The first direct measurement of the 17O(α,γ)21Ne reaction and its impact on heavy element production
\n

The ratio of the 17 O(α,γ)21 Ne reaction rate to that of the competing 17 O(α,n)20 Ne
\nreaction has long been thought to have significant consequences to the s-
\nprocess. Specifically in massive rotating stars at low metallicity, the abun-
\ndance of light isotopes toward the end of the helium burning stages can be
\nsufficient to compensate for their relatively low neutron capture cross sec-
\ntions. The high neutron absorption rate on 16 O can potentially reduce the
\nefficiency of the weak s-process unless there is a substantial recycling factor
\nthrough the 17 O(α,n)20 Ne reaction. The strength of the weaker (α,γ) channel,
\nrelative to that of the (α,n), is crucial to determining weak s-process elemen-
\ntal abundances due to the reduction in neutron availability a strong channel
\nwould cause.
\nPrior to this work all astrophysical calculations were based on sparse data on
\nthe (α,n) channel and two wildly conflicting theoretical models on the (α,γ)
\nchannel. Calculations based on predicted reaction rates have so far resulted
\nin a discrepancy of up to 104 in the abundance of elements from strontium
\nto barium. This work presents the results of the first direct measurement of
\nthe 17 O(α,γ)21 Ne reaction carried out on the DRAGON recoil separator at
\nthe TRIUMF laboratory in Vancouver, Canada. Cross section measurements
\nwere performed across the energy range of 0.6 < Ecm < 1.6 MeV. Data was
\nsuccessfully taken enabling the calculation of a resonance strength within
\nthe Gamow window of ωγ = 4±0.3 meV at Ecm = 0.621 MeV, as well as
\nconfirmed resonance strengths at 0.8 and 1.165 MeV and further upper limits.
\nThe calculated reaction rate appears to oppose the assignment of 16 O as a poi-
\nson, instead favouring neutron recycling via the (α,n) channel, the associated
\nefficiency of the s-process and resulting higher Sr-Ba abundances. However,
\nit should be highlighted that without further information covering the lower
\nportion of the Gamow window this can only represent a lower limit on the
\n17
\nO(α,γ)21 Ne reaction rate. The techniques, data and results relevant to this
\nreaction are presented within this thesis.
\n

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