Random numbers r can be used very simply to generate stochastic free flights, which duration will then be given by t r = − − τ τ 0 ln ⁡ ⁡ ( r ) . The computer time used for self-scattering is more than compensated for by the simplification of the calculation of the free-flight duration. To enhance the speed of free flight time calculation, several schemes such as “Constant Technique”, and “Piecewise Technique” are used to minimize the self-scattering events.