Radiation sources with photon energies up to few MeV are common in technology and medical applications. In this energy range, the relevant photon interactions are reduced to photo effect, coherent and incoherent scattering and pair production.
The Monte Carlo Transport can be carried out analogue or non-analogue. In the first case, all interactions are sampled according to the classical concept. If after an interaction, no secondary photon emerges, the particle history is ceased. This transport mode induces good statistics in those regions where in reality would be a high particle flux. But i.e. behind shieldings, where the physics cause the contrary, the simulation will as well take large calculation time to reach a statistically significant result.
To improve this situation, the non-analogue transport mode can be applied. In this case, all absorption interaction are handled implicitly, which means, that a history is not terminated after an absorption but instead only the weight is reduced according to the probability of this absorption reaction against the probability of a scattering reaction. Everey interaction is then forced to be a scattering interaction. Right now, only the photo effect is being handled implicitly.
As specialty, it is possible to apply the adjoint transport mode for photons. Adjoint transport may be pictured as backward simulation of the particle history, starting from the detector and scoring in the physical source. Such a simulation is beneficial if the volume of the detector is small compared to that of the source, which is to say if forward the the geometrical hit probability is small due to pure geometry.
The calculation of stray radiation fields is a special challenge at radiation transport simulations. As an example, the picture shows the resulting dose rate distribution on the roof of an X-Ray room where a 320 kV / 13 mA X-Ray tube is operated. This dose rate is due purely to stray radiation. Shown is the top view on the roof including the dose rate in colors as well as with iso-dose lines. Additionally, position and beam direction in the room underneath are sketched.