A measure of the @P04855@ of a specified interaction or reaction between an incident radiation and a target particle or system of particles. It is the reaction rate per target particle for a specified process divided by the @F02461@ of the incident radiation. In general, a specification is added of the type of radiation (e.g. @N04116@, photon), the @E02102@ of the incident radiation (e.g. thermal, epithermal, fast) and the type of interaction of reaction (e.g. @A00103@, @N04229@, @S05487@).