branching ratio

Short definition from Wiktionary

Noun

1. 1
   In radioactivity, the portion of all decays that take a given pathway. For example, californium-252 decays by alpha decay ~97% of the time, and by spontaneous fission ~3% of the time; so the branching ratio is about 0.97 in favor of alpha decay, and about 0.03 in favor spontaneous fission.