C12(α,γ)O16 E2 cross section: R-matrix fits combined with a microscopic cluster model

Details C12(α,γ)O16 E2 cross section: R-matrix fits combined with a microscopic cluster model C12(α,γ)O16 E2 cross section: R-matrix fits combined with a microscopic cluster model

Jul 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phrvc..78a5808d&link_type=abstract

Physical Review C, vol. 78, Issue 1, id. 015808

Mathematics

Logic

10

Cluster Models, Spectroscopic Factors And Asymptotic Normalization Coefficients, Many-Body Theory, Stellar Helium Burning

Scientific paper

The E2 component of the 12C(α,γ)16O cross section is investigated in two ways: by a microscopic cluster model, and by R-matrix fits. The α+12C microscopic calculation is performed in the framework of the generator coordinate method (GCM) by including all C12 states (T=0) within the p shell. Using different nucleon-nucleon interactions we find SE2(300keV)≈50 keV · b for ground-state transitions. We also study cascade transitions to the 02+ and 21+ excited states of O16. Then the S-factor is analyzed in the phenomenological R-matrix theory. We show that the background term plays a crucial role, and cannot be determined without ambiguity. Using the experimental phase shifts and capture cross sections, only an upper limit on the extrapolated S factor can be obtained [SE2(300keV)<190 keV · b]. To constrain the R-matrix analysis, we use the GCM asymptotic normalization constant (ANC) of the 21+ level, well known to be a cluster state. This procedure strongly reduces the uncertainties on the R-matrix fit, and we end up with a recommended value of SE2(300keV)=42±2 keV · b. We show that ANC values derived from indirect methods are not consistent with the 12C(α,γ)16O cascade transitions to the 21+ state, and suggest that a remeasurement of this cross section is desirable.

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