Testing PAI model

Hello, so I am trying to test the Ionization energy loss models for muons for example Em5. For normal muon ionization energy loss model using G4MuIonisation(). I tested it only for ionization energy loss and switched off other losses I get this result below:

Which is accurate based on PDG table.

Now I am not sure if I am implementing PAI model correctly.

Using some lines from TestEm8 I make some changes to the Physics list:

 else if (name == "pai") {
    G4EmParameters::Instance()->AddPAIModel("all","world","pai"); // adding PAI Model 

And add this line in macro:

/testem/phys/addPhysics  pai

However I still keep this lines in PhysListEmStandard.cc

 G4MuIonisation* muIoni = new G4MuIonisation();  // this is for muon ionization 
 muIoni->SetStepFunction(0.1, 50*um); 

This yields the result:

Which is slightly different result but is this the correct implementation of PAI model ?


PhysListEmStandard is so-called “local” physics configuration of EM physics. It is useful in many cases but comparison of results should be done first of all using referenced EM physics, because there is no guarantee that PhysListEmStandard is the same in different examples and is not fully coherent with the actual version of Geant4.

So, please, use first of all “emphysics_opt0” or “emphysics_opt4”. You may get the same or a bit different results but this may be subject of discussion. If you are trying to use the pai model, please, note that in recent releases there were a problem, which likely is fixed in Geant4 11.1patch2 and in coming 11.2beta versions.


Thank you for the reply ! Just out of curiosity what was the issue with PAI model ?

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