Total photoabsorption cross sections for H-1, H-2 and He-3from 200 to 800 MeV

The total photoabsorption cross sections for 1H, 2H, and 3He have been measured for incident photon energies ranging from 200 to 800 MeV. The 3He data are the first for this nucleus. By using the large acceptance detector DAPHNE in conjunction with the tagged photon beam facility of the MAMI accelerator in Mainz, cross sections of high precision have been obtained. The results show clearly the changes in the nucleon resonances in going from 1H to 3He. In particular, for the D13 region the behavior for 3He is intermediate between that for 1H, 2H, and heavier nuclei. This will provide a strong constraint to the theories that are presently being developed with a view to explaining the apparent ‘‘damping’’ of higher resonances in heavy nuclei.