Problem 1256

Summary: Coulomb Scattering forces very short steps for muons of 20 eV
Product: Geant4 Reporter: John Apostolakis <John.Apostolakis>
Component: processes/electromagneticAssignee: Vladimir.Ivantchenko
Status: RESOLVED FIXED    
Severity: normal    
Priority: P2    
Version: 9.4   
Hardware: All   
OS: All   
Attachments: Verbose1 per step output

Description John Apostolakis 2011-10-13 11:56:46 CEST 
During simulation low energy muons take a large number of steps, with the step size determined by Coulomb Scattering.

#Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  ProcName
 5001     -186      885 -3.94e+03  2.81e-05        0 4.18e-08       1.4           CoulombScat
 5002     -186      885 -3.94e+03  2.81e-05        0 8.64e-09       1.4           CoulombScat
 5003     -186      885 -3.94e+03  2.81e-05        0 1.43e-09       1.4           CoulombScat
 5004     -186      885 -3.94e+03  2.81e-05        0 5.98e-08       1.4           CoulombScat
 5005     -186      885 -3.94e+03  2.81e-05        0 9.99e-08       1.4           CoulombScat
...
 5491     -186      885 -3.94e+03  2.81e-05        0 8.17e-08       1.4     CoulombScat
 5492     -186      885 -3.94e+03  2.81e-05        0 8.42e-09       1.4     CoulombScat
 5493     -186      885 -3.94e+03  2.81e-05        0 5.42e-08       1.4     CoulombScat
 5494     -186      885 -3.94e+03  2.81e-05        0    2e-08       1.4    CoulombScat
 5495     -186      885 -3.94e+03  2.81e-05        0 4.83e-08       1.4     CoulombScat
 5496     -186      885 -3.94e+03  2.81e-05        0 5.57e-09       1.4     CoulombScat
 5497     -186      885 -3.94e+03  2.81e-05        0 2.44e-08       1.4     CoulombScat
 5498     -186      885 -3.94e+03  2.81e-05 1.71e-176 1.49e-08       1.4  CoulombScat
 5499     -186      885 -3.94e+03  2.81e-05        0 2.05e-08       1.4     CoulombScat
 5500     -186      885 -3.94e+03  2.81e-05        0 6.45e-08       1.4     CoulombScat

 All these steps occur in the volume is LArMgr::LAr::EMEC::Neg::OuterWheel::Absorber 
(which was deleted for readability).

It occurs with Geant4 9.4 patch 2.
Comment 1 John Apostolakis 2011-10-13 11:58:39 CEST 
Created attachment 138 [details]
Verbose1 per step output

The full output file is available (to verbose 6 level).
Comment 2 John Apostolakis 2011-10-13 16:24:57 CEST 
According to the experts, the volume LArMgr::LAr::EMEC::Neg::OuterWheel::Absorber
is made of std::Iron (density = 7.87)

https://svnweb.cern.ch/trac/atlasoff/browser/LArCalorimeter/LArGeoModel/LArGeoEndcap/trunk/src/EMECConstruction.cxx#L156

It depends on the geometry version - this is for Atlas geometry setup tagged ATLAS-GEO-18-01-XX.
Comment 3 Vladimir.Ivantchenko 2011-10-31 20:13:50 CET 
Hi John,

Thanks for pointing out to the problem. The problem is fixed both for 9.4 and 9.5 versions. 9.4 fix is waiting to be included in the patch.

VI