Problem 2249

Summary: Change in photon conversion physics
Product: Geant4 Reporter: Michael MacKenzie <michaelmackenzie>
Component: processes/electromagnetic/standardAssignee: Vladimir.Ivantchenko
Status: RESOLVED WONTFIX    
Severity: normal    
Priority: P4    
Version: 10.6   
Hardware: All   
OS: All   

Description Michael MacKenzie 2020-06-15 18:36:10 CEST 
The updated photon conversion code has changed the behavior found in 10.4.
In BetheHeitlerModel (and now G4PairProductionRelModel in 10.6) the correlation between the e+ and e- cos(theta) from the photon direction has changed fairly significantly.

This change is most clearly seen by looking at how cos(theta) is chosen:
10.4:
G4BetheHeitlerModel.cc:270-274 shows one random value is picked from the angular distribution which is applied for each daughter.

10.5 and 10.6:
G4ModifiedTsai.cc:129-135 shows a random value is picked separately for each daughter, therefore decorrelating the two.

This difference is most plainly seen by plotting KE(e-)*cos(e- angle from photon) vs KE(e+)*cos(e+ angle from photon) which goes from being a diagonal line (10.4) to a 2D distribution (10.5 and 10.6).

Is this an expected change in behavior solving a previous bug? I was unable to find a reference to any change here other than code cleaning in the release notes.

Best,
Michael
Comment 1 Vladimir.Ivantchenko 2020-06-30 11:35:50 CEST 
Hello,

since Geant4 10.5 we have introduced so called 5D model of gamma conversion which handles correctly nuclear recoil and final state e+e-. This model in Geant4 10.6 is included in Option4 (EMZ) physics. We also tried update Tsai angular generator for e+e- using comparisons with this 5D model. It cannot be as accurate as the 5D model. If this question is important for you, please, try to make a study of 3 existing generators, where 5D should be the reference.
Comment 2 Vladimir.Ivantchenko 2020-11-23 11:03:52 CET 
Hello,

Tsai generator is based on parameterisation of original Tsai angular distribution of bremsstrahlung. Extension of Tsai angular distribution to sampling of e+e- pair production is done using comparison of gamma conversion to e+e- pair with 5D model, which is fully based on QED differential cross section. In the current version of Tsai generator used for e+e- production there is less inaccuracy of the final state.
Comment 3 Vladimir.Ivantchenko 2020-11-23 11:04:55 CET 
It is not a bug but request to explain what was changed in Geant4.