Three α-particle decay of C 12

Abstract

The reaction B 11(p, 2α)He 4 has been studied at proton bombarding energies of 2.0, 2.65, 3.25, 3.73, 4.00, 5.08 and 5.64 MeV. These energies excite several resonantstates in C 12. The kinematics of the reaction were completely determined by detecting in coincidence two of the three final α-particles in solid-state detectors. This determination of the kinematics permitted discrimination between sequential two-body decay and direct three-body decay. The reaction mechanism was found to be sequential two-body decay involving the ground and the broad first-excited states of Be 8, with events involving the latter showing strong interference effects. These interference effects were attributable to the identity of the three final particles and to the ambiguity of the order of their emission. Not more than 5 % of the experimental cross section could be attributed to the direct three-body decay mechanism; indeed, the contribution from the direct process is probably much smaller than the quoted 5 %. Hence, the conclusion must be that, for C 12 with an excitation energy between 17 and 21 MeV, the decay mechanism is predominantly sequential two-body decay. Also the total cross section for this reaction is of the order of 100 to 200 mb; thus it is comparable to the geometric cross section. These two facts imply that, in this energy region, C 12 has a strong cluster configuration of an α-particle plus a Be 8 nucleus in either the ground or the first-excited state.