Birks constant for different materials

Hello all!

Does anybody know what does the Birks constant mean in Geant4?

I’ve found 4 examples:

extended/electromagnetic/TestEm3/… optical material is C9H10;

advanced/amsEcal/… optical material is C8H8;

extended/optical/OpNovice/ … optical material is water;

extended/optical/LXe/… optical matirial is LXe;

Although the optical materials are different in these examples, the Birks constant is the same – 0.126 mm/MeV.

Is this value specified in air equivalent or optical material’s equivalent?

For example, I have found in the literature that the value of the Birks constant for ZnS(Ag) is 1,37 сm air equivalent MeV^(-1). Is this mean that my code will be : ZnS->GetIonisation()->SetBirksConstant(1,37*сm/MeV)?
Or Birks constant will be : 1,37[сm/MeV]*0,00129[g/cm^3 – air density]/3,64[g/cm^3 – ZnS density]=0,00049[cm/MeV]?


so called Birks effect means saturation of signal in sensitive detector in the case of dense ionisation. This effect was first measured (likely by Birks) in organic scintillators, however, exist in one or another form in other detectors. The measurement of its absolute value is not simple, there are limited number of experiments which publish this number.

Note, that in Geant4 tracking Birks effect is not taken into account. In User actions you may apply Birks correction when create hits in sensitive detector.


Thanks for your help! :slight_smile: yes, I understand the physical meaning of the Birks effect, but I just don’t understand why the value of the constant is the same in different examples for different materials. And if the unit of the constant is mm/MeV, what substance is it? Is it mm air equivalent MeV^(-1)? Or mm scintillator material equivalent MeV^(-1)?

Please take the numerical values in the optical examples as examples only. In this case, wikipedia gives 0.126 mm/MeV for polystyrene based scintillators.

I don’t understand the second part of the question.