Ne19 level structure for explosive nucleosynthesis

Background: 19Ne is an important isotope in nuclear astrophysics due to its role in both the F-18(p, alpha)O-15 and O-15(alpha, gamma )19Ne reactions in novae and Type I x-ray bursts, respectively. The energy levels of 19Ne near the alpha and proton thresholds (S alpha = 3529 keV, Sp = 6410 keV) correspond to resonances in both of these reactions. Previous measurements to study the structure of Ne-19 have focused on both regions in an effort to constrain these reaction rates. Purpose: Discrepancies in the energies, spins, and parities for levels in Ne-19 from previous measurements contribute to the reaction-rate uncertainties. Gamma rays from the depopulation of excited states in Ne-19 were measured to reduce the level-energy uncertainties and inconsistencies in previous spin-parity assignments. Methods: The F-19(He-3, t)Ne-19 reaction was used to elucidate the structure of Ne-19 levels up to Ex = 6.9 MeV. The reaction products were measured using Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies-a coupling of the Oak Ridge Rutgers University Barrel Array and Gammasphere at Argonne National Laboratory. Tritons produced in the reaction were measured in coincidence with gamma rays from the deexcitation of Ne-19 energy levels. Results: Previously unobserved transitions allowed for discrepancies in the resonance properties relevant to these two reactions to be resolved. In total, 41 transitions from 21 energy levels were measured in Ne-19, with 21 of those transitions being previously unobserved. Of particular importance, transitions from two 3/2+ states with energies of 6423(3) and 6441(3) keV, crucial for accurate estimations of the F-18(p, alpha)15O reaction rate, were found. Conclusions: Energies and spin-parities of important energy levels near the proton and alpha thresholds were measured and some of the discrepancies in previous measurements were resolved. Measurement of the two near-threshold 3/2+ states reduced the calculated upper limit of the F-18(p, alpha)O-15 reaction rate by factors of 1.5-17 in the nova temperature range.