The photodisintegration of 12C leading to excited states of 11B and 11C

Abstract

Gamma-ray spectra from the reactions 12C(γ, nγ') and 12C(γ, pγ') have been measured at 112° using bremsstrahlung photons with end-point energies 24.5, 27, 33 and 42 MeV in order to study the transitions from the giant dipole resonance in 12C to excited states in 11B and 11C. The γ-rays were detected in a Ge(Li) detector. Using measured response functions of the detector to γ-rays the fractions of decays of the excited 12C nucleus which proceed through the various excited states of the residual nucleus were determined. The fractions of neutrons and protons emitted to the ground states of 11C and 11B were found by measuring the 11C β + activity, combining it with the known (γ, n o)/(γ, P o) integrated cross-section ratio. Approximately 90 % of the neutron and proton emission proceed through the ground states, with approximately 5 % to each of the first excited states and the remainder shared among the higher levels of odd and even parity. The behaviour of the cross sections for transitions to the excited states is deduced as a function of excitation energy, and it appears that most of the cross sections are concentrated into regions only a few MeV wide. The results are interpreted in terms of current models of the giant resonance in 12C and of the unified model for the states in 11B and 11C. It appears that only a quite complex structure of the 12C ground state can explain the excitation of these many states of negative and positive parity. Comparison of proton and neutron emitting reactions to analogue states also permits some conclusions to be drawn about isospin impurities in the giant resonance.