cd yrast structures. Some of the properties measured play an important role in calculations of the yields of elements and isotopes produced in r-process nucleosynthesis that takes place at high temperature in the presence of large densities of neutrons. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The anomalous proton-decay rates in 131Eu and 141Ho have been explained by the presence of large quadrupole deformation. Consistent with this hypothesis was the discovery of the decay branch in 131Eu to the 2+ state in the daughter nucleus. Direct evidence that 141Ho and 131Eu are deformed came from the observation of rotational bands. From the dynamic moment of inertia the deformation of β2=0.25(4) was deduced for the 141Ho ground state. The large signature splitting in the ground-state band in 141Ho indicates that Coriolis mixing plays an important role in this nucleus. The comparison between the Particle-Rotor Model and the data indicates that 141Ho has significant hexadecapole deformation and/or might be triaxial. The comparison between theory and experiment constrains other parameters used in the proton decay rate calculations from deformed nuclei such as the Coriolis interaction and pairing strength. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Excited structures in the proton-rich nucleus 140Dy were established following the decay of a 7.3±1.5 μs, Kπ = 8- isomer. The isomer decays into the yrast line at the 8+ state, revealing a rotational band with a deduced deformation of β2 = 0.24(3). The isotope 140Dy is the daughter of the deformed proton emitter 141Ho. The new information obtained here supports the role of deformation in proton emission and the previous assignments of single-particle configurations to the two proton emitting states in 141Ho. In addition, the reduced hindrance factor measured for the isomer is consistent with the trend observed in the N = 74 isotones and shows no deviation which could be attributed to the proximity of 140Dy to the proton drip line. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Several searches for new proton emitters were performed recently at ATLAS using the Argonne Fragment Mass Analyzer equipped with a Double-Sided Si Strip Detector. Proton emitting states were observed in odd-odd spherical nuclei [sup 164]Ir and [sup 170]Au. [sup 164]Ir is the fourth Ir proton emitter. A proton line was associated with the highly deformed odd-odd nucleus [sup 130]Eu. The proton emitter [sup 135]Tb was also observed. It is the first proton emitting Tb isotope. The estimated cross section for producing [sup 135]Tb is about 2 nb. The properties of the proton emitters deduced from this work helped to understand the role of core vibrations and neutrons in proton decay. In view of the present results the full delineation of the proton drip-line between Z=50 and Z=83 is feasible. In order to cope with small cross sections and short half lives expected for even more exotic proton emitters several modifications of the FMA implantation station were implemented. Measurements were also performed at the target position using a catcher to search for proton emitters with half lives down to 10 ns. [ABSTRACT FROM AUTHOR]

In a series of experiments, [sup 48]Ca induced cold fusion reactions on Pb targets were employed to populate states in [sup 253,254]No isotopes. Evidence for decays from a rotational band built on a 7/2[624] configuration was obtained in [sup 253]No. The ground-state band of [sup 254]No is identified up to spin 20[sup +]. The deduced quadrupole deformation is β=0.27(2). The initial angular momentum and excitation energy of [sup 253,254]No residues, after neutron emission, was determined. States up to spin 22h and excitation energy of E[sup *]=8.5 MeV were observed. The data provide direct information on the fission barrier at high spins B[sub f]>=5 MeV, and on the shell-correction energy E[sub shell][sup exp]>=4.1 MeV. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recent high-spin studies in the mass 60 region of highly deformed and superdeformed shapes are discussed. The experimental data are from reaction channel selected γ-spectroscopic measurements at Gammasphere. Points of special attention are rotational bands in nuclei below the Z=28 shell gap and measurements of transition quadrupole moments throughout the region. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]