Improvement of the correlated sampling method in Monte Carlo perturbation theory

We have developed an improved correlated sampling method by considering the neutron source change cased by the perturbation. The improved correlated sampling method is not limited for the small perturbation and this method is equal to the conventional correlated sampling method if the neutron source is not changed before and after the perturbations. By comparing the improved and the conventional correlated sampling methods, calculational results of the improved correlated sampling method are much closer to the reference calculation especially for the case of large perturbations. (author)