The photoneutron method of producing displacement radiation effects has been established. The displacement effects produced by recoils from $\mathrm{Si}(\gamma ,n)$ reactions have been detected by measurement of the change in excess-carrier lifetime. The change in the reciprocal of the lifetime is approximately proportional to the total number of primary reactions at various irradiating bremsstrahlung energies between 15 and 38 MeV, with a proportionality constant of (2.0×${10}^{-4\mathrm{}}$±25%)${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$. The data can be explained on the basis that the primary energy loss of the recoil atoms is due to ionization. Other possible explanations are offered and a comparison of the results with those of 30-MeV electron irradiations is made.

• Received 8 August 1963

DOI:https://doi.org/10.1103/PhysRev.133.A884