Key ^{19}Ne States Identified Affecting γ-Ray Emission from ^{18}F in Novae.

Detection of nuclear-decay gamma rays provides a sensitive thermometer of nova nucleosynthesis. The most intense gamma-ray flux is thought to be annihilation radiation from the beta(+) decay of F-18, which is destroyed prior to decay by the F-18(p, alpha)O-15 reaction. Estimates of F-18 production had been uncertain, however, because key near-threshold levels in the compound nucleus, Ne-19, had yet to be identified. We report the first measurement of the F-19(He-3, t gamma)Ne-19 reaction, in which the placement of two long-sought 3/2(+) levels is suggested via triton-gamma-gamma coincidences. The precise determination of their resonance energies reduces the upper limit of the rate by a factor of 1.5-17 at nova temperatures and reduces the average uncertainty on the nova detection probability by a factor of 2.1.