The alpha-particle breakup of the 12.71 and 11.83 MeV levels in C 12

Abstract

The alpha-particle decay of states in C 12, with special emphasis on those at 11.83 and 12.71 MeV excitation, has been studied using the B 11(d, n)C 12 reaction. Neutron time-of-flight spectra and semiconductor detector charged-particle spectra were analysed by a two-parameter pulse-height analyser. Angular and energy distributions of the alpha particles from the 12.71 and 11.83 MeV states indicate that these states decay sequentially through Be 8. The 1 + spin-parity assignment for the 12.71 MeV state is supported by the fact that no transitions to the ground state of Be 8 were observed. Approximately 90% of the transitions from the 11.83 MeV state proceed via Be 8(2.9 MeV) while the remaining 10% feed the ground state. There is some evidence to indicate that the 10.84 MeV state of C 12 decays exclusively to Be 8(0). The 3 − assignment for the 9.64 MeV state in C 12 is favoured over 1 − on the basis of the angular distribution of the alpha particles from this state to the Be 8 ground state. This work, together with other reported in the literature, justifies the representation of C 12 as a Be 8+α two-body cluster from the alpha-particle threshold to 16 MeV excitation. Such a representation is in accordance with the two-body cluster model of Phillips and Tombrello.