A bounded Monte-Carlo estimator for reaction rates in finite volume elements

The Monte-Carlo calculation of reaction rates becomes unreliable in regions where only very few particle histories contribute directly to the reaction. The integral formulation of the transport equation however, generates a sampling procedure which makes each collision point of a history ( in regions near the particle source) contribute to reactions in regions far from the source. Each collision-point contribution is an integral over the solid angle under which the detector region is seen from the collision point and its estimator is based on the uncollided flux. For large distances between collision point and flux point this new estimator reduces to the standard point flux estimator with its well known singularity behaviour. (author)