Radiative Capture Measurements of Astrophysically Important Reactions Using the DRAGON Separator


The rate of the 18F(p,g)19Ne reaction affects the final abundance of the g-ray observable radioisotope 18F, produced in novae. However the rate used is calculated from incomplete information on the contributing resonances. Of the two resonances thought to play a significant role, one has a radiative width estimated from the assumed analogue state in the mirror nucleus, 19F. The second, located at Ecm=665 keV, does not have an analogue state assignment at all, as such it's radiative width is extremely uncertain. This thesis presents the first successful direct measurement of the 18F(p,g)19Ne reaction, conducted at the recoil separator facility DRAGON. The strength of the 665 keV resonance (Ex=7.076 MeV) was found to be an order of magnitude weaker than currently assumed in nova models. Reaction rate calculations show that this resonance therefore plays no significant role in the destruction of 18F at any astrophysical energy. As part of an ongoing endeavour to expand DRAGON's capabilities to include heavy ion reactions, 76Se(a,g)80Kr was also measured at astrophysical energies. This reaction is relevant to the production of p-nuclei, which are the 35 proton rich nuclei with A>56 that cannot be created via the s- or r-processes. Although their production mechanism(s) remain ambiguous, one favoured scenario involves a series of (g,n) photo disintegration reactions from r/s seed nuclei. 80Kr represents a branching point in this process, with the uncertainty in its (g,a) rate having a large influence on abundance calculations of various astrophysical models. Observation of the 76Se(a,g)80Kr reaction can be used to better characterize the reverse (g,a) reaction rate. At DRAGON the cross section of the forward reaction was successfully measured at two astrophysical energies. Results agree well with theoretical predictions and represent the first time a recoil separator has been used for such a study

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White Rose E-theses Online

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This paper was published in White Rose E-theses Online.

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