Dr Lisa Morrison

Background: The fusion excitation function of C 12+ C 12 contains many resonances from above the Coulomb barrier towards sub-barrier energies. These oscillations might be linked to molecular states in the compound nucleus C 12 with drastic consequences for stellar carbon burning. Moreover, these structures render the common extrapolations from measurements into the astrophysics region of interest at deep sub-barrier energy extremely uncertain. Purpose: We have investigated the cross section around the lowest direct-coincident gamma-particle measurements, where previously only limits could be established with the aim of obtaining a detailed description of the excitation function. We have furthermore analyzed the ratio of extreme decay branching into the first excited state of daughter nuclei with alpha or proton emission at relative kinetic energies where previous measurements are in disagreement with each other. Methods: The experiments were carried out at the Andromède accelerator, IJCLab Orsay (France), with carbon-beam intensities of up to 3 p µ A on thin rotating carbon-target foils. Light charged particles and gamma decays were detected using silicon strip detectors and LaBr3(Ce) crystals, respectively, in coincidence measurements with nanoseconds precision. Results: We establish data points with highly improved accuracy at the high-energy tail of the lowest resonance detected so far in direct measurements, right in the astrophysics region of interest (RoI). The findings are in agreement with an earlier interpretation of the fusion excitation function composed of resonances on top of a global trend of empirically determined fusion hindrance behavior. At a relative energy between 3.1 MeV and 3.3 MeV, an oscillating behavior of the branching ratio into the first excited final state with alpha or proton emission is observed, with extreme values significantly deviating from data at higher energy. Conclusions: Our findings in the astrophysics RoI support reaction-rate models with a lower average S-factor trend, that deviates significantly from standard extrapolations between 2.2 MeV and 2.6 MeV, for stellar carbon-burning simulations of up to 25 M⊙ stars. Based on our data, an overall increase of the S-factor at deep sub-barrier energy cannot be confirmed. The extremely low ratio of the branching into the first excited state with proton over alpha emission of ≈2% at 3.23 MeV might indicate the presence of alpha-cluster compound states in C 12. This highly favors α emission with fundamental consequences in possible stellar carbon-burning sites.

The GRIFFIN (Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei) project is a major upgrade of the decay spectroscopy capabilities at TRIUMF-ISAC. GRIFFIN will replace the 8π spectrometer with an array of up to 16 large-volume HPGe clover detectors and use a state-of-the-art digital data acquisition system. The existing ancillary detector systems that had been developed for 8π, such as the SCEPTAR array for β-tagging, PACES for high-resolution internal conversion electron spectroscopy, and the DANTE array of LaBr3/BaF2 scintillators for fast γ-ray timing, will be used with GRIFFIN. GRIFFIN can also accommodate the new neutron detector array DESCANT (Deuterated Scintillator Array for Neutron Tagging), enabling the study of β-delayed neutron emitters. DESCANT consists of up to 70 detectors, each filled with approximately 2 liters of deuterated benzene, a liquid scintillator that provides pulse-shape discrimination (PSD) capabilities to distinguish between neutrons and γ-rays interacting with the detector. In addition, the anisotropic nature of n-d scattering as compared to the isotropic n-p scattering allows for the determination of the neutron energy spectrum directly from the pulse-height spectrum, complementing the time-of-flight (TOF) information. The installation of GRIFFIN is under way and first experiments are planned for the fall of 2014. The array will be completed in 2015 with the full complement of 16 clovers. DESCANT will be tested coupled with GRIFFIN in spring of 2015.

Isomeric states in 128In and 130In have been studied with the JYFLTRAP Penning trap at the IGISOL facility. By employing state-of-the-art ion manipulation techniques, three different beta-decaying states in 128In and 130In have been separated and their masses measured. JYFLTRAP was also used to select the ions of interest for identification at a post-trap decay spectroscopy station. A new beta-decaying high-spin isomer feeding the 15− isomer in 128Sn has been discovered in 128In at 1797.6(20) keV. Shell-model calculations employing a CD-Bonn potential re-normalized with the perturbative G-matrix approach suggest this new isomer to be a 16+ spin-trap isomer. In 130In, the lowest-lying (10−) isomeric state at 58.6(82) keV was resolved for the first time using the phase-imaging ion cyclotron resonance technique. The energy difference between the 10− and 1− states in 130In, stemming from parallel/antiparallel coupling of (π0g9/2−1)⊗(ν0h11/2−1), has been found to be around 200 keV lower than predicted by the shell model. Precise information on the energies of the excited states determined in this work is crucial for producing new improved effective interactions for the nuclear shell model description of nuclei near 132Sn.

Low-lying states in the isotope Xe130 were populated in a Coulomb-excitation experiment performed at CERN's HIE-ISOLDE facility. The magnitudes and relative signs of seven E2 matrix elements and one M1 matrix element coupling five low-lying states in Xe130 were determined using the semiclassical coupled-channel Coulomb-excitation least-squares search code gosia. The diagonal E2 matrix elements of both the 21+ and 41+ states were extracted for the first time. The reduced transition strengths are in line with those obtained from previous measurements. Experimental results were compared with the general Bohr Hamiltonian with the microscopic input from mean-field theory utilizing universal nuclear energy density functional (UNEDF0), shell-model calculations using the GCN50:82 and SN100PN interactions, and simple phenomenological models (Davydov-Filippov and γ-soft). The extracted shape parameters indicate triaxial-prolate deformation in the ground-state band. In general, good agreement between theoretical predictions and experimental values was found, while neither phenomenological model was found to provide an adequate description of Xe130.

Data analysis of the Coulomb excitation experiment of the exotic 206Hg nucleus, recently performed at CERN's HIE-ISOLDE facility, needs to account for the contribution to target excitation due to the strongly-present beam contaminant 130Xe. In this paper, the contamination subtraction procedure is presented.

ray spectrometer designed for use in decay spectroscopy experiments with low-energy radioactive ion beams provided by TRIUMF’s Isotope Separator and Accelerator (ISAC-I) facility. GRIFFIN is composed of sixteen Compton-suppressed large-volume clover-type high-purity germanium (HPGe) 𝛾-ray detectors combined with a suite of ancillary detection systems and coupled to a custom digital data acquisition system. The infrastructure and detectors of the spectrometer as well as the performance characteristics and the analysis techniques applied to the experimental data are described.

In recent years, a number of both theoretical and experimental investigations have been performed focusing on the zirconium isotopic chain. In particular, state-of-the-art Monte Carlo shell-model calculations predict shape coexistence in these isotopes. In this context, the 94Zr nucleus, which is believed to possess a nearly spherical ground state, is particularly interesting since the purported deformed structure is basedon the low-lying 02+ state, making it amenable for detailed study. In order to provide definitive conclusionson the shapes of the low-lying states, two complementary experiments to study 94Zr by means of low-energy Coulomb excitation were performed. This data will allow the quadrupole moments of the 21,2+ levels to be extracted as well as for the deformation parameters of the 01,2+ states to be determined and, thus, definitive conclusions to be drawn on the role of shape coexistence in this nucleus for the first time. The first experiment was performed at the INFN Legnaro National Laboratory with the GALILEO-SPIDER setup, which, for the first time, was coupled with 6 lanthanum bromide scintillators (LaBr3:Ce) in order to maximize the γ-ray detection effciency. The second experiment was performed at the Maier-Leibnitz Laboratory (MLL) in Munich and used a Q3D magnetic spectrograph to detect the scattered 12C ions following Coulomb excitation of 94Zr targets.

We report on high-statistics data from the beta(-) decay of the K-46 J pi = 2(-) ground state taken with the GRIFFIN spectrometer located at the TRIUMF-ISAC facility. In total, 199 gamma rays and 42 excited states were placed in the level scheme, and from the observed beta feeding and angular correlations of pairs of cascading gamma rays, it was possible to assign spins and parities to excited states and determine mixing ratios for selected gamma rays. The level structure of Ca-46 is compared to theoretical predictions from a microscopic valence-space Hamiltonian derived from two- and three-nucleon forces. These calculations are in reasonable agreement with the experimental data and indicate that the protons in this region are not as inert as would be expected for semimagic nuclei.

The low-lying structure of 13Be has remained an enigma for decades. Despite numerous experimental and theoretical studies, large inconsistencies remain. Being both unbound and one neutron away from 14Be, the heaviest bound beryllium nucleus, 13Be is difficult to study through simple reactions with weak radioactive-ion beams or more complex reactions with stable-ion beams. Here, we present the results of a study using the 12Be ⁢(𝑑, 𝑝)⁢ 13Be reaction in inverse kinematics using a 9.5 MeV per nucleon 12Be beam from the ISAC-II facility. The solid deuteron target of IRIS was used to achieve an increased areal thickness compared to conventional deuterated polyethylene targets. The 𝑄-value spectrum below −4.4 MeV was analyzed using a Bayesian method with geant4 simulations. A three-point angular distribution with the same 𝑄-value gate was fit with a mixture of 𝑠- and 𝑝-wave, 𝑠- and 𝑑-wave, or pure 𝑝-wave transfer. The 𝑄-value spectrum was also compared with geant simulations obtained using the energies and widths of states reported in four previous works. It was found that our results are incompatible with works that revealed a wide 5/2+ resonance but shows better agreement with ones that reported a narrower width.

The first low-energy Coulomb-excitation measurement of the radioactive, semi-magic, two proton-hole nucleus 206Hg, was performed at CERN's recently-commissioned HIE-ISOLDE facility. Two γ rays depopulating low-lying states in 206Hg were observed. From the data, a reduced transition strength B(E2;21+→01+)=4.4(6) W.u was determined, the first such value for an N=126 nucleus south of 208Pb, which is found to be slightly lower than that predicted by shell-model calculations. In addition, a collective octupole state was identified at an excitation energy of 2705 keV, for which a reduced B(E3) transition probability of 30−13+10 W.u was extracted. These results are crucial for understanding both quadrupole and octupole collectivity in the vicinity of the heaviest doubly-magic nucleus 208Pb, and for benchmarking a number of theoretical approaches in this key region. This is of particular importance given the paucity of data on transition strengths in this region, which could be used, in principle, to test calculations relevant to the astrophysical r-process.

Isomeric states in 128In and 130In have been studied with the JYFLTRAP Penning trap at the IGISOL facility. By employing state-of-the-art ion manipulation techniques, three different beta-decaying states in 128In and 130In have been separated and their masses measured. JYFLTRAP was also used to select the ions of interest for identification at a post-trap decay spectroscopy station. A new beta-decaying high-spin isomer feeding the 15− isomer in 128Sn has been discovered in 128In at 1797.6(20) keV. Shell-model calculations employing a CD-Bonn potential re-normalized with the perturbative G-matrix approach suggest this new isomer to be a 16+ spin-trap isomer. In 130In, the lowest-lying (10−) isomeric state at 58.6(82) keV was resolved for the first time using the phase-imaging ion cyclotron resonance technique. The energy difference between the 10− and 1− states in 130In, stemming from parallel/antiparallel coupling of (π0g9/2−1)⊗(ν0h11/2−1), has been found to be around 200 keV lower than predicted by the shell model. Precise information on the energies of the excited states determined in this work is crucial for producing new improved effective interactions for the nuclear shell model description of nuclei near 132Sn.

Monte Carlo simulations are widely used in nuclear physics to model experimental systems. In cases where there are significant unknown quantities, such as energies of states, an iterative process of simulating and fitting is often required to describe experimental data. We describe a Bayesian approach to fitting experimental data, designed for data from a 12Be(d,p) reaction measurement, using simulations made with GEANT4. Q-values from the 12C(d,p) reaction to well-known states in 13C are compared with simulations using BayesOpt. The energies of the states were not included in the simulation to reproduce the situation for 13Be where the states are poorly known. Both cases had low statistics and significant resolution broadening owing to large proton energy losses in the solid deuterium target. Excitation energies of the lowest three excited states in 13C were extracted to better than 90 keV, paving a way for extracting information on 13Be.

The first low-energy Coulomb-excitation measurement of the radioactive, semi-magic, two proton -hole nucleus 206Hg, was performed at CERN's recently-commissioned HIE-ISOLDE facility. Two gamma rays depopulating low-lying states in 206Hg were observed. From the data, a reduced transition strength B(E2; 2+1 -> 0+1 ) = 4.4(6) W.u. was determined, the first such value for an N = 126 nucleus south of 208Pb, which is found to be slightly lower than that predicted by shell-model calculations. In addition, a collective octupole state was identified at an excitation energy of 2705 keV, for which a reduced B(E3) transition probability of 30+10 -13 W.u. was extracted. These results are crucial for understanding both quadrupole and octupole collectivity in the vicinity of the heaviest doubly-magic nucleus 208Pb, and for benchmarking a number of theoretical approaches in this key region. This is of particular importance given the paucity of data on transition strengths in this region, which could be used, in principle, to test calculations relevant to the astrophysical r-process.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.