Search

The design and performance of a new Compton-suppressed HPGe and charged-particle array, CLARION2-TRINITY, are described. The TRINITY charged-particle array is comprised of 64 Cerium-doped Gadolinium Aluminium Gallium Garnet (GAGG:Ce) crystals configured into five rings spanning 7-54 degrees, and two annular silicon detectors that can shadow or extend the angular coverage to backward angles with minimal $\gamma$-ray attenuation. GAGG:Ce is a non-hygroscopic, bright, and relatively fast scintillator with a light distribution well matched to SiPMs. Count rates up to 40 kHz per crystal are sustainable. Fundamental characteristics of GAGG:Ce are measured and presented, including light- and heavy-ion particle identification (PID) capability, pulse-height defects, radiation hardness, and emission spectra. The CLARION2 array consists of up to 16 Compton-suppressed HPGe Clover detectors ($\approx4\%$ efficiency at 1 MeV) configured into four rings (eight HPGe crystal rings) using a non-Archimedean geometry that suppresses back-to-back coincident 511-keV gamma rays. The entire array is instrumented with 100- and 500-MHz (14 bit) waveform digitizers which enable triggerless operation, pulse-shape discrimination, fast timing, and pileup correction. Finally, two examples of experimental data taken during the commissioning of the CLARION2-TRINITY system are given: a PID spectrum from $^{16}$O + $^{18}$O fusion-evaporation, and PID and Doppler-corrected $\gamma$-ray spectra from $^{48}$Ti + $^{12}$C Coulomb excitation., Comment: 14 pages, 21 figures

Background: At energies above the capture barrier, coupled-channels (CC) calculations with a standard nuclear potential diffuseness (0.65 fm) increasingly overestimate experimental capture cross section as the charge product (${Z}_{1}{Z}_{2}$) of the colliding nuclei increases. It has been suggested this may be linked to energy dissipation outside the capture barrier.Purpose: We investigate quantitatively the role of energy dissipation in suppressing capture in heavy ion fusion reactions.Method: The yields of sequential fission, including that resulting from deep inelastic collisions, and of fission following capture were determined simultaneously for collisions of $^{18}\mathrm{O}, ^{30}\mathrm{Si}, ^{34}\mathrm{S}$, and $^{40}\mathrm{Ca}+^{232}\mathrm{Th}$ at a range of energies around the respective capture barriers.Results: The ratio of experimental to CC capture cross sections was found to decrease with increasing ${Z}_{1}{Z}_{2}$. Conversely, the ratio of sequential fission to capture-fission increased with increasing ${Z}_{1}{Z}_{2}$. The sum of sequential and capture fission agrees quite well with the CC cross sections.Conclusions: The experimental capture fission and sequential fission cross sections, and their comparison with CC calculations, give a consistent picture that the increase in density overlap at the capture barrier with increasing ${Z}_{1}{Z}_{2}$ of the colliding nuclei is correlated with increasing energy dissipative processes. These compete increasingly strongly with capture as the ${Z}_{1}{Z}_{2}$ of the reaction increases. For the $^{40}\mathrm{Ca}$ reaction, the total fission yield exceeds expectations from capture model calculations, indicating that deep inelastic processes occur both from trajectories that would have led to capture and also from more peripheral trajectories.

New 125 I atomic decay emission data of medical interest are presented. The calculations are based on two atomic structure codes that implement the multi-configuration Dirac-Hartree-Fock method. Radiative and non-radiative ransition rates are calculated in this method and then used to generate the atomic deexcitation cascade. Subshell transition rates, level widths and fluorescence yields are compared to the Evaluated Atomic Data Library. Coster-Kronig and Auger electron emission yields are also compared with results from other authors. The comparison with the experimental electron emission spectrum shows that the new calculations can reproduce very well the structure of the K-LL Auger electron peaks and improve the description of the M Auger peaks below 300 eV. The 125 I dose-point kernel is also simulated using the new data, resulting in higher values below 10 nm when compared those obtained with the Evaluated Atomic Data Library.

Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Lowexcitation energies favor SHE survival agains ...

Microstructural evolution during the early stages of ageing (less than one hour) in a Ni-Cr-Fe based superalloy Inconel 718 (IN718) has been investigated using Small-Angle X-ray Scattering (SAXS). The effects of precipitate kinetics on the precipitate size distribution are compared indirectly with SAXS measurements by using Vickers microhardness data. The microhardness increased after 4 min of ageing at a temperature of 760 degrees C, although the recorded SAXS data did not reveal the precipitate size distribution. This indicates that the precipitates had not evolved enough to be detected, but still a small number of precipitates increased the yield strength. After ageing the alloy for the shortest period for which data were available, 8 min, clear evidence of precipitates could be found from the SAXS data, showing that the gamma '' - precipitates are about 6 nm in width and 3 nm in height.

Superheavy elements are created in the laboratory by the fusion of two heavy nuclei. The large Coulomb repulsion that makes superheavy elements decay also makes the fusion process that forms them very unlikely. Instead, after sticking together for a short time, the two nuclei usually come apart, in a process called quasifission. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission. A systematic study of carefully chosen mass-angle distributions has provided information on the global trends of quasifission. Large deviations from these systematics reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and thus implicitly in hindering or favouring superheavy element production.

The SOLEROO ( sol enoidal e xotic r are is o t o pe) separator is under development for the production of light radioactive ion beams at the Australian National University. The separator is based on a single superconducting solenoid followed by two tracking detectors. Cross-section and beam purity measurements have been made to optimise the production of 6 He, 8 Li, 10 Be and 12 B beams. Having validated the trajectory simulation code SoliX, the optimal geometry for utilisation of the tracking detectors has also been explored.

The breakup mechanisms for 9 Be have been studied through coincident detection of the breakup α particles at sub-barrier energies, where the probability of capture of a charged breakup fragment by the target nucleus is negligible. Combining the reconstructed reaction Q-value and the relative energy of the two coincident breakup α particles a complete picture of breakup dynamics has been obtained. The measurements reveal breakup processes which are fast enough (~ 10-22 s) to affect fusion, called prompt breakup. It is shown that prompt breakup is triggered predominantly by the transfer of a neutron -a two-step process. The prompt breakup probabilities are shown to have an exponential dependence on the surface-to-surface separation of the interacting nuclei, but are independent of the target nuclei, ranging from 144 Sm to 209 Bi . These results provide significant input towards a complete quantal model aiming to describe the breakup of weakly bound stable and halo nuclei.

A low-mass radioactive ion beam capability in Australia has been developed using a 6.5 T superconducting solenoid as the separator element. The separator, called SOLEROO , separates the large background of primary-beam particles from the radioactive species of interest. A further rejection of remnant unwanted nuclear species leaving the solenoid is achieved by tracking each emerging particle and identifying them event-by-event using a pair of position sensitive parallel plate avalanche counters. With primary 7 Li beam current of 1 e μ A , a 6 He production rate of 1.2 × 10 5 s − 1 has been achieved. The tagged secondary beam will be combined with a high efficiency 512 pixel silicon detector array for nuclear experiments.

Using a back-angle detector array covering 117° to 167°, coincidence measurements of breakup fragments at sub-barrier energies have enabled the complete characterisation of the breakup processes in the reactions of 6,7 Li with 208 Pb. Those breakup processes fast enough ( ∼ 10 − 22 s ) to affect fusion are identified through the measured relative energy of the two breakup fragments. The majority of these prompt breakup events are triggered by transfer of a neutron from 6 Li, and of a proton to 7 Li. These mechanisms, rather than breakup following direct projectile excitation, should thus be responsible for the majority of the ∼ 30 % suppression of complete fusion observed at above-barrier energies. Breakup characteristics thus depend both on the properties of the initial nucleus and its neighbours. Quantitative modelling of this two-step process will require development of a complete reactions model, relevant for reactions involving both α -cluster nuclei, and exotic nuclei near the neutron and proton drip-lines.

A largely extended experimental knowledge of the Cu-61(29)32 nucleus has been obtained from three experiments. Excited states in Cu-61 were produced via the fusion-evaporation reaction Si-28(Ar-36, 3p)Cu-61. In addition to the Ge array GAMMASPHERE, neutron and charged-particle detectors placed around the target position were used for high-performance particle spectroscopy. The constructed level scheme includes more than 160 energy levels and 320 gamma-ray transitions belonging to both normally deformed as well as superdeformed rotational structures. The multipolarities have been determined for the gamma-ray transitions and as a result spin-parity assignments are given for nearly all energy levels. Experimental results in the normally deformed region are compared with predictions from large-scale shell model calculations. The collective structures are compared with results from cranked Nilsson-Strutinsky calculations. The results reveal the need to modify the standard Nilsson parameters in the mass A similar to 60 region. (Less)

Background: The fusion-evaporation reaction at energies around the Coulomb barrier is presently the only way to produce the heaviest elements. However, formation of evaporation residues is strongly hindered due to the competing fusion-fission and quasifission processes. Presently, a full understanding of these processes and their relationships has not been reached.Purpose: This work aims to use new fission measurements and existing evaporation residue and fission excitation function data for reactions forming Cf isotopes to investigate the dependence of the quasifission probability and characteristics on the identities of the two colliding nuclei in heavy element formation reactions.Method: Using the Australian National University's 14UD electrostatic accelerator and CUBE detector array, fission fragments from the ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$, ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{208}\mathrm{Pb}$, and ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reactions were measured. Mass and angle distributions of fission fragments were extracted and compared to investigate the presence and characteristics of quasifission.Results: Mass-angle-correlated fission fragments were observed for the ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reaction; no correlation was observed in the other reactions measured. Flat-topped fission-fragment mass distributions were observed for ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$ at compound-nucleus excitation energies from 28 to 52 MeV. Less pronounced flat-topped distributions were observed, with very similar shapes, for all three sulfur-induced reactions at excitation energies lower than 45 MeV.Conclusions: A high probability of long-time-scale quasifission seems necessary to explain both the fission and evaporation residue data for the ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$ and ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$ reactions. Flat-topped mass distributions observed for ${}^{12}\mathrm{C}$- and ${}^{34,36}\mathrm{S}$-induced reactions are suggested to originate both from late-chance fusion-fission at low excitation energies and the persistence of shell effects at the higher energies associated with quasifission.

Deep inelastic collisions (DICs) can compete strongly with fusion in collisions of heavy nuclei. However, standard coupled-channels calculations do not take DIC processes into account. As a result, calculations have been shown to overestimate the fusion cross-sections, resulting in a discrepancy between experimental data and theoretical calculations, particularly at energies above the fusion barrier. To investigate this discrepancy, we conducted a series of experiments using the ANU 14UD tandem accelerator and the CUBE 2-body fission spectrometer to examine the competition between transfer/DIC and fusion. In particular, fusion-fission and 3-body fission yields have been extracted for 34 S + 232 Th and 40 Ca + 232 Th systems. This work shows that the transfer-fission probability is enhanced relative to fusion-fission for 40 Ca + 232 Th, when compared to 34 S+ 232 Th. It is suggested that the enhancement of this DIC process in 40 Ca + 232 Th is linked to an increase in the density overlap of the colliding nuclei as a function of the charge product and contributes to fusion hindrance.

We examined the 40 Ca + 238 U reaction experimentally using the Mass-Angle Distribution (MAD) technique and within the Time Dependent Hartree Fock (TDHF) theory, using the TDHF3D code. A new, TDHF based approach has been developed to construct mass distributions as well. The results revealed that the orientation of the heavy deformed prolate nucleus plays a major role in the reaction outcome. It was found that aligned collisions lead to quasifission and short contact times of 5-10 zs, whilst anti-aligned collisions lead to longer contact times (> 23 zs). TDHF accurately predicted the presence of quasifission and the average mass splits in this reaction. The influence of shell effects around 208 Pb in the calculated quasifission characteristics was observed in both experiment and theory.

Mass-angle distributions carry detailed information on the characteristics of quasifission, and thus of the dynamics of heavy element formation reactions. Recent experimental results are presented and discussed.

This research was supported in part by the Swedish Research Council and the U.S. DOE grant DE-AC05-000R22725.

Novel experimental results on mirror nuclei in the sd and fp shells are presented. Their respective Mirror Energy Difference (MED) diagrams are interpreted by means of large-scale shell-model calculations. A unique way of extracting effective charges from isospin symmetry studies is also discussed.

Recent results and experiments are presented aiming at nuclear structure studies in the vicinity of the doubly-magic isotope Ni-56. The feature of prompt particle emission is revised, and new studies of mirror nuclei in the upper fp shell are briefly discussed.

We present the Pad Chamber detector system in the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. The three station system provides space points along each track in the spectrometer arms at mid-rapidity and covers a total area of 88 m(2). Its main functions are to provide the track coordinate along the beam and to ensure reliable pattern recognition at very high particle multiplicity. A new concept for two dimensional wire chamber readout via its finely segmented cathode was developed. The full readout system, comprising 172 800 electronic channels, is described together with the challenging design of the chambers. The electronics, mounted on the outer chamber face, together with the chamber itself amounts to 1.2% of a radiation length. Results from cosmic ray tests, showing an average efficiency better than 99.5% for all chambers are presented. The experiences from the full scale operation in the first run are reported. (C) 2002 Elsevier Science B.V. All rights reserved. (Less)

The quasi-fission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in $^{40}$Ca$+^{238}$U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed $^{238}$U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of $^{238}$U produce quasi-fission fragments in the magic $Z=82$ region, whilst collisions with the side are the only one which may result in fusion., 5 pages, 4 figures; Accepted for publication in Physical Review Letters

The formation of a compound nucleus in different projectile and target combinations is a powerful method for investigating the fusion process. Recently, the dominance of quasi-fission over fusion-fission has been inferred for 34 S+208 Pb in comparison to 36 S+206 Pb; both reactions lead to the compound nucleus 242 Cf* .The mass and angle distributions of the fission fragments from these reactions were studied in order to further investigate the presence of quasi-fission.

Background: The formation of superheavy elements by fusion of two massive nuclei is severely inhibited by the competing quasifission process.Purpose: Through extensive mass-angle distribution measurements, we map out the systematic dependence of quasifission characteristics as a function of the identity of the colliding nuclei.Methods: The Australian National University's Heavy Ion Accelerator Facility and CUBE spectrometer have been used to measure mass-angle distributions for 42 reactions forming heavy elements. Beam energies above their respective capture barriers were used to minimize the known influence of nuclear structure effects.Results: Different mappings of mass-angle distribution characteristics (including timescales) over the nuclear landscape show a systematic dependence on entrance channel and compound nucleus fissilities.Conclusions: The results provide an empirical baseline to assess effects of nuclear structure at lower beam energies, and motivate the testing and validation of complete dynamical models of heavy element fusion through comparison of mass-angle distributions.

Background: Quasifission, a fission-like reaction outcome in which no compound nucleus forms, is an important competitor to fusion in reactions leading to superheavy elements. The precise mechanisms driving the competition between quasifission and fusion are not well understood.Purpose: To understand the influence reaction parameters have on quasifission probabilities, an investigation into the evolution of quasifission signatures as a function of entrance channel parameters is required.Methods: Using the Australian National University's (ANU) CUBE detector for two-body fission studies, measurements were made for a wide range of reactions forming isotopes of curium. Important quasifission signatures---namely, mass-ratio spectra, mass-angle distributions, and angular anisotropies---were extracted.Results: Evidence of quasifission was observed in all reactions, even for those using the lightest projectile (${}^{12}\text{C}+{}^{232}\text{Th}$). But the observables showing evidence of quasifission were not the same for all reactions. In all cases, mass distributions provided some evidence of the possible presence of quasifission but were not sufficient in most cases to clearly identify reactions for which quasifission was important. For reactions using light projectiles (${}^{12}$C, ${}^{28,30}$Si, ${}^{32}$S), experimental angular anisotropies provided the clearest signature of quasifission. For reactions using heavier projectiles (${}^{48}$Ti, ${}^{64}$Ni), the presence of mass-angle correlations in the mass-angle distributions provided strong evidence of quasifission and also provided information about quasifission timescales.Conclusions: The observable offering the clearest signature of quasifission differs depending on the reaction timescale.

The mass distributions of the fission fragments were measured in reactions induced by ${}^{48}$Ti beams on ${}^{144,154}$Sm, ${}^{162}$Dy, ${}^{174}$Yb, ${}^{186}$W, ${}^{192}$Os, ${}^{196}$Pt, ${}^{200}$Hg, and ${}^{208}$Pb targets at near-barrier energies. The systematic tendencies of the mass widths have been explored. For the reactions with the lighter or spherical target nuclei, the mass widths increase with increasing bombarding energy, a behavior expected for fusion-fission. For the heavier and statically deformed nuclei, the mass widths increase rapidly as energies fall close to and below the capture barriers. Well above the barrier energies, the mass widths and estimated minimum quasifission contributions increase smoothly with increasing fissility of the composite nucleus. The observed behavior is a complex function of the fissility, deformation, and energy with respect to the capture barrier. Comparisons with dynamical quasifission model calculations for these reactions will be a good test of models of fusion in heavy-element formation reactions.

The effects of nuclear structure can be manifested at three stages of the reaction dynamics: through the effect of channel couplings in overcoming the entrance channel potential barrier; in damping of the relative motion leading to capture; and in diffusion over the potential energy surface resulting in fusion or quasifission outcomes. This sequence of coupled processes offers many and subtle opportunities for nuclear structure to play a role in affecting the dynamics, and thus the outcomes. Recent experimental results, in particular mass-angle distributions (MAD), have isolated different ways in which nuclear structure has been shown to play a significant role in determining reaction outcomes.

The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger $N/Z$ asymmetries., accepted for publication in Physics Letters B

The reaction ${}^{16}$O $+$ ${}^{208}$Pb is a benchmark in nuclear reaction studies as it involves two doubly magic nuclei. New measurements of back-scattered projectile-like fragments at sub-barrier energies show that the probability of two-proton ($2p$) transfer is much larger than that of $\ensuremath{\alpha}$-particle transfer. At energies around the fusion barrier the probability for $2p$ transfer is $\ensuremath{\sim}10%$, similar to that for one-proton transfer. The $2p$ transfer probabilities are enhanced by up to an order of magnitude compared to calculations based on an independent particle picture as simulated by the fully microscopic time-dependent Hartree-Fock model (TDHF). Since beyond mean-field correlations like nucleon pairing are not included in the TDHF model, the enhancement indicates strong pairing correlations between the transferred protons. $2p$ transfer leads to excitation energies (most likely in the target-like nucleus) up to $\ensuremath{\sim}13$ MeV, indicating that it may represent an effective doorway for the dissipation of energy and thus provide a microscopic mechanism toward understanding the inhibition of fusion and energies both above and below the barrier.

The time scales of the dynamical nuclear rearrangement processes of fission following fusion, and of quasifission, are significant tests of models of nuclear dynamics. Experimental approaches to determine characteristic times, and their consistency, are discussed.

Below-barrier no-capture breakup measurements of the weakly bound $^{9}\mathrm{Be}$ nucleus, incident on targets ranging in atomic number from 62 to 83, have been carried out using a large-area high-resolution back-angle detector array. It is shown that the three-body reconstructed reaction $Q$-value and relative energy of the breakup fragments together reveal the full dynamics of the breakup mechanism, identifying all physical processes that lead to the breakup of the projectile-like nucleus. Contrasting with the simple expectation of direct breakup into the most energetically favored clusters, the data show that breakup following $n$-transfer dominates the total breakup yield. Breakup from long-lived states in the projectile-like nucleus, which on the reaction time scale may be considered stable, has been isolated from the prompt breakup yield. It has been shown that the prompt breakup probability essentially depends on the surface separation of the interacting nuclei. The measured prompt breakup probability functions for each target have been used together with a classical trajectory model to predict the above-barrier suppression of complete fusion. The suppression factor, expressed as the fraction of incomplete fusion, is nearly independent of target charge.

The sub-barrier excitation function of the octupole vibrational state at 2.615 MeV in {sup 208}Pb is analyzed within the coupled-channels framework. It is shown that the position of the minimum in the excitation function, which is due to the destructive interference of the Coulomb and nuclear scattering amplitudes, is very sensitive to the nuclear potential for both the ground state and the octupole state. A different nuclear potential for the 3{sup -} state may arise from changes in the matter distribution of {sup 208}Pb due to the particle-hole excitations. The present analysis places a strong limit on the 3{sup -} nuclear potential diffuseness, giving a difference of only {Delta}a{sub 0}=0.011+-0.004 fm between the ground state and 3{sup -} state diffuseness.

Combined data from four fusion-evaporation reaction experiments were utilized to investigate deformed and superdeformed structures in Zn-62(30)32. Combination of the Gammasphere gamma-ray spectrometer and ancillary particle detection systems allowed for the connection of rotational bands to well-known, low-lying excited states in Zn-62, as well as spectroscopy of discrete high-spin states reaching excitation energies of E-x=42.5 MeV. Four well- or superdeformed bands in Zn-62 are characterized and described by means of cranked Nilsson-Strutinsky calculations.

The combined data from three fusion-evaporation reaction experiments have been utilized to investigate the semi-magic nucleus Ni-58(28)30. To detect gamma rays in coincidence with evaporated particles, the Ge-detector array Gammasphere was used in conjunction with the charged-particle detectors Microball and LuWuSiA (the Lund Washington University Silicon Array), and a neutron detector array. The results yield a significantly extended level scheme of Ni-58 comprising some 340 gamma-ray transitions and include a total of at least 14 discrete particle decays into excited states of the daughter nuclei Fe-54 and Co-57. The level scheme is compared with large-scale shell-model calculations and cranked Nilsson-Strutinsky calculations.

The Zn-61(30)31 nucleus has been studied via the combined data of two fusion-evaporation reaction experiments using a Ar-36 beam and a Si-28 target foil. The experimental setups involved the Ge array GAMMASPHERE and neutron and charged particle detectors placed around the target position. The resulting level scheme comprises about 120 excited states connected via some 180 gamma-ray transitions. In total, seven rotational structures were identified up to I similar to 25 or higher and compared with predictions from cranked Nilsson-Strutinsky calculations.

Mass‐angle distributions carry detailed information on the characteristics of quasi‐fission, and thus of the dynamics of heavy element formation reactions. Recent experimental results are presented and discussed.

Data from three experiments using the heavy-ion fusion evaporation-reaction Ar-36+Si-28 have been combined to study high-spin states in the residual nucleus Ni-60, which is populated via the evaporation of four protons from the compound nucleus Ge-64. The GAMMASPHERE array was used for all the experiments in conjunction with a 4 pi charged-particle detector arrays (MICROBALL, LUWUSIA) and neutron detectors (NEUTRON SHELL) to allow for the detection of. rays in coincidence with the evaporated particles. An extended Ni-60 level scheme is presented, comprising more than 270 gamma-ray transitions and 110 excited states. Their spins and parities have been assigned via directional correlations of gamma rays emitted from oriented states. Spherical shell-model calculations in the fp-shell characterize some of the low-spin states, while the experimental results of the rotational bands are analyzed with configuration-dependent cranked Nilsson-Strutinsky calculations. (Less)

Comprehensive fission measurements, including mass-angle distributions, for the reaction of 32S with the prolate deformed nucleus 232Th at near-barrier energies show two distinct components in both mass and angle; surprisingly, both have characteristics of quasifission. Their relative probabilities vary rapidly with the ratio of the beam energy to the capture barrier, suggesting a relationship with deformation aligned (sub-barrier), or antialigned (above-barrier), configurations at contact.

High-spin states in the doubly magic N=Z nucleus Ni-56 have been investigated with three fusion-evaporation reaction experiments. New gamma-ray transitions are added, and a confirmation of a previously suggested prompt proton decay from a rotational band in Ni-56 into the ground state of Co-55 is presented. The rotational bands in Ni-56 are discussed within the framework of cranked Nilsson-Strutinsky calculations.

Comprehensive fission measurements, including mass-angle distributions, for the reaction of 32S with the prolate deformed nucleus 232Th at near-barrier energies show two distinct components in both mass and angle; surprisingly, both have characteristics of quasifission. Their relative probabilities vary rapidly with the ratio of the beam energy to the capture barrier, suggesting a relationship with deformation aligned (sub-barrier), or antialigned (above-barrier), configurations at contact.

The formation of new, superheavy elements [1] requires the fusion of two massive nuclei. After contact of the two colliding nuclei, the dinuclear system is pictured as moving in deformation space over a potential energy surface (PES). The system may reach compact (mononuclear) shapes inside the fission barrier, and thus have the possibility of forming a heavy element in its ground state. Alternatively, before reaching a compact shape, the system may reseparate into two heavy fragments, intermediate in mass between the projectile and target nuclei. This is called quasifission [2,3]. The probability of fusion (forming a new nucleus with a compact near-equilibrium shape) is determined by many variables. Those believed to play a major role include the mass-asymmetry of the two colliding nuclei, the charge of the heavy element being formed, and the neutron and proton shell structure encountered during the fusion process [4]. The detailed role and significance of shell structure is the most complex and open question. Shell effects after contact can be categorized as dynamic or static. Nonequilibrium dynamic effects, associated with the diabatic motion of the nucleons at avoided crossings of their molecular orbitals, may effectively result in timedependent shell structures [5] that can influence the early collective motion of the dinuclear system. After thermal equilibration, the well-known static shell structures affect the PES, and also the inertia and the strength of energy dissipation (and thus the magnitude of shape fluctuations), which together determine the motion over the PES. In spontaneous and low energy fission of heavy nuclei, the role of (static) shell structures is well known. Fission mass yields and total kinetic energy distributions, and the existence and properties of fission isomers, can be understood as resulting from local structure in the PES caused by energetically favorable shell structures in the nascent fragments, both for spherical and deformed shapes [6]. It is expected (and observed) that as the excitation energy Ex increases, these shell effects are exponentially attenuated. This strong dependence on Ex may be a key to isolating their role in quasifission dynamics. However, there are uncertainties regarding the effective energy scale of the attenuation [7], which is critical to predict the influence of shell structure on quasifission, where Ex is often >20 MeV. In collisions of heavy nuclei, a further complication often arises, resulting from nuclear structure effects in the entrance channel. The heavy reaction partner may have a large static deformation. This is caused by unfavored shell effects for a spherical shape, compared with a gain in energy (shell stability) for a deformed shape. This deformation affects the entry point into the PES. For nuclear collisions involving prolate nuclei, when the deformation axis is aligned with the projectile nucleus, the dinucleus is very elongated at contact, whereas it is more compact if antialigned. In experiments, by choice of the bombarding energy, only the aligned orientation (low beam energy) or all orientations (higher energy) can be selected [8–10]. Measurements have consistently shown that the lower energies are associated with the largest deviation of fission properties from the expectations for fusion-fission [10 –14], as well as showing suppression of the yield of heavy elements following capture [15,16]. Both observables should be associated with an increased probability of quasifission [10,11].

Muon production at forward rapidity (1.5

Measurements of neutral pion (pi(0)) production at midrapidity in root s(NN)=200 GeV Au+Au collisions as a function of transverse momentum, p(T), collision centrality, and angle with respect to reaction plane are presented. The data represent the final pi(0) results from the PHENIX experiment for the first RHIC Au+Au run at design center-of-mass energy. They include additional data obtained using the PHENIX Level-2 trigger with more than a factor of 3 increase in statistics over previously published results for p(T)>6 GeV/c. We evaluate the suppression in the yield of high-p(T) pi(0)'s relative to pointlike scaling expectations using the nuclear modification factor R-AA. We present the p(T) dependence of R-AA for nine bins in collision centrality. We separately integrate R-AA over larger p(T) bins to show more precisely the centrality dependence of the high-p(T) suppression. We then evaluate the dependence of the high-p(T) suppression on the emission angle Delta phi of the pions with respect to event reaction plane for seven bins in collision centrality. We show that the yields of high-p(T) pi(0)'s vary strongly with Delta phi, consistent with prior measurements 1,2. We show that this variation persists in the most peripheral bin accessible in this analysis. For the peripheral bins we observe no suppression for neutral pions produced aligned with the reaction plane, whereas the yield of pi(0)'s produced perpendicular to the reaction plane is suppressed by a factor of similar to 2. We analyze the combined centrality and Delta phi dependence of the pi(0) suppression in different p(T) bins using different possible descriptions of parton energy loss dependence on jet path-length averages to determine whether a single geometric picture can explain the observed suppression pattern.

Longitudinal density correlations of produced matter in Au+Au collisions at sqrt(s{sub NN})=200 GeV have been measured from the inclusive charged particle distributions as a function of pseudorapidity window sizes. The extracted {alpha}{xi} parameter, related to the susceptibility of the density fluctuations in the long-wavelength limit, exhibits a nonmonotonic behavior as a function of the number of participant nucleons, N{sub part}. A local maximum is seen at N{sub part}{approx}90, with corresponding energy density based on the Bjorken picture of {epsilon}{sub Bj}{tau}{approx}2.4 GeV/(fm{sup 2}c) with a transverse area size of 60 fm2. This behavior may suggest a critical phase boundary based on the Ginzburg-Landau framework.

Emission source functions are extracted from correlation functions constructed from charged pions produced at midrapidity in Au+Au collisions at s(NN)=200 GeV. The source parameters extracted from these functions at low k(T) give first indications of a long tail for the pion emission source. The source extension cannot be explained solely by simple kinematic considerations. The possible role of a halo of secondary pions from resonance emissions is explored.

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 similar to 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 Cu-58.

Excited states have been observed in the N=Z-2 odd-odd nucleus 48Mn for the first time. Through comparison with the structure of 48V, a first high-spin study of an odd-odd mirror pair has been achieved. Differences between the T=1 analogue states in this pair have been interpreted in terms of Coulomb effects, with the aid of shell-model calculations in the full pf valence space. Unlike other mirror pairs, the energy differences have been interpreted almost entirely as due to a monopole effect associated with smooth changes in radius (or deformation) as a function of angular momentum. In addition, the large energy shift between analogue negative-parity states is interpreted in terms of the electromagnetic spin-orbit interaction in nuclei.

Excited states have been observed in the N=Z-2 odd-odd nucleus 48Mn for the first time. Through comparison with the structure of 48V, a first high-spin study of an odd-odd mirror pair has been achieved. Differences between the T=1 analogue states in this pair have been interpreted in terms of Coulomb effects, with the aid of shell-model calculations in the full pf valence space. Unlike other mirror pairs, the energy differences have been interpreted almost entirely as due to a monopole effect associated with smooth changes in radius (or deformation) as a function of angular momentum. In addition, the large energy shift between analogue negative-parity states is interpreted in terms of the electromagnetic spin-orbit interaction in nuclei.