Example GB02

Force collision biasing

This example illustrates how to make a force collision biasing in a way that is essentially the same than the MCNP one.

Generally speaking, the scheme consists of a G4VBiasingOperator that takes decisions on what sort of biasing is to be applied. The operator makes these decision on requests of the G4BiasingProcessInterface process. This process wraps an actual physics process and asks to the operator about what sort of biasing it should apply. This operator selects G4VBiasingOperation objects that implement the actual biasing content.

In the present case, we make use of the biasing operator

• G4BOptrForceCollision that implements an "a la MCNP" force collision scheme for one particle type. This operator is defined in processes/biasing/generic. It is a non-trivial operator. It starts by "splitting" the track at the volume entrance. Then this track is forced to fly through the volume with no interaction. The G4OptnForceFreeFlight biasing operation is used for that. The second copy is then forced to interact within the volume, which is handled by the G4BOptnForceCommonTruncatedExp operation : it is common as it takes care of several processes by itself, and it applies a truncated exponential law : ie and exponential law limited to the [0,L] range, L being the volume width along the track flight.

To allow several particle types to undergo this force interaction scheme, an other operator is defined

• GB02BOptrMultiParticleForceCollision which holds one G4BOptrForceCollision per particle type, and which delegates then everything to it.

The geometry is simple : a single volume to which an instance of GB02BOptrMultiParticleForceCollision is attached to.

The wrapping of physics processes by G4BiasingProcessInterface processes is simply handled by the G4GenericBiasingPhysics physics constructor, as shown in the main program.

Then, at whatever level (stepping action, or sensitive detector) the statistical weight of the track can be obtained as:

w = track->GetWeight() ;