Nuclear Photodissociation by High Energy Synchrotron Gamma-Rays

Abstract

The stars and single proton tracks produced in photographic emulsions exposed to a beam of high energy synchrotron gamma-rays have been analyzed. The maximum energy of the bremsstrahlung spectrum was varied between 150 and 300 Mev. The following subjects were studied: 1. the cross section for star production as a function of the excitation energy; 2. the energy distribution of the protons from stars as well as single protons; 3. the angular distribution of star protons as well as single protons; and, 4. the relative number of stars associated with a meson coming out.The cross section for the nuclear photoevents increases with increasing energy above the meson threshold. There seem to be two sorts of processes taking place in competition with each other. One is the so called free meson effect; namely, a free meson is produced inside the nucleus by the interaction of a photon with a nucleon and is then absorbed in the same nucleus. The other effect is a process in which a photon is absorbed directly by a group of nuclear particles without emitting a real meson. Evidence for the free meson effect is seen in the fact that the angular distribution of star protons of energy between 20 and 60 Mev in the case of 300-Mev excitation shows a strong forward peak. Evidence for the existence of the direct absorption of photons comes from the fact that the angular distribution of star protons of high energy, say about 100 Mev, shows a forward asymmetry.The cross section for direct absorption is much larger than expected from Levinger's theory of nuclear photodissociation. The cross section should be at least of the same order of magnitude as the free meson effect.