Problem 2585

Hi,

I found out that the energy loss formula used in the ATIMA model may not be correct when applied to compound materials.
Specifically, in G4AtimaEnergyLossModel::ComputeDEDXPerVolume, the stopping power is calculated from the effective charge (zt) of the input material and its associated mass (at) from NIST:
"
G4double zt = material->GetIonisation()->GetZeffective();
zt = std::min(zt,93.); 
G4double at = nist->GetAtomicMassAmu(G4lrint(zt));
G4double dedx = StoppingPower(p->GetPDGMass(), p->GetPDGCharge(), kineticEnergy/(MeV), at, zt) *material->GetDensity()/(g/cm3);
"
(from source/processes/electromagnetic/standard/src/G4AtimaEnergyLossModel.cc)

While this works well with simple materials, it seems wrong when dealing with compounds. This effect can be observed by calculating some ion ranges in a compound (e.g. plastic) and comparing ATIMA with the default energy loss model.

I suppose a possible fix (if this is confirmed to be a bug) could be to use Bragg's rule, as in the C++ ATIMA library (catima).

Best regards,
Valentin