Dale Visser

Reactions through resonances in 19Ne are thought to be important participants in explosive stellar environments. In x-ray bursters, 15O(alpha,gamma) has been suggested as a possible pathway out of the hotCNO “bottleneck” to processes that generate more energy. At the lower temperatures and densities in novae, in particular oxygen-neon novae, 18F(p,gamma) and 18F(p,alpha) play a role that is important in gamma-ray astronomy. The amount of 18 F remaining in a nova’s expanding envelope of ashes depends sensitively on the rates of these two reactions.

It is difficult to directly measure astrophysical reaction rates, even at the relatively high energies of novae (∼500 keV) and x-ray bursters (∼1–2 MeV). For the reactions considered here, the participant nuclei are radioactive, creating more challenges. At the Wright Nuclear Structure Laboratory, indirect methods are employed for measuring astrophysical reaction rates. Nuclei are populated through alternate reactions, and the properties of their states are measured. For this study, 19Ne was populated via the 19F(3He,t) reaction with a 25 MeV 3He beam incident on 80 mug/cm2 of CaF2, deposited on 10 mug/cm2 of carbon. A large area segmented silicon array, the Yale LampShade Array (YLSA), was assembled for the purpose of measuring proton- and alpha-decays from astrophysically relevant states in 19Ne. The reaction rates are linearly dependent on the branching ratios to these channels. YLSA detected the decay particles at backward lab angles in coincidence with tritons at theta = 0°. Angular distributions were fit to the coincidence data, and integrated in order to determine branching ratios to the proton and alpha channels.

These results were compared to similar efforts to measure branching ratios using different reactions and techniques, and also to a previous ( 3He,t) effort that used different instruments. In addition to standard statistical methods, a Bayesian technique was employed for averaging the data due to the physical constraints on allowed values.