The helicity dependences of the differential cross sections for the semi- inclusive gamma He-3 -> pi(0) X and gamma He-3 -> pi(+/-) X reactions have been measured for the first time in the energy region 200 <E-gamma <450 MeV. The experiment was performed at the tagged photon beam facility of the MAMI accelerator in Mainz using a longitudinally polarised high-pressure He-3 gas target. Hadronic products were measured with the large-acceptance Crystal Ball detector complemented with additional devices for charged-particle tracking and identification. Unpolarised differential cross sections and their helicity dependence are compared with theoretical calculations using the Fix-Arenhovel model. The effect of the intermediate excitation of the Delta(1232) resonance can be clearly seen from this comparison, especially for the polarised case, where nuclear effects are relatively small. The model provides a better theoretical description of the unpolarised charged pion photoproduction data than the neutral pion channel. It does significantly better in describing the helicity-dependent data in both channels. These comparisons provide new information on the mechanisms involved in pion photoproduction on He-3 and suggest that a polarised He-3 target can provide valuable information on the corresponding polarised quasi-free neutron reactions.
- TAGGED PHOTON SPECTROMETER
- 1ST MEASUREMENT