Problem 1363

Summary: Incorrect normal in G4Para - vector is not normalised
Product: Geant4 Reporter: John Apostolakis <John.Apostolakis>
Component: geometry/solidsAssignee: Gabriele Cosmo <Gabriele.Cosmo>
Status: RESOLVED FIXED    
Severity: normal    
Priority: P4    
Version: 9.5   
Hardware: All   
OS: All   

Description John Apostolakis 2012-10-03 18:16:44 CEST 
A G4Para with the following parameters
   fDx = 80
   fDy = 100
   fDz = 120
   fTalpha = 0.57735
   fTthetaCphi = 0.5
   fTthetaSphi = 0.866025
responds incorrectly to SurfaceNormal() method for the following point:
   dx=  72.45979294818625
   dy= -13.0600217133
   dz=  93.3122548334

The problem comes from the incorrect calculation of 
   salpha ( = -sin(alpha) ) 
which affects the normal nX value:

nX
  (double) dx = 0.654654
  (double) dy = -1.13389
  (double) dz = 0.654654

The value in this case is unphysical
salpha   = -1.5          <===  it should be 0.5
Comment 1 John Apostolakis 2012-10-03 18:31:24 CEST 
There is a simple correction.
 
Revise the following line:
  if (fTalpha) {salpha = -calpha/fTalpha;} // NOTE: using MINUS std::sin(alpha)
to this:
  if (fTalpha) {salpha = -calpha*fTalpha;} // NOTE: using MINUS std::sin(alpha)

Below is a patch, which can be applied to the source.

Index: src/G4Para.cc
===================================================================
--- src/G4Para.cc	(revision 62306)
+++ src/G4Para.cc	(revision 62307)
@@ -495,7 +495,7 @@
   newpy  = p.y()-fTthetaSphi*p.z();
 
   calpha = 1/std::sqrt(1+fTalpha*fTalpha);
-  if (fTalpha) {salpha = -calpha/fTalpha;} // NOTE: using MINUS std::sin(alpha)
+  if (fTalpha) {salpha = -calpha*fTalpha;} // NOTE: using MINUS std::sin(alpha)
   else         {salpha = 0.;}
   
   //  xshift = newpx*calpha+newpy*salpha;