Energy Deposition Spike at Multilayer interface

Hi all,
When analysing an energy deposition profile in a multilayer object I have been finding that there is consistently a spike in energy deposited at the interface of the two layers, regardless of the defined materials.

For example, a Si-Si block with a combined thickness of 2um and interface at 0.4um there will be a spike of energy deposited at 0.4um.

Initially I believed this to be a factor of changing densities between two materials, however, the test with two Silicon blocks has pointed to it being something more.

Is this a common issue in GEANT? Any help would be appreciated.

My suggestion is to limit the step. That should be smaller than the size of the voxel you are using (let’s 1/4 or 1/5). If that does not work, you can randomize the position of the edep along the step as follows:

G4double edep = theStep->GetTotalEnergyDeposit();
G4ThreeVector prePoint = theStep->GetPreStepPoint() ->GetPosition();
G4ThreeVector postPoint = theStep->GetPostStepPoint()->GetPosition();
G4ThreeVector point = prePoint + G4UniformRand()*(postPoint - prePoint);

Then you record edep and the point.


Not really my expertise, but I think it may be a feature rather than a bug. When a geometrical boundary limits a step, you have to be careful that multiple scattering does not change the end position so that it steps over - or does not step far enough to get to - the boundary. This is very tricky, and may result in several very small steps just before the boundary. Then there is the question of calculating an energy loss (with fluctuations) for those steps. Ultimately it all levels out, but it may depend on which precision of electromagnetic physics you are using - opt1/2/3/4 or EMX/Y/Z, etc. Try posting this on the physics forum, giving information about your setup, your Geant4 version, and the physics you are selecting.
Incidentally, Geant4, not GEANT4, nor GEANT.

Another thought just occurs to me…I don’t know how you are recording the energy deposition, but the energy deposition you get at the end of a step should be thought of as being distributed over the step. If you record it as being at the “post-step-point” it might appear to be lumped near the boundary.