On photonuclear reaction mechanisms at intermediate energies

Measurements of 12C $(\vec{\gamma},pp)$ photon asymmetries for $E_{\gamma}=200$ –450 MeV

“…3 for both E γ = 200-310 MeV and E γ = 320-450 MeV. Both distributions rise smoothly from threshold as in previous measurements [16,5] at similar photon energies. The small shoulder at E m ∼ 35 MeV found at E γ = 200-250 MeV in ref.…”

“…Comparisons of previously measured 12 C(γ,pp) E m spectra [5,14] with predictions from the Valencia model [4] suggested that for E m > 70 MeV direct two-proton emission makes a very small contribution. Above 100 MeV multistep processes contribute to the reaction cross section and for E γ < 250 MeV photon absorption on a nucleon pair followed by FSI is thought to be the dominant process.…”

“…Despite its approximations, the Valencia model gives a good account of the shape and strength of the 12 C(γ,pn) missing energy spectrum and the shape of the 12 C(γ,pp) spectrum [5], suggesting that the relative contributions from different processes and FSI are reasonably well described. These studies indicated that for the mechanism to be predominantly two-body, a restriction has to be made to the low energy part of the residual excitation spectrum.…”

“…The low strength just above threshold indicates that the residual nucleus is not often left in or near its ground state. Most of the 12 C(γ,pp) yield is found at higher missing energies where much of the strength is thought to come from initial pion production, multi-nucleon knockout and mechanisms with FSI [5,14]. At high E m the spectra for both photon energy ranges fall off smoothly up to the limits determined by the useful energy range of the detectors.…”

“…At high E m both reaction channels are thought to have similar mechanisms [5]. Powrie et al [12] suggested the substantial asymmetry values observed above 300 MeV may be due to pion production reactions followed by pion reabsorption.…”