Perturbation of phase space downstream by parameters upstream

The development of a radiation history from the source particle to the end-of-life of all its progenies is of essence in Monte Carlo simulations. The progression from upstream to downstream is demonstrated here for the case of a target system of a megawatt spallation neutron source. The target system is assembled component-by-component starting with the target, followed by the moderator, the inner and outer reflectors, the de-coupler and the lining. At each stage the energy and time spectra are scored, underlining the function of each component. Perturbations are then introduced to the beam energy and beam footprint to gauge the resultant changes in the escaping neutron flux, which could have major implications on the shielding design. Such a gauge is necessary as for most facilities the application for licensing takes place concurrently with the design of the target system. The study is therefore instructive in applying for a license before the full and final specification of the target system becomes available. (authors)