Key ${}^{19}$Ne states identified affecting ${\hat{I}}^3$-ray emission from ${}^{18}$F in novae

Abstract

Detection of nuclear-decay ${\hat{I}}^3$ rays provides a sensitive thermometer of nova nucleosynthesis. The most intense ${\hat{I}}^3$-ray flux is thought to be annihilation radiation from the $β^+$ decay of ${}^{18}$F, which is destroyed prior to decay by the ${}^{18}$F($p$,$\hat{I}\pm$)${}^{15}$O reaction. Estimates of ${}^{18}$F production had been uncertain, however, because key near-threshold levels in the compound nucleus, ${}^{19}$Ne, had yet to be identified. This Letter reports the first measurement of the ${}^{19}$F(${}^3$He,$t{\hat{I}}^3$)${}^{19}$Ne reaction, in which the placement of two long-sought 3/2${}^{+}$ levels is suggested via triton-${\hat{I}}^3$-${\hat{I}}^3$ 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.

6 pages, 4 figures