Coupling of forward and adjoint Monte Carlo calculations for continuous energy by next-event estimation

Coupling of forward and adjoint Monte Carlo calculations can be applied in several ways. Here we consider the coupling at a surface that divides the total problem space in a part containing the total neutron source and a part containing the detector. Up to now this coupling required a segmentation of the coupling surface in small areas, but also with respect to energy and directional intervals. In this paper it is described how the coupling can be realised for a continuous energy treatment of the neutron transport without any segmentation of the coupling surface. This requires the introduction of a next-event estimator to match not only the crossing point of the coupling surface for a forward and an adjoint history, but also the particle direction and energy. Given a collision site of a forward particle at one side of the coupling surface and a collision site and direction and energy of an adjoint particle at the other side of the surface, the next-event estimator has to determine the next collision site for the adjoint particle such that it could have a collision thereafter at the collision site of forward particle, while its energy matches that of the forward particle. (author)