The High Energy Nuclear Photoeffect

The nuclear photoeffect for photons of energy greater than 150 Mev is calculated assuming the two-nucleon model used by Heidmann. The main features of the nuclear photoeffect are then quite similar to those of the deuteron photoeffect. The cross section for nuclear absorption of a high energy photon is about 1.6A times the cross section for the deuteron photoeffect. The deuteron photoeffect gives a very strongly forward angular distribution for protons of a given energy, observed in the laboratory system. The angular distribution for protons from the nuclear photoeffect is almost as strongly forward: for 90-Mev protons in the laboratory system the ratio of the differential cross section at 60° to that at 90° is about 3. The proton energy spectrum decreases rapidly with proton energy, and becomes steeper for observations at large angles. The calculated angular distributions and proton energy spectra are in fair agreement with measurements by Walker. The absolute value of the differential cross section for 90-Mev protons from carbon at 30° (laboratory system) is about 0.2 μbarn/Mev steradian per $Q$, or about 20 μbarn/steradian per photon. This absolute value is about one-third the absolute value measured by Walker, and is somewhat larger than the absolute value measured by Levinthal and Silverman.