Photonuclear Yields from Arsenic at 140 Mev and 320 Mev

Yields of a number of radioactive nuclei produced by synchrotron irradiations of arsenic have been determined at two energies, 140 Mev and 320 Mev. By comparing the yields at both energies to each other and to other data, it is possible to show that the ($\gamma , 3n$) and ($\gamma , \alpha n$) reactions proceed predominantly by the absorption of a photon to form an excited compound nucleus from which particles are emitted by evaporation. On the other hand, reactions in which more than one charged particle is emitted are often produced by photons having much more energy than would be required for particle evaporation. The average cross section for the production of a number of such nuclides by photons of energy between 140 and 320 Mev has been measured. For some gallium and copper isotopes the cross section is as large as a few tenths of a millibarn.