  Background: The complete event fission simulation code FREYA is used to study
correlations in fission. To make the best possible simulations, FREYA one must
find the optimized values of the five physics-based parameters which
characterize the events produced by FREYA. So far this has been done only
empirically or by brute force computational techniques. Purpose: We seek to
develop a method of parameter optimization for FREYA that would include
uncertainty quantification and an analysis of the correlations between the
parameters. Methods: We focus on spontaneous fission as a simpler test case of
the method. We first check the results of previous optimizations for
$^{252}$Cf(sf) and then go on to develop a simulated annealing approach to
optimize the parameters. Although $^{252}$Cf(sf) has the most measured
observables, we are able to apply the technique to all spontaneously fissioning
isotopes in the current version of FREYA. Results: We find optimal values of
the five FREYA parameters, with uncertainties, for all isotopes studied. Our
results are compared with available data. The parameter values are physically
reasonable and in accord with intuition, even in cases where the data for
optimization are sparse to non-existent. Conclusions: The simulated annealing
method provides a way to determine the best parameters for the
phenomenologically-successful fission model FREYA. We have developed an
algorithm that can be updated to include additional isotopes or more data sets
for the current isotopes as they become available.
