  Many neutron transport Monte-Carlo codes can randomly sample fission neutron
energies from a Watt spectrum. The quality of simulations depends on how well
the Watt spectrum represents the true energy spectrum of the fission neutrons,
and on one's choice of the Watt parameters a and b. The energy spectra of
fission neutrons have been calculated and tabulated for the neutron induced
fission of 235,238U and 239Pu as a function of incoming neutron energy by
Madland using the Los Alamos Model. Each of these energy spectra are mapped
into time-of-flight space and fitted with a Watt spectrum. A subroutine getab
has been written to interpolate these results, so that Watt a and b parameters
can be estimated for all incoming neutron energies up to ~16 MeV.
