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

Toolkit Fundamentalsvolume or of a user defined sphere. The adjoint source should contain one or several sensitive volumes and shouldbe small compared to the entire geometry. The user can set the minimum and maximum energy of the adjointsource. After its generation the adjoint primary particle is tracked backward in the geometry till a user definedexternal surface (spherical or boundary of a volume) or is killed before if it reaches a user defined upper energylimit that represents the maximum energy of the external source. During the reverse tracking, reverse processestake place where the adjoint particle being tracked can be either scattered or transformed in another type of adjointparticle. During the reverse tracking the G4AdjointSimulationManager replaces the user defined primary, run,stepping, ... actions, by its own actions. A reverse tracking phase corresponds to one Geant4 event.3.7.3.1.2. Forward tracking phaseWhen an adjoint particle reaches the external surface its weight, type, position, and direction are registered and anormal primary particle, with a type equivalent to the last generated primary adjoint, is generated with the sameenergy, position but opposite direction and is tracked in the forward direction in the sensitive region as in a forwardMC simulation. During this forward tracking phase the event, stacking, stepping, tracking actions defined by theuser for his forward simulation are used. By this clear separation between adjoint and forward tracking phases,the code of the user developed for a forward simulation should be only slightly modified to adapt it for an adjointsimulation (see Section 3.7.3.2). Indeed the computation of the signals is done by the same actions or classes thatthe one used in the forward simulation mode. A forward tracking phase corresponds to one G4 event.3.7.3.1.3. Reverse processesDuring the reverse tracking, reverse processes act on the adjoint particles. The reverse processes that are at themoment available in Geant4 are the:• Reverse discrete ionization for e-, proton and ions• Continuous gain of energy by ionization and bremsstrahlung for e- and by ionization for protons and ions• Reverse discrete e- bremsstrahlung• Reverse photo-electric effect• Reverse Compton scattering• Approximated multiple scattering (see comment in Section 3.7.3.4.3)It is important to note that the electromagnetic reverse processes are cut dependent as their equivalent forwardprocesses. The implementation of the reverse processes is based on the forward processes implemented in the G4standard electromagnetic package.3.7.3.1.4. Nb of adjoint particle types and nb of G4 events of an adjoint simulationThe list of type of adjoint and forward particles that are generated on the adjoint source and considered in thesimulation is a function of the adjoint processes declared in the physics list. For example if only the e- and gammaelectromagnetic processes are considered, only adjoint e- and adjoint gamma will be considered as primaries. Inthis case an adjoint event will be divided in four G4 event consisting in the reverse tracking of an adjoint e-, theforward tracking of its equivalent forward e-, the reverse tracking of an adjoint gamma, and the forward trackingof its equivalent forward gamma. In this case a run of 100 adjoint events will consist into 400 Geant4 events. If theproton ionization is also considered adjoint and forward protons are also generated as primaries and 600 Geant4events are processed for 100 adjoint events.3.7.3.2. How to update a G4 application to use the reverse MonteCarlo modeSome modifications are needed to an existing Geant4 application in order to adapt it for the use of the reversesimulation mode (see also the G4 example examples/extended/biasing/ReverseMC01). It consists into the:• Creation of the adjoint simulation manager in the main code• Optional declaration of user actions that will be used during the adjoint tracking phase• Use of a special physics lists that combine the adjoint and forward processes• Modification of the user analysis part of the code61