LET calculation using G4EmCalculator

Hello everyone!
I am trying to set up a simple proton LET calculation within my water phantom, based on the example TestEm0. I have two questions in this regard.

  1. In the example TestEm0, the calculation of the restricted stopping power (with cut) considers only electromagnetic process without multiple scattering (MSC) and I am wondering why MSC should not be considered:
     if (((*plist)[j]->GetProcessType() == fElectromagnetic) &&
         (procName != "msc"))

I have an assumption that it is due to the continuous energy loss along step for the multiple scattering. And since the continuous energy loss corresponds to the energy deposition lower than the delta value (aka not enough energy to create a delta ray), therefore it is not related to discrete ionizations => we need to switch off the multiple scattering to avoid the continuous process along the step in the restricted stopping power. Am I following the correct logic?

  1. Could I calculate the LET (in a basic way) by simply employing the following function:
G4double G4EmCalculator::ComputeElectronicDEDX(G4double kinEnergy,
const G4ParticleDefinition* part, const G4Material* mat, G4double cut)

since it corresponds to the restricted electronic stopping power, aka restricted LET? Does the function ComputeElectronicDEDX() also exclude the MSC case?

Thank you very much in advance!


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if you look at the definition of restricted collision stopping power, the multiple scattering is not considered. It considers only ionisations caused by the incident particle. The hypothesis is that the incident particle moves with a constant direction. It does not deflect.

The ComputeElectronicDEDX excludes multiple scattering as well.


Hello Susanna,
Thank you a lot for the prompt answer!

I guess I was mistaken with the MSC concept. Am I right that the MSC model does not consider the continuous energy loss since it just combines multiple elastic scattering events? If so, why did they have to impose such restriction on the MSC in TestEm0 example?

There is still something that I did not quite catch, let me show you:
Imagine I have a proton travelling through a water phantom. I also have a production cut set up (say, it equals 10 um). In case, a delta-ray produced (with a range higher than 10 um), I understand that it is treated as a discreet process.

However, what is not clear to me is what happens when an energy is deposited locally, aka continuously (for low energies imparted to the medium, when delta rays are not produced). Say, mean free paths are sampled and during the next step a proton is supposed to lose energy continuously => does it mean that continuous energy deposition cannot overlap with multiple scattering, thus, no multiple scattering can be involved in a continuous energy loss process?

Thank you very much in advance!

If so, why did they have to impose such restriction on the MSC in TestEm0 example?
The pre-built physics constructors of Geant4 activate all the physics processes. Then, depending on the specific physical quantity to calculate, it may be necessary to switch off some processes, such as the msc in the stopping power calculation.

To understand the role of msc, please read this document, I found it very clear and I suggest its reading: