Abstract
The 12C\((\vec{\gamma},pp)\) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry \(\Sigma\) has been measured over a wider \(E_{\gamma}\) range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the \(\Sigma\) data exhibit a strong angular dependence, similar to deuteron photodisintegration.
Highlights
This paper reports measurements of the 12C(γ,pp) reaction made with linearly polarised photons in the energy range Eγ = 200–450 MeV
An understanding of which specific mechanisms contribute to (γ,NN) reactions at different missing energies has been obtained from the Valencia model [4] which uses a microscopic treatment of photon-nucleus coupling at intermediate energies including pion Final-State Interactions (FSI)
The comparison between the calculations and the previous measurements suggested the magnitude of Σ predicted by the calculations was too large in the photon energy range Eγ = 240–290 MeV, this comparison was hampered by the poor statistical accuracy of the earlier data
Summary
In recent years polarised photons have provided access to observables sensitive to the details of the reaction process, whose effects are hidden when averaged over the total response. Cross sections for reactions in which the polarisation of the incident photon is either parallel (σ ). Perpendicular (σ⊥) to the plane of the emitted protons have different sensitivity to the reaction mechanisms that contribute. This difference is emphasised through the photon asymmetry, Σ. Polarised photon measurements provide a powerful tool to study details of two-nucleon emission reactions and to test models of the reaction process. This paper reports measurements of the 12C(γ,pp) reaction made with linearly polarised photons in the energy range Eγ = 200–450 MeV. The near 4π angular coverage of the Crystal Ball (CB) detector, used to detect protons, allowed investigation of the photon asymmetry Σ over a wide range of proton angles
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