Before revision: is the number of neutrons produced, on average, by a fission event—it is between 2 and 3 for both 235U and 239Pu .
After revision: (e.g., for thermal neutrons in 235U, = 2.4355 ± 0.0023 )is the number of neutrons produced, on average, by a fission event—it is between 2 and 3 for both 235U and 239Pu .