The rule that the electron-hole-pair creation energy is 3 times the semiconductor band gap is extended to include a large group of insulators. It is used, together with the free-particle approximation, to describe cathodoluminescent phosphor efficiencies and the escape probabilities of secondary electrons.

• Received 16 September 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.1522