Spectroscopy and excited-state g factors in weakly collective Cd 111: Confronting collective and microscopic models

Background: The even cadmium isotopes near the neutron midshell have long been considered among the best examples of vibrational nuclei. However, the vibrational nature of these nuclei has been questioned based on E2 transition rates that are not consistent with vibrational excitations. In the neighboring odd-mass nuclei, the g factors of the low-excitation collective states have been shown to be more consistent with a deformed rotational core than a vibrational core. Moving beyond the comparison of vibrational versus rotational models, recent advances in computational power have made shell-model calculations feasible for Cd isotopes. These calculations may give insights into the emergence and nature of collectivity in the Cd isotopes.This research was supported in part by the Australian Research Council Grants No. DP120101417, No. DP130104176, No. DP140102986, No. DP140103317, No. DP170101673, and No. LE150100064. B.J.C., A.A., J.T.H.D., M.S.M.G., and T.J.G. acknowledge support of the Australian Government Research Training Program. Support for the ANU Heavy Ion Accelerator Facility operations through the Australian National Collaborative Research Infrastructure Strategy (NCRIS) program is acknowledged

Perturbed angular distributions with LaBr3 detectors: The g factor of the first 10+ state in Cd 110 reexamined

The time differential perturbed angular distribution technique with LaBr3 detectors has been applied to the Iπ = 11-/2 isomeric state (Ex=846 keV, τ=107 ns) in 107Cd, which was populated and recoil-implanted into a gadolinium host following the 98Mo(^12C, 3n)^107Cd reaction. The static hyperfine field strength of Cd recoil implanted into gadolinium was thus measured, together with the fraction of nuclei implanted into field-free sites, under similar conditions as pertained for a previous implantation perturbed angular distribution g-factor measurement on the Iπ=10+ state in 110Cd. The 110Cdg(10+) value was thereby reevaluated, bringing it into agreement with the value expected for a seniority-two vh11/2 configuration.This research was supported in part by the Australian Research Council Grants No. DP120101417, No. DP130104176, No. DP140102986, No. DP140103317, No. DP170101673, No. LE150100064, and No. FT100100991, and by The Australian National University Major Equipment Committee Grant No. 15MEC14

Solenogam: A new detector array for γ-ray and conversion-electron spectroscopy of long-lived states in fusion-evaporation products

A new detector array, Solenogam, has been developed at the Australian National University Heavy Ion Accelerator Facility. Coupled initially to the SOLITAIRE 6.5 T, gas-filled, solenoidal separator, and later to an 8 T solenoid, the system enables the study of long-lived nuclear states through -ray and conversion-electron spectroscopy in a low-background environment. The detector system is described and results from the commissioning experiments are presented.The authors are grateful to the academic and technical staff of the Department of Nuclear Physics (Australian National University) and the Heavy Ion Accelerator Facility for their continued support. This re- search was supported by the Australian Research Council through grant numbers FT100100991, DP120101417, DP14102986, and DP140103317. M.S.M.G., A.A., B.J.C., J.T.H.D., T.J.G., B.Q.L., and T.P. acknowledge the support of the Australian Government Research Training Program. Support for the ANU Heavy Ion Accelerator Facility operations through the Australian National Collaborative Research Infrastructure Strategy (NCRIS) program is acknowledge

Emerging nuclear collectivity in 124-130Te

The emergence of nuclear collectivity near doubly-magic 132Sn was explored along the stable, eveneven 124−130Te isotopes. Preliminary measurements of the B(E2; 41+ → 21+) transition strengths are reported from Coulomb excitation experiments primarily aimed at measuring the g factors of the 41+ states. Isotopically enriched Te targets were excited by 198-205 MeV 58Ni beams. A comparison of transition strengths obtained is made to large-scale shell-model calculations with successes and limitations discussed

Evidence for shape coexistence in 52Cr through conversion-electron and pairconversion spectroscopy

Electric monopole (E0) transitions are a highly sensitive probe of the charge distribution of an atomic nucleus. A large E0 transition strength (ρ2(E0)) is a clear indicator of nuclear shape coexistence. In the region between doubly magic 40Ca and 56Ni, E0 transitions have never been observed in the Ti or Cr isotopes, nor in the heavier iron isotopes (56,58Fe). We have performed the first measurements of the E0 transitions in 52Cr via conversion-electron and pair-conversion spectroscopy using the Super-e spectrometer at the Australian National University Heavy Ion Accelerator Facility. We present the first spectra obtained for 52Cr, including the first observation of the E0 transition from the first-excited 0+ state in 52Cr, in both electron-positron pairs and conversion-electron spectroscopy. The preliminary values for the E0 strength in the 1531keV 2+ → 2+ transition in 52Cr is ρ2(E0) � 103 = 470(190), and for the 1728-keV 23+ → 21+ transition, it is ρ2(E0) 103 = 1800(1200). The large E0 strengths observed are consistent with shape coexistence in this region. However, despite the relatively precise observation of the conversion-electron and electron-positron pair intensities, the E0 strengths have large uncertainties. More precise determinations of relevant spectroscopic quantities, such as the state lifetimes and transition mixing ratios for mixed M1 + E2 transitions, are needed to determine the E0 strength more precisely.This research was supported in part by the Australian Research Council grant numbers DP140102986 and DP170101673, and was partially supported by the International Joint Research Promotion Program of Osaka University and JSPS KAKENHI Grant Number JP17H02893. This work is also based on the research supported partly by National Research Foundation of South Africa (118645, 90741). J.T.H.D., A.A., B.J.C., M.S.M.G., T.J.G., B.P.M., and B.P.E.T. acknowledge support of the Australian Government Research Training Program. Support for the ANU Heavy Ion Accelerator Facility operations through the Australian National Collaborative Research Infrastructure Strategy program is acknowledged

Improved precision on the experimental E0 decay branching ratio of the Hoyle state

Background: Stellar carbon synthesis occurs exclusively via the 3α process, in which three α particles fuse to form 12C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the α threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the E0 decay branching ratio of the Hoyle state, and the current 10% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The rate of the 3α process is an important input parameter in astrophysical calculations on stellar evolution, and a high precision is imperative to constrain the possible outcomes of astrophysical models.The project was supported by the Australian Research Council Discovery Grants No. DP140102986, No. DP170101673, and No. DP170102423. Operation of the ANU Heavy Ion Accelerator Facility is supported by the NCRIS HIA capability. The support from technical staff for the development of the pair spectrometer, as well as during the long experimental runs, is greatly appreciated. This work was partially supported by the International Joint Research Promotion Program of Osaka University and JSPS KAKENHI Grant No. JP 17H02893, the Natural Sciences and Engineering Research Council of Canada, the National Research Foundation (NRF), South Africa, under Grants No. 93533 and No. 118645

Investigating nuclear structure through gamma-ray and electron spectroscopy with Solenogam

This thesis reports on the continued development of the Solenogam system following the installation of a new 8-T solenoid. Solenogam is an array designed at the Australian National University Heavy Ion Accelerator Facility (ANU-HIAF) for gamma-ray and electron spectroscopy. Coupled to a gas-filled magnetic solenoid (SOLITAIRE), the system enables the study of long-lived nuclear states in a low-background environment. The combination of gamma-ray and electron detectors allows a wide range of spectroscopic information to be obtained with the energy and ordering of nuclear states extracted from gamma-gamma coincidences and transition multipolarities assigned using conversion coefficients extracted from the electron data. Furthermore, the collection of gamma-e- coincidences allows conversion coefficients to be extracted for complex level schemes as contaminating electron lines can be removed using gamma-ray gates. In order to better understand the behaviour of the array, a simulation of Solenogam has been developed using the Geant4 libraries. Alongside this, two measurements were conducted with the array. In the first, electric monopole (E0) transitions were investigated in $^{184}$Pt and $^{190}$Pt in order to benchmark Solenogam's capabilities for characterising these transitions. As E0 transitions are sensitive to changes in the nuclear charge distribution and hence the nuclear shape, they are an ideal probe of shape coexistence, a phenomenon where competing nuclear shapes exist at similar excitation energies. The known 184Pt level scheme contains several coexisting structures and mixed M1/E2/E0 transitions, making for an ideal test case. The results of this measurements were consistent with previously reported values; however, no new spectroscopic information was obtained from the measured data. In the second measurement, the decay of a (49/2+) isomer in 145Sm was studied. High-spin isomers have been reported in almost all the N=83 isotones near Z=64 and, while these states have been interpreted as deformed shape isomers, in several cases the spin and parity assignments remain tentative. A revised half life of 3.52(16) mus has been measured and firm spin and parity assignments were made based on conversion coefficients. These results indicated that the previously reported 145Sm level scheme was incorrect and a new state at 8815-keV has been proposed as the high-spin isomer. This new level scheme was understood using shell-model calculations and analysis of the systematics of the N=83 isotone chain

Arousal and activation? Using the electroencephalogram to investigate the cognitive-energetic model of reinforcement learning in complex tasks.

The Cognitive-Energetic Model (CEM; Sergeant, 2000) of ADHD has shown promise as a valid model of the disorder (and healthy behaviour), however electroencephalogram (EEG) methodologies are not well represented in investigations of relationships between the three theoretical levels of the model and behavioural patterns. Four experimental studies are presented that track the development of simple and complex reinforcement learning (RL) and its neurophysiological correlates in the context of the CEM. In the first three studies, event-related potential (ERP) and behavioural measures of performance from healthy controls are analysed in a relatively simple, deterministic RL task (DLT) assessing reward valence, and a complex, probabilistic association learning task (i.e. Weather Prediction Task; WPT). The final experiment compares behaviour, ERPs and spectral EEG measures from an adult clinical group with a matched, healthy control group in the WPT. Low-level processes of stimulus identification (N1, N2), feature analysis (P2) and manipulation (LPP), differentiated value conditions in early learning on the DLT, and showed a general increase in amplitude with training. Stimulus processing (N1, N2) components showed a similar enhancement with learning, but only when tracked over extended periods of training on the WPT. Control positive feedback components (e.g. FRN) displayed decreases in amplitude in both extended and brief learning periods on the WPT, however the opposite pattern was observed in the ADHD group. ADHD feedback components showed a particular sensitivity to rewarding feedback and were topographically shifted from controls. Negative feedback components were not modulated by training. Resting state spectral EEG analysis revealed relative power differences between groups in global eyes closed alpha power, and topographically in delta, theta, and beta frequencies, when transitioning to an eyes open state. Generally, the ADHD group displayed abnormal energy distribution in order to meet WPT demands. ERP and EEG findings are discussed in relation to the ‘energetic pools’ described in the CEM. Recommendations for revisions to CEM pathways, and future directions for research are discussed