Two proton knockout from carbon using linearly polarised photons.
PhD thesis, University of Glasgow.
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The ^(12)C([gamma],pp) reaction has been studied in the photon energy range E_=200-450MeV at the Mainz Microtron, MAMI. The linearly polarised photon beam was produced via the coherent bremsstrahlung technique with a diamond radiator and tagged with the Glasgow Tagging Spectrometer. The beam was incident on a ^(12)C target and the reaction products were detected in the 4pi Crystal Ball detector. The experimental study examines the photon asymmetry Sigma over a wider photon energy range than previous measurements and presents the first measurement of the angular dependence of Sigma.
The photon asymmetry has a negative magnitude for missing energies Em<70MeV where direct emission of nucleon pairs is expected. A strong peak at low Em is observed in Sigma for photon energies above and below Delta resonance energies. The asymmetry is studied in two missing energy regions Em<40MeV and E_m=40-70MeV where direct knockout from (1p)(1p) and (1s)(1p) shells is expected. For both missing energy regions the photon energy dependence of Sigma is rather flat, and the magnitude of Sigma([gamma],pp) generally exceeds Sigma([gamma],pn) for photon energies below 300MeV. Similar values are observed for photon energies less than 300MeV$. At low Em and photon energies below 300MeV, the results suggest that different mechanisms contribute to ^(12)C([gamma],pp) and ^(12)C([gamma],pn) reactions. The similarity in Sigma above photon energies of 300MeV suggests that both channels are dominated by contributions from isobaric currents. A strong angular dependence of Sigma is presented which follows a trend remarkably similar to deuteron photodisintegration. Theoretical calculations using an unfactorised distorted wave treatment of direct two-nucleon emission do not agree with the magnitude of the photon asymmetry. For Em above 100MeV and photon energy > 300MeV, Sigma has a substantially negative value which is attributed to two-step reactions following initial quasifree pion production.
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