10.3.2} Secondary neutrons

 In the multigroup transport scheme, the production of secondary
 neutrons via (n,xn) reactions is taken into account implicitly by the
 so-called "non-absorption probability", a group-dependent factor
 by which the weight of a neutron is multiplied after exiting a
 collision. If the only possible reactions are capture and scattering,
 the non-absorption probability is < 1, but at energies above the
 threshold for (n,2n) reaction it can take values larger than 1.
 Fission neutrons, however, are treated separately and created
 explicitly using a group-dependent fission probability. They are
 assumed to be emitted isotropically and their energy is sampled from
 the fission spectrum appropriate for the relevant isotope and neutron
 energy. The fission neutron multiplicity is obtained separately from
 data extracted from European, American and Japanese databases.

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