Present experimental constraints on the total S-factor of ^(12)C(α,γ)^(16)O and proposals for future experiments

Abstract

Because of the critical importance of the ^(12)/^(16)O ratio resulting from helium burning to the later evolution of massive stars, R-matrix fits have been made to the available angular distribution data from radiative α-capture and elastic α-scattering on ^(12)C to estimate the total ^(12)C(α,γ)^(16)O rate at stellar energies. Largely primary data, i.e. energy-dependent differential cross sections, are used in the analysis with all relevant partial waves being fitted simultaneously (surface fits). It is shown that while the E1 part of the reaction is well constrained by a recent experiment on the β-delayed α-particle decay of ^(16)N only upper limits can be placed on the E2 ground state transition which we take conservatively as S_(E2)(300) <140keV b. Monte-Carlo simulations were subsequently carried out to explore what kind of future data could lead to better restrictions on S_(E2)(300). We find that improved elastic scattering data may be the best candidate for such restrictions while improving S(300) with new radiative capture data seems to be much more difficult.