The emission of high‐energy secondary electrons from thin targets of low atomic number has been calculated. The targets are assumed to be bombarded with 25‐MeV electrons, 600‐kVp x rays, and prompt fission gamma radiation. The results give maximum efficiency of high‐energy secondary electron emission of 8% for 25‐MeV electrons, 0.30% for prompt fission gamma radiation, and 0.05% for 600‐kVp x rays.