Dr Charlie Paxman

The collective structure of low-lying states in the N = 50 nucleus 92Mo (Z = 42) has been investigated through sub-barrier Coulomb excitation using the CHICO2 device coupled to the gamma-ray energy tracking in-beam nuclear array (GRETINA). Quadrupole and octupole collective properties are measured for the 2+1, 2+2 , 2+3 , 2+4 , and 3-1 states. The spectroscopic quadrupole moment for the 2+1 state is measured to be Qs = 0.23(8) eb, showing a weakly deformed oblate nature in good agreement with recent state-of-the-art Shell-Model calculations. A modest reduced electric-octupole transition strength of B(E 3; 3-1-* 0+1 ) = 15(3) W.u. is obtained, in disagreement with the literature, revealing an emerging tension in the strength of octupole collectivity for the N = 50 nuclei between historic (e, e') and (p, p') experiments and more recent experiments utilizing Coulomb excitation and lifetime plus branching-ratio data.

Recent models of the rapid proton (rp) capture process indicate that a competition between the 59Cu(p, gamma)60Zn and 59Cu(p, alpha)56Ni reactions may result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between 56Ni and 60Zn. Here, we report the identification of 15 proton-unbound levels in 60Zn, populated via 59Cu(d, n) transfer, which govern the rate of the 59Cu(p, gamma)60Zn reaction in XRBs. Precise excitation energies for levels in 60Zn were obtained from observed gamma decays, and spectroscopic factors were determined from angle-integrated cross sections. Incorporating these results into stellar-model calculations, we find that with experimentally constrained uncertainties a NiCu cycle in XRBs is indeed possible, though we limit its branching strength to less than 38%. While modest, such a branching has significant impact on the light curve, motivating further studies of the relevant rates. Our calculations also indicate that a significant NiCu cycle leads to an increase in the amount of odd-A nuclei in the burst ashes, which may affect Urca cooling processes in neutron star crusts.

The observation of active Ti-44 in supernova remnants offers the potential to solve one of the most debated questions in modern astrophysics, the exact underlying explosion mechanism of core collapse supernovae (CCSNe). In particular, a comparison between the predicted synthesized yield of Ti-44 and the ejected flux is expected to allow for a determination of the mass cut of the star. Unfortunately, such comparisons are currently severely hindered by large uncertainties in the nuclear reactions governing the production and destruction of Ti-44. On this note, the astrophysical V-45(p, gamma)Cr-46 reaction is expected to play a decisive role. We present the first experimental information on resonant states in the V-45(p, gamma)Cr-46 reaction. Excitation energies have been measured with high precision for ten previously unknown, low-spin, proton-unbound states in Cr-46, and spin-parity-assignment constraints have been made using observed gamma-decay patterns and shell-model calculations. We find that an l = 1 resonance at 714 keV dominates the V-45(p, gamma)Cr-46 reaction for the entire temperature range of alpha-rich freezeout. Moreover, the nominal rate is found to be similar to 25-50% of previous estimates for T > 1.2 GK, indicating an increase in Ti-44 production in CCSNe of similar to 20-35%.

The excited states of N = 44 74Zn were investigated via gamma-ray spectroscopy following 74Cu beta decay. By exploiting gamma-gamma angular correlation analysis, the 2 thorn 2 , 3 thorn 1 , 0 thorn 2 , and 2 thorn 3 states in 74Zn were firmly established. The gamma-ray branching and E2/M 1 mixing ratios for transitions deexciting the 2 thorn 2 , 3 thorn 1 , and 2 thorn 3 states were measured, allowing for the extraction of relative BoE2 thorn values. In particular, the 2 thorn 3 -0 thorn 2 and 2 thorn 3 -4 thorn 1 transitions were observed for the first time. The results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the N = 40 gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize 74Zn in its ground state. Furthermore, an excited K = 0 band with a significantly larger softness in its shape is identified. A shore of the N = 40 "island of inversion" appears to manifest above Z = 26, previously thought as its northern limit in the chart of the nuclides.

The 25 Al(p, γ) reaction has long been highlighted as a possible means to bypass the production of 26 Al cosmic γ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26 Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed γ-ray spectroscopy study of 26 Si and present evidence for the existence of a new, likely ℓ = 1, resonance in the 25 Al + p system at Er = 153.9(15) keV. This state is now expected to provide the dominant contribution to the 25 Al(p, γ) stellar reaction rate over the temperature range, T ∼ 0.1 − 0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26 Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, Galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed Galactic abundance of 26 Al is ∼0.2 M⊙. Finally, we briefly highlight the important role that Super-AGB stars may play in the production of 26 Al.

A high-precision branching ratio measurement for the superallowed Fermi beta(+) emitter Ga-62 was performed with the Gamma-Ray Infrastructure for Fundamental Investigations of Nuclei (GRIFFIN) spectrometer at the Isotope Separator and Accelerator (ISAC) radioactive ion beam facility at TRIUMF. The high efficiency of the GRIFFIN spectrometer allowed 63 gamma -ray transitions, with intensities down to approximate to 1 part per million (ppm) per Ga-62 beta(+) decay, to be placed in the level scheme of the daughter nucleus Zn-62, establishing the superallowed beta branching ratio for Ga-62 decay to be 99.8577(-0.0029)(+0.0023)%, a factor of 4 more precise than the previous world average. For several cascades, gamma-gamma angular correlation measurements were performed to assign spins and/or determine the mixing ratios of transitions. In particular, the spin of the 2.342 MeV excited state in the daughter nucleus Zn-62 was definitively assigned as J = 0. This assignment resolves a discrepancy between previous measurements and has important implications for the isospin symmetry breaking correction, delta(C1), in Ga-62 superallowed Fermi beta decay.

We have performed the first direct measurement of the Rb-83(p, gamma) radiative capture reaction cross section in inverse kinematics using a radioactive beam of Rb-83 at incident energies of 2.4 and 2.7A MeV. The measured cross section at an effective relative kinetic energy of E-cm = 2.393 MeV, which lies within the relevant energy window for core collapse supernovae, is smaller than the prediction of statistical model calculations. This leads to the abundance of Sr-84 produced in the astrophysical p process being higher than previously calculated. Moreover, the discrepancy of the present data with theoretical predictions indicates that further experimental investigation of p-process reactions involving unstable projectiles is clearly warranted.