Calculations of Neutron Time-Energy Distributions in Heavy Moderators
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The task of making detailed neutronics calculations for heavy moderating isotopes has always been a most difficult problem for a variety of reasons. Consider, for instance, the problem of calculating the time and energy (or lethargy) dependent neutron density N(E,t) that develops in a homogeneous moderator during and following the introduction of a source, S(E,t) in the MeV range. Monte Carlo techniques are very impractical because of the large number of collisions required per history. The usual multi group stepping matrix techniques 1 are inapplicable for at least two reasons: (a) inordinately large matrices are required when collision energy losses are small and (b) scattering through the inelastic region may take but a microsecond while moderation to thermal energies may require times of the order of a millisecond, thus making it virtually impossible to choose a suitable time step upon which to define the matrix.
The article of record as published may be found at http://dx.doi.org/10.13182/NSE70-A19333