Problem 506

Summary:	optical photons propopgating at wrong velocity
Product:	Geant4	Reporter:	ps
Component:	processes/transportation	Assignee:	John Apostolakis <John.Apostolakis>
Status:	RESOLVED FIXED
Severity:	major
Priority:	P2
Version:	5.1
Hardware:	PC
OS:	Linux

Description ps 2003-07-09 06:09:03 CEST

I have a simulation of a water tank. The interior surface of the tank is a
diffuse reflector, and in the roof are three photomultiplier tubes. The tank is
part of an experiment detecting cosmic rays (primarily muons passing through the
tank producing Cerenkov photons). For test purposes I have a particle gun that
fires optical photons into the tank (with specified energy, position, direction
and polarisation). Such photons typically bounce around the tank many times
before being absorbed or detected by a PMT. I found that for the first step
(from the start point to the first wall), the speed of the photon is correct
(c/1.33 - using 1.33 as the refractive index of the water), but after that step
the speed becomes c/1.165, and likewise when the photon moves into the casing of
the PMT (refractive index 1.49) is moves with speed c/1.245 - the effective
refractive index appears to be 1+(x/2) instead of 1+x for all steps apart from
the first.

This was deduced by using the time for a step as
PostStepPoint->GetGlobalTime()-PreStepPoint->GetGlobalTime() and the distance
moved during the step as Step->GetStepLength(). Using verbose output from the
tracking information gives the same picture - the overall track length and the
track time don't match for the expected velocity of the photon. Source code
(about 2000 lines in total) is available if required.

Comment 1 ps 2003-07-16 08:17:59 CEST

Linux tar file has been added which contains a short version of the program
which also shows the bug. Just under 600 lines of code in total. Sample output
is also included, showing the problem.

Comment 2 gum 2003-07-16 21:41:59 CEST

I believe I have traced the problem to a calculation in:

 G4VParticleChange* G4Transportation::AlongStepDoIt

In it the time is updated with the following logic:

     G4double finalVelocity   = track.GetVelocity() ;
     G4double initialVelocity = stepData.GetPreStepPoint()->GetVelocity() ;
     G4double stepLength      = track.GetStepLength() ;

     if (finalVelocity > 0.0)
     {
        G4double meanInverseVelocity ;
        // deltaTime = stepLength/finalVelocity ;
        meanInverseVelocity = 0.5
                            * ( 1.0 / initialVelocity + 1.0 / finalVelocity ) ;
        deltaTime = stepLength * meanInverseVelocity ;
     }

The problem is that the velocity of the PreStepPoint at a boundary is the
velocity in optical medium #1 while the velocity of the PostStepPoint is that of
medium #2 upon refraction. Similarly, upon reflection, the PreStepPoint velocity
is that of medium #2 (remember there is a 'null' step - zero length step - upon
reflection and then the PreStepPoint of the next step points to medium #2),
while the PostStepPoint is back in medium #1.

The optical photon, of course, moves with the velocity of medium #2 upon
refraction and the velocity of medium #1 upon reflection and not their mean! The
fix requires special logic for the optical photon, just like in
G4Track::GetVelocity(), such that only the PostStepPoint velocity is used to
update the time-of-flight.

Thank you for being diligent.

Peter Gumplinger

Comment 3 John Apostolakis 2003-10-15 11:52:59 CEST

For optical photons we now use the velocity to use the one for the relevant
medium - not the average of initial and final values.

This change was included in Geant4 5.2 patch 1 (27 August 2003)

Thanks again for your report. Best regards, John Apostolakis