An increase of the $e^{+}-e^{-}$ pair-creation rate above the Bethe-Heitler rate has been observed for photons incident along the $〈110〉$ axis of a Ge crystal at 100 K. The enhancement increases with increasing energy and the rate is more than doubled at 100 GeV. The observed features appear to be consistent with the theory of crystal-assisted pair creation, but the magnitude is less than predicted. Pair-creation enhancement is also observed off axis and this is attributed to coherent pair creation.

• Received 13 August 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.2371