Total photoabsorption cross sections forH1,H2, andHe3from 200 to 800 MeV

The total photoabsorption cross sections for ${}_{}{}^1{}_{}{}^{}\mathrm{H}$, ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been measured for incident photon energies ranging from 200 to 800 MeV. The ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ 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 ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ to ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. In particular, for the ${\mathit{D}}_{13}$ region the behavior for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is intermediate between that for ${}_{}{}^1{}_{}{}^{}\mathrm{H}$, ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, 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. © 1996 The American Physical Society.