  Employing a recently developed Monte Carlo model, we study the fission of
240Pu induced by neutrons with energies from thermal to just below the
threshold for second chance fission. Current measurements of the mean number of
prompt neutrons emitted in fission, together with less accurate measurements of
the neutron energy spectra, place remarkably fine constraints on predictions of
microscopic calculations. In particular, the total excitation energy of the
nascent fragments must be specified to within 1 MeV to avoid disagreement with
measurements of the mean neutron multiplicity. The combination of the Monte
Carlo fission model with a statistical likelihood analysis also presents a
powerful tool for the evaluation of fission neutron data. Of particular
importance is the fission spectrum, which plays a key role in determining
reactor criticality. We show that our approach can be used to develop an
estimate of the fission spectrum with uncertainties several times smaller than
current experimental uncertainties for outgoing neutron energies up to 2 MeV.
