Reuben Russell

We have performed a detailed γ-ray spectroscopy study of the nucleus, 49Mn, using the GRETINA tracking array and FMA recoil separator. With this powerful new setup, low-spin excited states, which are most relevant for astrophysical processes, have been identified for the first time, including four proton-unbound levels, corresponding to key astrophysical resonances in the 48Cr(p,γ)49Mn reaction. Of these four levels, two were found to dominate the 48Cr(p,γ)49Mn reaction for temperatures, T = 0.2 − 1.4 GK, and uncertainties in the rate have been reduced by more than 3 orders of magnitude. Specifically, γ decays were observed from 1/2+ and 3/2− excited states at Ex = 2570.9(26) keV and 2595.8(21) keV, corresponding to an ℓ = 0 and ℓ = 1 resonance in the 48Cr + p system at Er = 482.9(84) keV and 507.9(83) keV, respectively. Present simulations of Type-I X-ray burst nucleosynthesis indicate that the newly defined 48Cr(p,γ) reaction rate is sufficiently fast to drive the flow of material towards higher masses in such environments. Consequently, despite the relatively long half life of 48Cr, we now do not expect a strong waiting point in the rp process at A = 48.

The properties of a nanosecond isomer in 32Si, disputed in previous studies, depend on the evolution of proton and neutron shell gaps near the island of inversion. We have placed the isomer at 5505.2(2) keV with J7` = 5-, decaying primarily via an E3 transition to the 2+1 state. The E3 strength of 0.0841(10) W.u. is unusually small and suggests that this isomer is dominated by the (vd3/2)-1 circle times (vf7/2)1 configuration, which is sensitive to the N = 20 shell gap. A newly observed 4+1 state is placed at 5881.4(13) keV; its energy is enhanced by the Z = 14 subshell closure. This indicates that the isomer is located in a yrast trap, a feature rarely seen at low mass numbers.

The neutron -rich strontium, zirconium, and molybdenum nuclei have been observed to undergo a dramatic evolution, becoming strongly deformed around N = 60, sometimes interpreted as a quantum phase transition between "normal" and intruder configurations. Key to understanding this evolution is to understand the configurations in isolation, in regions where interference can be neglected. A deformed coexisting configuration is inferred from the presence of a 0 2 state which decreases in excitation energy with increasing neutron number, becoming the first -excited state at 98Mo. We present here the results of a low -energy Coulomb -excitation measurement of the nucleus 96Mo, extracting B(E2) values and quadrupole moments. It is found that, while the B(E2) values agree with those found in the literature, there is a significant disagreement with literature spectroscopic quadrupole moments. The results are compared with shell -model calculations using a 88Sr core with good agreement found, likely indicating that intruder structures do not significantly impact the ground -state structure, in contrast with the heavier molybdenum isotopes.