Coherent photon scattering on nuclei in the Δ(1232) resonance region

Abstract

Angular distributions for photon scattering from ¹²C have been measured at the Saskatchewan Accelerator Laboratory (SAL) high duty factor bremsstrahlung facility. Data were taken at 160, 200, 250 and 290 MeV endpoint energies and at angles ranging from 20 degrees to 150 degrees with the 53 cm diameter x 56 cm long Boston University Sodium Iodide (BUNI) detector. Coherently scattered photons were easily distinguished from neutral pion decay photons in detected energy spectra. The excellent resolution of the detector and the high duty cycle at SAL have made it possible to obtain new data on the angular distributions for elastic and, in some cases, for inelastic scattering as well. It was essential to distinguish between elastic scattering and inelastic scattering to the 4.44 MeV state in ¹²C. This was further complicated by the continuous nature of bremsstrahlung such that only a region within about 4 MeV of the endpoint could be used. Hence, precise energy calibration and stability of the calibration were required. Since elastic cross sections can be small, pileup transferring inelastic events into the elastic region was a concern and required careful attention. The angular distributions for elastic scattering were compared with a simple calculation of a dipole distribution weighted by the charge form factor and scaled by forward cross sections derived from total absorption cross sections with dispersion relations. This calculation badly underestimates cross sections at back angles. The more serious calculation with the Δ-hole model at 200 MeV overestimates the measurements at all angles. Some cross sections for inelastic scattering leading to the 4.44 MeV first excited state in carbon have been extracted. These cross sections are small compared to the elastic channel at forward angles, but are dominant at backward angles. Since this experiment was the first to separate elastic from inelastic photon scattering at these energies, these new data should stimulate further theoretical study.