Geant4 User's Guide for Application Developers - Geant4 - CERN

Toolkit Fundamentals3.7.3.4. Known issues in the Reverse MC mode3.7.3.4.1. Occasional wrong high weight in the adjoint simulationIn rare cases an adjoint track may get a wrong high weight when reaching the external source. While this happensnot often it may corrupt the simulation results significantly. This happens in some tracks where both reversephoto-electric and bremsstrahlung processes take place at low energy. We still need some investigations to removethis problem at the level of physical adjoint/reverse processes. However this problem can be solved at the level ofevent actions or analysis in the user code by adding a test on the normalized signal during an adjoint simulation.An example of such test has been implemented in the Geant4 example extended/biasing/ReverseMC01 . In thisimplementation an event is rejected when the relative error of the computed normalized energy deposited increasesduring one event by more than 50% while the computed precision is already below 10%.3.7.3.4.2. Reverse bremsstrahlungA difference between the differential cross sections used in the adjoint and forward bremsstrahlung models isthe source of a higher flux of >100 keV gamma in the reverse simulation compared to the forward simulationmode. In principle the adjoint processes/models should make use of the direct differential cross section to samplethe adjoint secondaries and compute the adjoint cross section. However due to the way the effective differentialcross section is considered in the forward model G4eBremsstrahlungModel this was not possible to achieve for thereverse bremsstrahlung. Indeed the differential cross section used in G4AdjointeBremstrahlungModel is obtainedby the numerical derivation over the cut energy of the direct cross section provided by G4eBremsstrahlungModel.This would be a correct procedure if the distribution of secondary in G4eBremsstrahlungModel would matchthis differential cross section. Unfortunately it is not the case as independent parameterization are used inG4eBremsstrahlungModel for both the cross sections and the sampling of secondaries. (It means that in the forwardcase if one would integrate the effective differential cross section considered in the simulation we would notfind back the used cross section). In the future we plan to correct this problem by using an extra weight correctionfactor after the occurrence of a reverse bremsstrahlung. This weight factor should be the ratio between the differentialCS used in the adjoint simulation and the one effectively used in the forward processes. As it is impossibleto have a simple and direct access to the forward differential CS in G4eBremsstrahlungModel we are investigatingthe feasibility to use the differential CS considered in G4Penelope models.3.7.3.4.3. Reverse multiple scatteringFor the reverse multiple scattering the same model is used than in the forward case. This approximation makesthat the discrepancy between the adjoint and forward simulation cases can get to a level of ~ 10-15% relativedifferences in the test cases that we have considered. In the future we plan to improve the adjoint multiple scatteringmodels by forcing the computation of multiple scattering effect at the end of an adjoint step.64