Problem 1114

Summary:	reflectivity is calculated twice when using groundbackpainted or polishedbackpainted
Product:	Geant4	Reporter:	Akira Okumura <oxon>
Component:	processes/optical	Assignee:	gum
Status:	RESOLVED FIXED
Severity:	normal
Priority:	P5
Version:	9.3
Hardware:	All
OS:	All
Attachments:	test code for groundbackpainted and reflectivity

Description Akira Okumura 2010-04-17 14:30:29 CEST

When I set the finish of a surface to groundbackpainted or polishedbackpainted, its reflectivity is calculated twice, in other words, effective reflectivity is square of the given parameter.

In the attached source code which is modified from ExN06, I set the reflectivity to 20%. But 9613 of 10000 photons are absorbed at a boundary (effective reflectivity is 20% x 20% = 4%!). Please try

$ tar zxvf N06.tgz
$ cd N06
$ make
$ exampleN06
Idle> /control/execute optPhoton.mac
Idle> /run/beamOn 10000

Comment 1 Akira Okumura 2010-04-17 14:31:43 CEST

Created attachment 72 [details]
test code for groundbackpainted and reflectivity

Comment 2 gum 2010-04-20 02:10:33 CEST

Thanks Akira,

for pointing this out. You are absolutely right. The bug was introduced when I allowed:

5th  Nov 2009 Peter Gumplinger (op-V09-02-03)
              G4OpBoundary::PostStepDoIt - all dielectric_dielectric
              surfaces may now have a reflectivity <1; not only
              'frontpainted' as was the case in the past.

most certainly upon a user's request to have a simple dielecric interface which was nevertheless not 'perfect'.

As you point out, not only 'frontpainted' surfaces have had historically a reflectivity attached to them but also 'backpainted' and the latter decision was/is implemented in the code of DielectricDielectric, while the calling code in G4OpBoundaryProcess.cc was changed to read:

        else if (type == dielectric_dielectric) {

.....

          else {
                  if( !G4BooleanRand(theReflectivity) ) {
                    DoAbsorption();
                  }
                  else {
                    DielectricDielectric();
                  }

when it simply read in the past:

          else {
               DielectricDielectric();
          }

I don't want to change the code in DielectricDielectric(), instead (more spaghetti) I'll change to:

          else if ( theFinish == polishedbackpainted ||
                    theFinish == groundbackpainted ) {
                    DielectricDielectric();
          }
          else {
                  if( !G4BooleanRand(theReflectivity) ) {
                    DoAbsorption();
                  }
                  else {
                    DielectricDielectric();
                  }
          }

or a permutation which requires fewer else ifs.

Please, try it out.

Peter

Comment 3 gum 2010-04-20 02:58:33 CEST

There is a new tag op-V09-03-01 which will be included in an upcoming patch to 9.3. In the meantime, the new code for (type == dielectric_dielectric) reads:

        else if (type == dielectric_dielectric) {

          if ( theFinish == polishedbackpainted ||
               theFinish == groundbackpainted ) {
             DielectricDielectric();
          }
          else {
             if ( !G4BooleanRand(theReflectivity) ) {
                DoAbsorption();
             }
             else {
                if ( theFinish == polishedfrontpainted ) {
                   DoReflection();
                }
                else if ( theFinish == groundfrontpainted ) {
                   theStatus = LambertianReflection;
                   DoReflection();
                }
                else {
                   DielectricDielectric();
                }
             }
        }