Calculating stopping power

Hi all,

I am trying to calculate the scintillation yield of a detector by summing the calculated light yield (given a known Birks value and using the eDep and dE/dx evaluated along the trajectory) after each step, but am a bit confused by the values for stopping power I am seeing.

My approach was to simply divide the energy deposited within a step by that step’s length to get the stopping power evaluated at that energy; however, this seems to depend heavily on the step size. If the step length is on the order of one micron or greater, the Geant4 stopping power values seem in rough agreement with the expected values from NIST ASTAR. However, lowering that step limit down to tens of nanometers results in large fluctuations (hundreds of MeV/mm) in dE/dx values that don’t seem physical.

Ordinarily, a step size of microns would be sufficient. However; there are some thin (sub-micron) layers of the detector, and so the particle just takes a single step in those volumes if the step limit isn’t lowered. So I was hoping someone would have some advice on how to correctly approach the issue of energy loss in thin layers.

The physics lists being used are EmStandard_option4 and StepLimiterPhysics.

Thank you!

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Hi! These tens of nanometers are probably too little for physics models to work. We are talking about some atomic layers, which are rarely used for the measured data. I expect odd stuff there. Do you have a scintillation detector of nanometer thickness? Or is it coating? What is the target task of calculating the losses in the thin layers?

One workaround for thin foils is to make it thicker and less dense.

to reproduce the results of ASTAR, the multiple scattering and the energy loss fluctuations should be switched off. The cut should be enough high not to produce secondary particles. The step should be small and the target semin-infinite. These are the conditions to reproduce the CSDA which is adopted in ASTAR.