Abstract
The carbon/oxygen (C/O) ratio at the end of stellar helium burning is the single most important nuclear input to stellar evolution theory. However, it is not known with sufficient accuracy, due to large uncertainties in the cross-section for the fusion of helium with 12C to form 16O, denoted as 12C(α, γ)16O. Here we present results based on a method that is significantly different from the experimental efforts of the past four decades. With data measured inside one detector and with vanishingly small background, angular distributions of the 12C(α, γ)16O reaction were obtained by measuring the inverse 16O(γ, α)12C reaction with gamma-beams and a Time Projection Chamber (TPC) detector. We agree with current world data for the total reaction cross-section and further evidence the strength of our method with accurate angular distributions measured over the 1− resonance at Ecm ~ 2.4 MeV. Our technique promises to yield results that will surpass the quality of the currently available data.
Highlights
Events recorded by the O-Time Projection Chamber (TPC) include Compton electrons, cosmic rays, 14N(γ, p), 16,18O(γ, α), and 12C(γ, α)8Be reactions
Detailed angular distributions of the reaction were measured with an unprecedented event-by-event resolution of ~2∘, and over almost a full range of polar angles, using a TPC detector with an optical readout (O-TPC)
We measure the tracks of the emanating α and 12C in a TPC detector operating with CO2 gas[21]
Summary
Events recorded by the O-TPC include Compton electrons, cosmic rays, 14N(γ, p), 16,18O(γ, α), and 12C(γ, α)8Be reactions. All background events from Compton electrons, cosmic rays, 14N(γ, p) and 18O(γ, α) were removed. Compton electrons deposit up to 100 keV total energy in the O-TPC, and were removed by an 800 keV electronics threshold on the anode signal. The majority of the cosmic events were removed by inspecting the track image and requiring an interaction point which is within ± 6 mm of the centre of the gamma-beam position. Events from the 14N(γ, p) reaction were removed by identifying the dE/dx lineshape of the proton, which differs significantly from the α particles. The 18O(γ, α) reaction events deposit 934.95 keV more energy than the 16O(γ, α) events, and were removed by measuring the total energy deposited in the O-TPC
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