[en] The approximate charge symmetry and charge independence of the nuclear force results in striking symmetries in nuclear behavior between isobaric analogue states (IAS) -- states of the same isospin quantum number in a set of nuclei of the same total number of nucleons (an isobaric multiplet). The Coulomb force breaks this symmetry and is the dominating factor in producing the large shifts in absolute binding energy between IAS. The recent development of large gamma-ray spectrometers has resulted in a wealth of information on IAS at high excitation energy and high angular momentum. The very small differences in excitation energy between the IAS can be interpreted, principally, in terms of Coulomb effects. The analysis of these Coulomb differences has been shown to be a remarkably sensitive probe of nuclear structure effects as well as providing stringent tests of state-of-the-art shell-model calculations. Some specific structural phenomena examined through the study of such energy differences are presented in this review, and include the evolution of nuclear radius/deformation as a function of spin and the electromagnetic spin-orbit effect

[en] The nucleus ⁶⁸Ge has been studied by gamma-ray spectroscopy following its population at high spin in the reaction ⁴⁰Ca(³²S,4p)⁶⁸Ge. The reaction channel was selected with the Microball array and gamma rays were detected with the Gammasphere array. The level scheme is very complex, reflecting the many different, and presumably mixed, excitation modes in this nucleus. Nevertheless, there appear to be some simplifications in the spin range above 18ℎ where we have identified a superdeformed band and several terminating bands. The results are compared with a cranked Nilsson-Strutinsky model without pairing

[en] Gamma decays from excited states in the T_z=-1/2 nucleus ⁵¹Fe have been observed for the first time. The differences in excitation energies as compared with those of the mirror partner, ⁵¹Mn, have been interpreted in terms of Coulomb effects and the resulting Coulomb energy differences (CED) can be understood intuitively in terms of particle-alignment effects. A new CED effect has been observed, in which different CED trends have been measured for each signature of the rotational structures that characterize these mid-f_7/2 shell nuclei. Large-scale fp-shell model calculations have been used to compute the trends of the CED as a function of spin. The result of comparing these calculations with the data demonstrates an ability to reproduce the fine details of the Coulomb effects with a precision far greater than has been previously achieved

[en] An experiment utilizing a double fragmentation reaction was performed to study isobaric analogue states in A ∼ 50 nuclei approaching the proton drip-line. γ-ray spectroscopy will be used to identify excited states in the neutron-deficient nuclei produced in the second fragmentation reaction. Excited state level schemes will be obtained, through comparison with states in their well-known mirror partners, along with information on Coulomb effects through measurements of the Coulomb energy differences between isobaric analogue excited states. The validity of isospin symmetry for nuclei approaching the proton drip-line can also be investigated and the information gained will aid in testing and improving fp shell model calculations. The analysis of the collected data is at a preliminary stage and current status of this work is reported

[en] Gamma decays have been observed for the first time in the T_z = -3/2 nucleus ⁵³Ni. This represents the first gamma-spectroscopy of a T_z = -3/2 nucleus heavier than A = 33. The nucleus was produced via a two-step fragmentation process, along with its mirror ⁵³Mn. Differences in excitation energy between isobaric analogue states have been calculated and a preliminary interpretation attempted; shell model calculations are required to further understand these results. This work represents the first study of isobaric analogue states via mirrored fragmentation reactions and demonstrates the power of this new technique

[en] The ¹⁸F(p,α)¹⁵O reaction rate is crucial for understanding the final abundance of ¹⁸F predicted by nova models. The γ-ray emission in the first few hours after a nova outburst is expected to be dominated by 511 keV annihilation photons from the decay of ¹⁸F, and so understanding its production can provide important constraints on the conditions during the outburst when compared with observations. Results are presented from the lowest-energy direct measurement to date, performed at the Isotope Separator and Accelerator radioactive beam facility at the TRIUMF laboratory, Canada. Cross section measurements at center-of-mass energies of 250, 330, 453, and 673 keV are obtained and the results compared to previous data and R-matrix calculations. The implications for the overall reaction rate in the context of nova explosions have been discussed.

[en] Isospin-breaking effects have been studied for the first time in T=(3/2) isobaric analog states. Gamma decays have been observed from T_z=-(3/2) nuclei, ⁴⁹Fe and ⁵³Ni, presented here in new level schemes, and mirror energy differences have been computed following observation of analog states in ⁴⁹V and ⁵³Mn, respectively. Shell-model calculations in the fp shell are in good agreement with the data and reveal the importance of non-Coulomb isospin-breaking effects in T=(3/2) isobaric analog states. A two-step fragmentation process was developed to allow access to highly proton-rich nuclei and to produce each member of a mirror pair via mirrored fragmentation of a ⁵⁶Ni secondary beam. This work represents the first study using this technique and demonstrates the power of this approach for future studies of isobaric analog states in very proton-rich systems.

[en] The β-delayed γ and proton decay of ²³Al has been studied with an alternative detector setup at the focal plane of the momentum achromat recoil separator MARS at Texas A and M University. We could detect protons down to an energy of 200 keV and determine the corresponding branching ratios. Contrary to results of previous β-decay studies, no strong proton intensity from the decay of the isobaric analog state (IAS) of the ²³Al ground state at E_x=7803 keV in ²³Mg was observed. Instead we assign the observed low-energy group E_p,c.m.=206 keV to the decay from a state that is 16 keV below the IAS. We measured both proton and gamma branches from the decay of this state at E_x=7787 keV in ²³Mg, which is a very rare case in the literature. Combining our data with its measured lifetime, we determine its resonance strength to be ωγ=1.4_-0.4^+0.5 meV. The value is in agreement with older direct measurements, but disagrees with a recent direct measurement. This state is the most important resonance for the radiative proton capture ²²Na(p,γ)²³Mg in some astrophysical environments, such as novae.

[en] Electron capture on neutron-rich medium-mass nuclei is a key process where the electrons that impede the collapse of the core of massive stars are captured, thereby producing very neutron-rich nuclei. As the core collapses, the supernova is then initiated. For the electron capture to proceed, however, the allowed Gamow-Teller (GT) transition must be unblocked either by thermal excitations or by mixing of proton configurations from a higher-lying shell into the ground-state configuration of the nucleus. The present paper presents an experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University, in which we study the configuration mixing in the neutron-rich⁷⁶Zn isotope. The experiment utilised single-proton and single-neutron knockout with detection of reaction-residue γ rays and measurement of the parallel momentum of the residue. Through this we investigate the proton components of the ⁷⁶Zn ground state, with a particular interest in π-g_9/2, which may unblock the GT electron capture even at low temperatures and thereby open a new pathway for the initiation of the collapse of the pre-supernova stellar core.

[en] We have studied the effects of electrode fabrication and detector capacitance on the time resolution of large area electronic grade polycrystalline chemical vapour deposited diamond sensors that are suitable for time of flight measurements of heavy ions at relativistic velocities. Sensors were prepared both in house, with Al or Au metal contacts, and commercially fabricated with Au/diamond-like carbon contacts. ³He, ⁴⁰Ar and a mixture of ²⁰Ne and ¹⁶O beams at 16.3, 33.5 and 21–23 MeV/u, respectively were used on these devices whilst arranged in transmission geometry. Signal processing only began over one meter away from the sensors. The present approach, where we have large-area/large capacitance multi-strip detectors with processing electronics at some distance from the target, is compatible with anticipated space limitations in particle-identification and tracking setups at existing and planned nuclear fragmentation facilities. In a systematic study under these conditions, we demonstrate that the time resolution is limited by detector capacitance and energy deposition in the sensors. An intrinsic time resolution σ_t = (44±5) ps was achieved for a diamond detector of ∼ 14 pF capacitance. We conclude that, once further refinements are made, a large area time of flight detection system using polycrystalline diamond detectors would be able to provide time resolutions better than 40 ps, approaching the requirement for particle-identification in relativistic fragmentation experiments, such as those at the facility for antiproton and ion research, FAIR.