A study of the 12C(γ-vector,pp) reaction

This thesis describes the first experimental study of the 12C(γ-vector,pp) reaction with linearly polarised tagged photons of energies 170-350 MeV. The photon beam was produced using the MAMI electron accelerator at the Institut fuer Kernphysik, Mainz, Germany, with the polarisation of the photons achieved by use of coherent bremsstrahlung on a diamond radiator. The photon energy was determined to within ∼2 MeV by momentum analysing the recoil electrons in the Glasgow tagging spectrometer, and the total tagged photon flux rate was 108 s-1. The polarised photons impinged on the 12C target and reaction products were detected in two scintillation detector systems, PiP and TOF. The PiP detector is a large solid angle (∼1 sr) hodoscope used for the detection of protons. The TOF detector system consists of an array of plastic scintillation detectors, which determine both proton and neutron energy from the time-of-flight. The combined detector system had an overall energy resolution of ∼7.5 MeV, sufficient to allow nucleon emission from the (1p)2 and (1p)(1s) shells to be resolved. The events were selected on the basis of angular correlation and excitation energy. Asymmetries and cross sections for the reaction with the photons polarised parallel and perpendicular to the reaction plane were determined as a function of photon energy, particle angle and recoil momentum. Comparisons are made with previous experimental results on 12C(γ-vector,pn and 16O(γ-vector,pp) reactions to help shed light on the processes occurring in the 12C(γ-vector,pp) reaction. Additionally, the 12C(γ-vector,pp) results are compared in detail to the predictions of a theoretical model developed by the Gent group, modified to sample the present detector acceptances by using a Monte Carlo technique. At low missing energies, the experimental results for the asymmetry suggest that absorption on Δ currents make a large contribution to the 12C(γ-vector,pp) reaction. This is more apparent in the measured cross sections, which display a resonance structure. Also, the similarity in the asymmetry for 12C(γ-vector,pn) and 16O(γ-vector,pp), suggests that the three reactions all have a contribution from Δ currents. Asymmetries calculated using the Gent model were in broad agreement with experiment in this low missing energy region although there are some significant differences in detail. Little evidence was found in the results for a large contribution from final state interaction (FSI) effects. This lends support to the idea that at low missing energy, the (γ,pp) reaction is not strongly fed from initial pn absorption of the photon followed by charge exchange FSI. However, at higher missing energies, the 12C(γ-vector,pp) results suggested a larger contribution from FSI and pion production processes in addition to direct two nucleon knockout. This conclusion was supported by the comparison to the Gent model, which significantly underpredicted the cross sections. The model is based purely on direct two nucleon knockout, and the lack of agreement between experiment and theory gives support to the idea of strength from the above mentioned processes at higher missing energies. (author)