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Title: Alpha capture reactions for abundance observations in nuclear astrophysics
Author: Frost-Schenk, Joseph William
ISNI:       0000 0004 9358 5174
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2020
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Heavy element production in the Universe is dependent upon α capture reactions. Their measurement can help to explain discrepancies in stellar models and observation. In this thesis two key alpha capture reactions have been investigated, 15N(α,γ)19F and 17O(α,n)20Ne. The latter through 20Ne(d,p)21Ne for the study of the 17O(α,n)20Ne / 17O(α,γ)21Ne reaction rate ratio. 19F can be observed in galactic absorption spectra, its abundance is not however well understood. The first directly measured direct capture measurement of 15N(α,γ) has been conducted in inverse kinematics using the DRAGON recoil separator. A measurement of the 1.323 MeV 15N+α resonance was made extracting a resonance strength of 0.92±0.11 eV. In massive stars heavy elements are formed through the s-process, the rate of which is dependent on the neutron flux. The 16O(n,γ) reaction is known to occur at a significant rate. Hence, the neutron poisoning effectiveness of 16O is dependent upon the reaction rate ratio of 17O(α,n)20Ne and 17O(α,γ)21Ne. The 20Ne(d,p) transfer reaction has been used as a mechanism for populating states with large neutron widths in 21Ne; those important for 17O(α,n)20Ne. The measurement was conducted using TUNLs split-pole spectrograph to populate states inside the Gamow window. Significant reductions of state energy uncertainties inside the Gamow window have oc- curred. Transferred angular momenta were found by comparison with states of known Jπ as well as comparison with outputs from FRESCO. Partial widths were extracted using a weakly bound extrapolation. Reaction rates were calculated using the RatesMC reaction rate code. Presented is a revision to the 17O(α,n)20Ne and 17O(α,γ)21Ne reaction rate ratio. A decrease in the previously accepted effectiveness of 16O as a neutron poison is found, suggesting an increased neutron flux within massive stars.
Supervisor: Laird, Alison ; Diget, Christian Sponsor: STFC ; University of York
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available