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E1 and E2 S factors of 12C(alpha, gamma)16O from gamma-ray angular distributions with a 4pi-detector array

A new experiment to determine the thermonuclear cross section of the 12C(alpha, gamma)16O reaction has been performed in regular kinematics using an intense alpha-particle beam of up to 340 &micro ; ; A from the Stuttgart DYNAMITRON. For the first time, a 4pi-germanium-detector setup has been used to measure the angular distribution of the gamma rays at all angles simultaneously. It consisted of an array of nine EUROGAM high-purity Ge detectors in close geometry, actively shielded individually with bismuth germanate crystals. The 12C targets were isotopically enriched by magnetic separation during implantation. The depth profiles of the implanted carbon in the 12C targets were determined by Rutherford backscattering for purposes of cross-section normalization and absolute determination of the E1 and E2 S factors. Angular distributions of the gamma decay to the 16O ground state were measured in the energy range Ec.m.=1.30– 2.78 MeV and in the angular range (lab.) 30°– 130°. From these distributions, astrophysical E1 and E2 S-factor functions vs energy were calculated, both of which are indispensable to the modeling of this reaction and the extrapolation toward lower energies. The separation of the E1 and E2 capture channels was done both by taking the phase value 12 as a free parameter and by fixing it using the results of elastic alpha-particle scattering on 12C in the same energy range.

the thermonuclear cross section of the 12C(alpha; gamma)16O reaction; absolute determination of the E1 and E2 S factors

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