Measurement of the Astrophysical S Factor (E2) of 12C(a,g)16O Reaction in 16O(e,e' a)12C Experiment.

Evgeni Tsentalovich

MIT-Bates~Linear~Accelerator~Center

The 12C(a,g)16O reaction at Ec.m.=0.3 MeV is one of the most important processes in nuclear astrophysics [1]. It plays a major role in the He burning stage of massive star evolution, and affects the predicted abundance distribution of elements. The reaction is also extremely important for supernova models. Due to the Coulomb barrier, the cross-section of the reaction is very low at stellar energies. It is expected to be about 10-8nb, well beyond the possibilities of laboratory measurements. However it is possible to measure the cross-section at higher energies, and to extrapolate to the effective energy of 0.3 MeV using appropriate theoretical models. Since the cross-section varies very rapidly with energy, it is most important to extend the measurements to the lowest energies possible.

Direct measurements of the 12C(a,g)16O reaction produce formidable problems due to low yield, difficulties with g detection and a high neutron-induced g-background. Most of experimental data [2-4] have been taken at Ec.m.> 1.0 MeV and an extrapolation over a long range produces significant errors. The knowledge of the E2 part of the cross-section is especially poor.

The E1 part of the cross section was also measured via a related reaction of a-decay of excited 16O nuclei following b-decay of 16N [5,6]. Unfortunately these experiments don't provide information about the E2 part of the cross-section.

The proposed experiment also uses the inverse reaction of a-decay of excited 16O states, and inelastic electron scattering is used to prepare these excited states. The superthin internal target will be used in Bates South Hall Ring. Scattered electrons will be detected in BLAST detector, and silicon strips will be used to detect a-particles and 12C nuclei in triple coincidence with electrons. The data allows extraction of the E2 part of the cross-section of the direct 12C(a,g)16O reaction with very small theoretical errors. The simulations show that the experiment can extend the measurements down to Ec.m.=0.6-0.8 MeV and provide unprecedentedly accurate data for the E2 part of the cross-section.

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5. L.Buchmann et al. Phys.Rev.Let. 70(1993)726.

6. Z.Zhao et al. Phys.Rev.Let. 70(1993)2066.