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

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.