Some geometric modeling and tracking codes for Monte Carlo particle transport including direction biasing techniques

In Monte Carlo simulation, one is interested in the particle tracking process in two major ways. The first of these is in simply keeping track of where the particle is in relation to the various homogeneous zones that are used to simulate the spatial domain of interest through which the particle moves randomly in phase space. The second of these, which is becoming more and more important, is in forcing particles either in a preferred general direction (direction biasing) or to exactly intersect a particular zone. This latter process is essential in the statistical estimation variance reduction process in which the result of interest (e.g., particle detection) is forced to occur at each particle interaction point. These processes have many factors in common when treated mathematically. This paper describes experience in developing and using mathematical algorithms for these general tracking and forcing processes in the Monte Carlo modeling of nuclear well-logging tools and industrial tomographic devices