D.J. Mountford

Is γ -Ray Emission from Novae Affected by Interference Effects in the F 18 ( p , α ) O 15 Reaction?

The (18)F(p,α)(15)O reaction rate is crucial for constraining model predictions of the γ-ray observable radioisotope (18)F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, (19)Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between J(π)=3/2(+) states near the proton threshold (S(p)=6.411 MeV) and a broad J(π)=3/2(+) state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the (19)F((3)He,t)(19)Ne reaction. In stark contrast to previous assumptions we find at least 3 resonances between the proton threshold and E(cm)=50 keV, all with different angular distributions. None of these are consistent with J(π)=3/2(+) angular distributions. We find that the main uncertainty now arises from the unknown proton width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects (18)F production by at least a factor of two in the model considered.

Resonances in 19Ne with relevance to the astrophysically important 18F( p,α)15O reaction

The most intense γ-ray line observable from novae is likely to be from positron annihilation associated with the decay of 18F. The uncertainty in the destruction rate of this nucleus through the 18F(p,α)15O reaction presents a limit to interpretation of any future observed γ-ray flux. Direct measurements of the cross section of both this reaction and the 18F(p,p)18F reaction have been performed between center of mass energies of 0.5 and 1.9 MeV. Simultaneous fits to both data sets with the R-matrix formalism reveal several resonances, with the inferred parameters of populated states in 19Ne in general agreement with previous measurements. Of particular interest, extra strength has been observed above E c.m.∼1.3 MeV in the 18F(p,p)18F reaction and between 1.3-1.7 MeV in the 18F(p,α)15O reaction. This is well described by a broad 1/2 + state, consistent with both a recent theoretical prediction and an inelastic scattering measurement. The astrophysical implications of a broad subthreshold partner to this state are discussed.

Resonances in ^{19}Ne with relevance to the astrophysically important ^{18}F(p,\alpha )^{15}O reaction

The main source of gamma ray emission from novae below 511 keV is likely to be associated with the beta+ decay of 18F. The main uncertainty in the abundance of this nucleus comes from the 18F(p,alpha)15O reaction. In 2006, through microscopic techniques, two previously unseen energy levels in the compound 19Ne nucleus, at 6 and 7.9 MeV were proposed by Dufour and Descouvemont. In light of this there have been two published attempts in the search for the higher of these states, by Murphy et al. and Dalouzy et al.. The aim of this work is to address the contradiction between these works and come to a conclusion as to the possible existence of this state. An experiment has taken place that utilised a 4 MeV/u 18F beam, degraded to 1.9 MeV/u, incident upon a thick CH2 target. The data has been analysed within the R-matrix formalism and preliminary results are presented here.