If the reaction is a fission, Np neutrons are emitted according to the value of v(En), the mean number of neutrons per fission (given in cross-section files). The Np neutrons are chosen as

Np = I + 1 if C < v(En )-I

Np = I if C>v(En)-I

where I is the largest integer smaller than v(En) . The energies of outgoing neutrons are chosen using a Maxwell fission spectrum, or an energy — dependent Watt spectrum, or an evaporation spectrum. However, such treatment is not totally correct, because the NP obey a Poissonian distribu­tion; for example, if v = 2.5, MCNP gives two or three neutrons (in order to have 2.5 on average), but it never gives bigger and smaller neutron number (with the same average).