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Title: The first direct measurement of the 17O(α,γ)21Ne reaction and its impact on heavy element production
Author: Taggart, Matthew
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2012
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The ratio of the 17 O(α,γ)21 Ne reaction rate to that of the competing 17 O(α,n)20 Ne reaction has long been thought to have significant consequences to the s- process. Specifically in massive rotating stars at low metallicity, the abun- dance of light isotopes toward the end of the helium burning stages can be sufficient to compensate for their relatively low neutron capture cross sec- tions. The high neutron absorption rate on 16 O can potentially reduce the efficiency of the weak s-process unless there is a substantial recycling factor through the 17 O(α,n)20 Ne reaction. The strength of the weaker (α,γ) channel, relative to that of the (α,n), is crucial to determining weak s-process elemen- tal abundances due to the reduction in neutron availability a strong channel would cause. Prior to this work all astrophysical calculations were based on sparse data on the (α,n) channel and two wildly conflicting theoretical models on the (α,γ) channel. Calculations based on predicted reaction rates have so far resulted in a discrepancy of up to 104 in the abundance of elements from strontium to barium. This work presents the results of the first direct measurement of the 17 O(α,γ)21 Ne reaction carried out on the DRAGON recoil separator at the TRIUMF laboratory in Vancouver, Canada. Cross section measurements were performed across the energy range of 0.6 < Ecm < 1.6 MeV. Data was successfully taken enabling the calculation of a resonance strength within the Gamow window of ωγ = 4±0.3 meV at Ecm = 0.621 MeV, as well as confirmed resonance strengths at 0.8 and 1.165 MeV and further upper limits. The calculated reaction rate appears to oppose the assignment of 16 O as a poi- son, instead favouring neutron recycling via the (α,n) channel, the associated efficiency of the s-process and resulting higher Sr-Ba abundances. However, it should be highlighted that without further information covering the lower portion of the Gamow window this can only represent a lower limit on the 17 O(α,γ)21 Ne reaction rate. The techniques, data and results relevant to this reaction are presented within this thesis.
Supervisor: Laird, Alison Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available