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If two elements with particle / matter interaction are closer to each other than a single time step then the particles drift for one time step after leaving the first element and before entering the second element. In #308 (closed) the variable tau was introduced in the class CollimatorPhysics in order to get rid of a time structure. The quantity tau is computed for the first and the last time step. The meaning of tau is the fraction of a time step the current position of a particle is away from the edge. tau has to between 0.0 and 1.0. However this isn't true if a particle hops from one element with particle / matter interaction to another. Currently this isn't handled correctly yet. Instead the particles drifts for at least one time step before it can enter another element with particle / matter interaction.