A Survey of Photonuclear Reactions on 12C

Watts, Daniel (1997) A Survey of Photonuclear Reactions on 12C. PhD thesis, University of Glasgow.

Abstract

This thesis describes an experimental study of the C12(gamma,2N) and C12(gamma,3N) reactions using tagged photons of energies 150-700 MeV. The photon beam was produced using the MAMI-B electron accelerator at the Institut fur Kernphysik at Mainz. Bremsstrahlung photons were produced by the electron beam impinging on a thin nickel foil radiator. The photon energies were determined with a resolution of MeV by analysis of the residual electrons in the Glasgow tagging spectrometer. Reaction products were detected in two detector systems PiP, TOF. The PiP detector is a large solid angle (~1 sr) plastic scintillator hodoscope used to detect protons. The TOF detector system comprised a versatile array of plastic scintillators which determined both proton and neutron energies from time of flight information. The detector system had a combined missing energy resolution of ~8 MeV. The particle detectors were positioned to cover kinematics both in and away from the back-to-back kinematics of previous measurements. The more unusual detector geometry gave more sensitivity to multiparticle/FSI processes and also allowed the study of direct 2 nucleon emission in regions of high initial pair momenta. The results were compared with a model developed by the Valencia group, which accounts for all the main photon absorption processes and therefore gives predictions of the cross section for all missing energy Em. The predictions of the Valencia model (VM) give reasonable agreement with the shape of the measured Em distributions for both 12C(gamma,pn) and 12C(gamma,pp) reactions up to 700 MeV, even in the more unusual kinematics. The magnitude of the cross section for the 12C(gamma,pp) was however overestimated by a photon energy and angular dependant factor of A smaller overestimation of the strength of the cross section was also observed in the 12C(gamma,pn) reaction for photon energies around the A resonance. The comparison of the low missing energy data with a model of direct 2N emission from a spectating nucleus (2N model) showed the dominance of the 2N process in the 12C(gamma,pn) reaction for Em ≤70 MeV and photon energies up to 600 MeV, even with the detectors positioned to be sensitive to more complex mechanisms. The 12C(gamma,pp) reaction was shown to be reasonably well described by direct emission of (1p)2 proton pairs for Em ?40 MeV and E? ?500 MeV, although the agreement with the 2N model was not as good as observed for the (gamma,pm) data. A large contribution of FSI/multiparticle events was observed for this reaction in the Em=40-70 MeV region although some evidence for the existence of photon absorption on (1s)(1p) proton pairs was found. The (gamma,3N) measurements allowed a more kinematically complete study of multiparticle/FSI processes and gave valuable information on the mechanisms involved. The first measurement of the 12C(gamma,ppn) and 12C(gamma,pnn) final states showed cross sections of similar magnitude and shape. The 12C(?, ppp) cross section was found to be ≤~8% of the ppn or pnn. The Valencia model predicted that the dominant mechanism for all the 3N final states was the reabsorption of photoproduced pions by nucleon pairs, with lesser contributions from 2N/3N+FSI processes. Although giving predictions in good agreement with the data for the ppn and pnn final states, the VM over-estimated the ppp final state by an energy dependent factor of ~5 which was attributed to the calculation of either the magnitude or angular distribution of the charge exchange FSI processes.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Douglas MacGregor
• Keywords: Nuclear physics and radiation, High energy physics
• Date of Award: 1997
• Depositing User: Enlighten Team
• Unique ID: glathesis:1997-75878
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 17:40
• Last Modified: 19 Nov 2019 17:40
• URI: https://theses.gla.ac.uk/id/eprint/75878