Search

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.

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.

Examples are presented to illustrate the power of prompt γ‐ray spectroscopy of fission fragments from compound nuclei with A∼200 formed in fusion‐evaporation reactions in experiments using the Gammasphere Ge‐detector array. Complementary methods, such as Coulomb excitation and deep‐inelastic processes, are also discussed. In other cases (n,xnγ) reactions on stable isotopes have been used to establish neutron excitation functions for γ‐rays using a pulsed “white”‐neutron source, coupled to a high‐energy‐resolution germanium‐detector array. The excitation functions can unambiguously assign γ‐rays to a specific reaction product. Results from all these methods bridge the gaps in the systematics of high‐spin states between the neutron‐deficient and neutron‐rich nuclei. Results near shell closures should motivate new shell model calculations.

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.

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.

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.

The high spin states of neutron‐rich nuclei in A∼110 region have been carefully investigated by measuring prompt γ‐γ‐γ coincident measurements populated in the spontaneous fission of 252Cf with the Gammasphere detector array. Many new collective bands have been discovered. In this proceeding paper, we introduce some interesting new band structures recently observed by our cooperative groups, that is, the one‐phonon‐ and two‐phonon γ‐vibrational bands in odd‐A 103Nb, 105Mo and 107Tc, the chiral doublet bands in even‐even 106Mo, 110Ru and 112Ru, and the pseudospin partner bands with in 108Tc. The characteristics of these band structures have been discussed.The high spin states of neutron‐rich nuclei in A∼110 region have been carefully investigated by measuring prompt γ‐γ‐γ coincident measurements populated in the spontaneous fission of 252Cf with the Gammasphere detector array. Many new collective bands have been discovered. In this proceeding paper, we introduce some interesting new band structures recently observed by our cooperative groups, that is, the one‐phonon‐ and two‐phonon γ‐vibrational bands in odd‐A 103Nb, 105Mo and 107Tc, the chiral doublet bands in even‐even 106Mo, 110Ru and 112Ru, and the pseudospin partner bands with in 108Tc. The characteristics of these band structures have been discussed.

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.

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.

This work focus on the electromagnetic decay properties of the heaviest known mirror pair 67As and 67Se, produced in the fusion evaporation reaction 32S+40Ca, with a E = 90 MeV beam provided by the ATLAS accelerator at Argonne National Laboratory. The high selectivity required to investigate these proton‐rich nuclei was achieved with the concurrent employment of Gammasphere, Neutron‐Shell, and Microball. The full array allowed to obtain the level scheme of the 67Se nucleus up to 5.56 MeV, and 5 new transitions were assigned to the known 67As level‐scheme. The level spin and parities were assigned using the ADO analysis and symmetry arguments.

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.

The experimental knowledge of the Cu-61(29)32 and Zn-61(30)31 nuclei has been largely extended via the joint results from three experiments. The fusion-evaporation reaction used a Ar-36 beam and a Si-28 target foil to produce the two nuclei via the evaporation of either three protons (Cu-61) or two protons and a neutron (Zn-61). 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 similar to 60 region. (Less)

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 45ℏ 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 65ℏ. This talk reviews the status of the spectroscopy and understanding of the observed structures in these Er and neighbouring nuclei.

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.

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.

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‐s...

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).

The use of the so called Hk‐technique (γ‐energy sum and multiplicity data) is checked with simulations of the multi‐detector γ‐arrays GAMMASPHERE and SPINSPECTROMETER. Their response to typical γ‐cascades is obtained and compared.

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.

Gamma‐rays from the nuclide 138Ce have been measured with the Gammasphere‐array following Coulomb excitation. Beams of 138Ce ions were focussed on a carbon target at beam energies of 480 and 400 MeV. Gamma‐ray yields and relative Coulomb excitation cross sections were measured in singles‐mode. M1, E2, and E3 transition matrix elements from low‐spin states were measured relative to the known B(E2; 01+ → 21+). The 2+ → 21+ M1 strength distribution from the lowest six 2+ states up to 2.7 MeV enables us to identify the 24+ state of 138Ce as the dominant fragment of the one‐phonon 21,ms+ mixed‐symmetry state with F‐spin quantum number Fmax − 1. From its mixing with the nearby 23+ state an F‐spin mixing matrix element of 44 keV can be estimated.

Gamma‐ray spectroscopy measurements with Gammasphere and multi‐nucleon transfer reactions with heavy beams provide a means of accessing deformed nuclei at stability, or on the neutron‐rich side of the stability line. New results on the discovery of high‐K isomers in the Er‐Tm‐Yb‐Lu region are discussed with a focus on the energy systematics and the factors which may govern K‐purity and therefore K‐hindrances. These include random‐mixing in regions of high level density, specific Coriolis mixing in configurations involving the i13/2 neutron configuration, and chance degeneracies between an individual high‐K state and a specific collective state which provide an opportunity to extract effective mixing matrix elements between states of very different‐K.

High‐spin states in 3674Kr38 were studied using the 40Ca(40Ca,α2p)74Kr fusion evaporation reaction at a beam energy of 165 MeV with GAMMASPHERE and MICROBALL and at a beam energy of 185 MeV with EUROBALL and ISIS multi‐detector arrays. Lifetimes of the high‐spin states for the ground‐state band and the favoured negative‐parity band have been determined using the Doppler‐shift attenuation method. The deduced transition quadrupole moments show a marginal decrease as a function of spin, suggesting that the rotational bands do not terminate at the maximum spin Imax.

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 ⊗ νd3/2 ⊗ πd5/2 configuration. A previously unobserved isomer has been identified in 99Mo at an en...

Two experiments to identify hyperdeformation in Xe‐nuclei have been made using the reaction 82Se(48Ca,xn)130−xXe, the first one with the EUROBALL‐IV array equipped with a BGO calorimeter and the second one with the Gammasphere array. A bombarding energy of 195 MeV was used with the VIVITRON and 206 MeV with the ATLAS accelerator. No discrete hyperdeformed bands have been identified, but a ridge structure with 4 ℏ2/J ∼ 0.048 MeV corresponding to a very large deformation was observed. A fluctuation analysis revealed that these ridges are composed of more than 10 rotational bands with ⩾ 5 transitions each. Seven and four discrete bands extending from I ∼ 20 to I ∼ 60 have been identified in 125Xe and in 126Xe respectively, with a ΔEγ of 100 – 120 keV. These bands are most probably related to a potential energy minimum at e ∼ 0.35 and a small γ‐deformation predicted by Ultimate Crancker calculations. The transition quadrupole moments, crossing frequencies and alignments observed in the bands compare very well...

With the coupling of large γ‐ray arrays to recoil mass spectrometers, in‐beam gamma‐ray studies can be performed on nuclei lying at the very edges of stability. We have recently performed in‐beam experiments using Gammasphere coupled to the FMA to measure excited states in proton unbound Au isotopes and neighboring Hg isotopes. These studies have allowed us to characterize the evolution of nuclear shapes as one approaches the proton drip line. In two of the isotopes studied, namely, 175Au and 179Hg, structures built on three distinct shapes at low excitation energy have been observed.

The results from two surrogate reaction experiments using the STARS (Silicon Telescope Array for Reaction Studies) spectrometer are presented. The surrogate method involves measuring the particle and/or γ‐ray decay probabilities of excited nuclei populated via a direct reaction. These probabilities can then be used to deduce neutron‐induced reaction cross sections that lead to the same compound nuclei. In the first experiment STARS coupled to the GAMMASPHERE γ‐ray spectrometer successfully reproduce surrogate (n,γ), (n,n′γ) and (n,2nγ) cross sections on 155,156Gd using 3He‐induced reactions. In the second series of experiments an energetic deuteron beam from the ESTU tandem at the Wright Nuclear Structure Lab at Yale University was used to obtain the ratio of fission probabilities for 238U/236U and 237U/239U populated using the 236,238U(d,d′f) and 236,238U(d,pf) reactions. Results from these experiments are presented and the implications for the surrogate reaction technique are discussed.

We have performed three separate experiments at LBNL’s 88‐Inch Cyclotron over a period of several years in which 235U (thick target) was Coulomb‐excited. It involved stand‐alone experiments with Gammasphere and with the 8PI Spectrometer using 136Xe beams at 720 MeV, and a CHICO‐Gammasphere experiment with an 40Ar beam at 180 MeV. In addition to extending the known negative‐parity bands to high spin, we have assigned levels in some seven positive‐parity bands which are in some cases ( e.g. [631]1/2, [624]7/2 and [622]5/2) strongly populated by E3 excitation. The CHICO data has been analyzed to extract E2 and E3 matrix elements from the observed yields. Additionally, many E1 and M1 matrix elements could be extracted from the γ‐ray branching ratios.

Several experiments were performed in recent years with the Euroball and Gammasphere spectrometers to search for high‐spin hyperdeformation in atomic nuclei. Possible evidence for hyperdeformation are the ridge structures with small energy separations observed in the A = 125 region in quasi‐continuum γ‐ray correlation spectra. These ridges are only found when the highest‐multiplicity events are selected in the data from experiments with the highest bombarding energies. A statistical analysis shows that the ridges are formed by ten or more bands and that the intensity of the individual bands is below 10−6 of the reaction‐channel intensity.

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...

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.

Two Gammasphere experiments have been performed in order to establish the possible triaxial nature of strongly deformed (SD) bands in 174Hf. A lifetime measurement confirmed the large deformation of the four previously observed bands in this nucleus. In addition, a thin‐target, high‐statistics experiment was carried out to search for linking transitions between the SD bands. No such transitions, which represent an experimental signature for wobbling modes, were observed. Four new SD bands were found in 174Hf together with a single SD band in 173Hf. These results indicate that the strongly deformed sequences of N ≈ 102 Hf isotopes behave differently than the TSD bands found in Lu nuclei near N = 92. The interpretation of these bands in terms of possible stable triaxial deformation is confronted with the experimental findings and UC predictions.

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....

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.

A high‐efficiency evaporation‐residue counter (named HERCULES) has been built. It consists of 64 thin fast‐plastic scintillators arranged in four angular rings. Its design has been optimized for use with GAMMASPHERE. Presented are: the design and performance of the detection system, the most important results from its commissioning run (using 48Ti + 126Te and 48Ti + 142Nd reactions), and a brief outlook for spectroscopic studies in the actinide region.

The possible occurrence of sub‐shell gaps at N = 32 and N = 34 in neutron‐rich titanium isotopes is discussed in light of new experimental results from (i) deep‐inelastic reactions measured with Gammasphere at the ATLAS facility at Argonne National Laboratory and from (ii) intermediate‐energy Coulomb excitation performed at the National Superconducting Cyclotron Laboratory at Michigan State University.

Recent experiments with Gammasphere and the Microball charged‐particle detector array have led to the identification, and have enabled the detailed spectroscopic study, of highly‐deformed and superdeformed rotational bands in a number of A ∼ 40, N ≈ Z nuclei. These rotational bands are built on multi‐particle multi‐hole excitations across the N,Z = 20 shell gap from the sd to the pf shell and, in the context of the Nilsson scheme, are associated with the shell gaps formed at large prolate deformation for particle numbers N,Z = 16, 18, 20 as pf‐shell intruder orbitals are occupied. In analogy with well‐developed rotational motion in heavy nuclei, these highly collective rotational structures involve active valence particles in two major shells for both protons and neutrons, yet the A ∼ 40 bands possess the unique advantage that the valence space dimension remains small enough to be approached from a shell‐model perspective. These rotational bands thus provide an ideal opportunity for detailed comparisons between experimental data and both microscopic and macroscopic models of collective motion in nuclei.

Neutron‐rich nuclei around A∼100 were populated as fission fragments produced by the 238U(α,f) fusion‐fission reaction. The deexcitation γ rays were detected by Gammasphere in coincidence with the detection of both fission fragments by the Rochester 4π heavy‐ion detector array, CHICO. This technique allows Doppler‐shift corrections to be applied for the observed γ rays on an event‐by‐event basis thus establishing the origin of γ rays from either fission fragment. In addition, it allows observation of γ‐ray transitions from states with short lifetimes and offers the opportunity to study nuclear species beyond the reach of the spontaneous fission process. With these advantages, one can extend the spectroscopic study to higher spins than those derived using the thick‐target technique, and to more neutron‐rich nuclei than those derived from spontaneous fissions. Among the new and interesting phenomena identified in this rapid shape‐changing region, the most distinct result is the evidence for a prolate‐to‐obl...

The present status of prompt particle emission from high‐spin states in the mass A ∼ 60 region is shortly summarized. The focus lies on recent results from a Gammasphere experiment, which aimed at the combined in‐beam high‐resolution spectroscopy of light charged particles and γ rays. More detailed studies of known cases as well as the identification of new cases of discrete prompt proton and α‐particle emission from highly‐ and superdeformed states have been possible. Finally, future directions are briefly outlined.

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 12C(12C,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 Ebeam=15.8 MeV. Favored population of the K=2 rotational band in 24Mg 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 1μb.

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.

New analyses of spontaneous-fission gamma data from our 252Cf Gammasphere runs of 1995 and 2000 have identified new transitions and given improved transition energy and intensity values for known transitions in even-odd fission-product nuclei just beyond the 82-neutron closed shell. New level schemes of N=83 isotones 139Ba83 and 135Te83 and of N=85 isotones 141Ba85, and 139Xe85 are established. Single-particle (hole) excitations and couplings, core excitations, and octupole excitations are assigned to these isotones. A regular sequence found in 135Te is interpreted as a shears band within the tilted-axis cranking model of Frauendorf. The level systematics of N=85 isotones indicate that the octupole excitation moves to higher excitation energy as the Z=50 shell is approached.

The creation of a specific heavy ion residue via heavy ion fusion can usually be achieved through a number of beam and target combinations. Sometimes it is necessary to choose combinations with rare beams and/or difficult targets in order to achieve the physics goals of an experiment. A case in point was a recent experiment to produce 152Dy at very high spins and low excitation energy with detection of the residue in a recoil mass analyzer. Both to create the nucleus cold and with a small recoil-cone so that the efficiency of the mass analyzer would be high, it was necessary to use the 80Se on 76Ge reaction rather than the standard 48Ca on 108Pd reaction. Because the recoil velocity of the 152Dy residues was very high using this symmetric reaction (5% v/c), it was furthermore necessary to use a stack of two thin targets to reduce the Doppler broadening. Germanium targets are fragile and do not withstand high beam currents, therefore the 76Ge target stacks were mounted on a rotating target wheel. A descrip...

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 220Th at beam energies of 206 MeV and ...

The results of lifetime measurements on low-lying superdeformed (SD) states in the mass-190 region are summarized. Recoil distance Doppler-shift experiments were performed with the large gamma-ray arrays GASP and Gammasphere employing the Cologne plunger device. The results show that decay out of the SD bands is governed by a weak mixing of cold SD with hot normal deformed (ND) states. Estimates for squared ND mixing amplitudes an2 and the maximum spreading width Γmax↓ of the SD states are presented and compared to theoretical predictions. The known linking transitions in 194Hg and 194Pb show no enhanced transition strength, supporting the assumption of a statistical decay of the hot ND states. Monte Carlo calculations based on a nearest neighbor mixing approach are presented and the spin dependence of Γ↓ is discussed.

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 {gamma}-ray studies which extended one's 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 {sup 141}Ho and {sup 131}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 {sup 141}Ho and {sup 131}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 {sup 131}Eu is only about 50 nb, and it represents the weakest channel ever studied in an in-beam experiment.

The high spin states of 240Pu and 244Pu have been investigated with GAMMASPHERE at ATLAS, using Coulomb excitation with a 208Pb beam at energies above the Coulomb barrier. Data on a transfer channel leading to 242Pu were obtained as well. In the case of 244Pu, the yrast band was extended to 34ℏ, revealing the completed πi13/2 alignment, a “first” for actinide nuclei. The yrast sequence of 242Pu was also extended to higher spin and a similar backbend was delineated. In contrast, while the ground state band of 240Pu was measured up to the highest rotational frequencies ever reported in the actinide region (∼300 keV), no sign of particle alignment was observed. In this case, several observables such as the large B(E1)/B(E2) branching ratios in the negative parity band, and the vanishing energy staggering between the negative and positive parity bands suggest that the strength of octupole correlations increases with rotational frequency. These stronger correlations may well be responsible for delaying or supp...

Three long, strongly coupled (ΔI=1) sequences have been identified in 110,112Te by using the GAMMASPHERE array in conjunction with the MICROBALL charged-particle array. These bands are interpreted in terms of deformed proton 1-particle–1-hole bands that reach termination at I∼40ℏ. This is the first observation of such collective dipole structures in this mass region. In contrast, many shorter dipole sequences have been associated with weakly deformed structures that generate angular momentum by the shears mechanism (magnetic rotation).

Excited states in the N = Z+1 nucleus Y-79 Were identified using the reaction Si-28(Fe-54, p2n)Y-79 at a 200 MeV beam energy and an experimental set up consisting of an array of Ge detectors and the Recoil Mass Spectrometer at Oak Ridge National Laboratory. With the help of additional gamma-gamma coincidence data obtained with Gammasphere, these gamma-rays were found to form a strongly-coupled rotational band with rigid-rotor-like behavior. Results of conventional Nilsson-Strutinsky cranked shell model calculations, which predict a deformation of beta(2)similar to 0.4, are in excellent agreement with the properties of this band. Similar calculations for the neighboring N = Z and N = Z + 1 nuclei are also in good agreement with experimental data. This suggests that the presence of the putative T = 0 neutron-proton pairing does not significantly affect such simple observables as the moments of inertia of these bands at low spins. (Less)

Superdeformation has been established for over a decade in the mass 150 region and nearly as long in the A=190 region. The first measurements directed at nuclei in these regions concentrated on mapping out the superdeformed (SD) islands by identifying SD rotational bands in γ-ray coincidence data. These early studies provided new insights into the physics of superdeformation, but also raised unexpected issues. The new gamma-ray arrays (Gammasphere, Eurogam/Euroball and Gasp) have provided a wealth of new data on properties of SD states in these two mass regions. This paper highlights some of the more recent results from the large arrays which have addressed the outstanding issues in the field, namely, ΔI=4 staggering, identical bands, SD vibrational bands, and questions about the feeding into and the decay out of the SD well.

High-spin states in proton-rich A∼60 nuclei on and near the N=Z line have been studied in a series of experiments performed with Gammasphere and the Microball charged-particle detector array. These experiments have led to the identification of more than 40 deformed and superdeformed rotational bands in more than a dozen different Ni, Cu, and Zn isotopes. Because of the limited spin content of the single-particle configurations in these relatively light nuclei, many of these bands are observed to, or close to, their terminating states. Lifetime measurements for two sets of strongly coupled rotational bands in 62Zn have confirmed the predicted loss of collectivity associated with the phenomenon of smooth band termination. Superdeformed bands have been observed in 60,61,62,65Zn, firmly establishing an island of superdeformation for nuclei with particle numbers N,Z≈30. Linking transitions connecting the doubly-magic SD band in the N=Z nucleus 60Zn to the yrast line have been identified. These linking transiti...