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None Optically conected volumes 

Forum: Processes Involving Optical Photons
Date: 24 Mar, 2010
From: Robert Penny <Robert Penny>


This is a pretty basic question about what counts as connected volumes
for the purpose of tracking optical photons.

I'm trying to model a cylindrical scintillator, with a flat cut on one
side, and a silicon detector mated to this flat.  I've started with
examples/novice/N06, then:

1. changed the water tank from a box to a cylinder

2. subtracted a box from the cylinder (using a G4SubtractionSolid) to
create a cylinder with a flat (this is my scintillator)

3. created a box-shaped physical volume (this is my silicon detector)

4. placed the silicon detector so that one of its faces is flush against
the flat in the scintillator

5. defined an optical surface for the interface between the two

6. set the material properties to be the same (for the moment) in both
the scintillator and the silicon detector (both with an index of 2.1)

7. changed the particle gun to fire optical photons, and fire single
optical photons from the scintillator to the silicon detector

I'm getting inconsistent results.  

If I fire photons at a 45 degrees from the scintillator to the silicon I
was seeing total internal reflection, and no transmission, on the
initial bounce.

If I fire at a steeper angle, I get transmission into the silicon at an
angle characteristic of changing from an interface of 2.1 to 1.  I have
tried to set the 

HOWEVER... when I fire at a shallow angle and get total internal
reflection on the first bounce, I sometimes get multiple bounces around
the scintillator that return and hit the silicon a second time.  Some of
these photons travel from the scintillator to the silicon showing no
refraction, which is what I'd expect because I have tried to set the
indexes to be matched between the two materials.

That's the problem I'm trying to debug.  Here's the question.

In N06 there's a clear parent/daughter relationship between all of the
optical surfaces.  The "water tank" is a daughter volume contained
within the parent "world", and the "bubble" is a daughter volume
contained within the parent "water tank".  The logical hierarchy makes
it clear that leaving a daughter volume, you unambiguously move into the
parent volume.

In my case, I have placed two daughter volumes adjacent to each other
within a parent volume, and I'm trying to make the coincident planar
surface region common to the two daughter volumes be an optical
connection.  I don't want an optical photon leaving one volume to
believe it has transitioned out to the world volume, then after
traveling an infinitesimal distance, transition back into the second
daughter volume.  As far as the final trajectory of the photon is
concerned, there will be no significant difference.  There will be a
difference though in reflection coefficient.

a. Does Geant decide on boundaries based not just on parent/daughter
relationships but also on the geometric position of aligned surfaces of
daughter volumes within the same parent?  

b. If (a) is true, how close must the two surfaces be to each other
before Geant merges them into one surface without an intermediate
boundary going out to the parent volume?  

c. If (b) is true, is the threshold for a common surface determined by a
tolerance setting in the geometry manager?  (How well does this play
with the concept of avoiding overlapping volumes.)

d. Is the definition of a G4LogicalBorderSurface a hint of some sort
that the two surface mentioned are adjacent and a single common boundary
in the overlap region?

I know that this probably won't solve all of my problems with the
geometry... but at least it'll help me go in the right direction in
debugging the problem.


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1 None: RE: Optically conected volumes   (Penny, Robert D. - 24 Mar, 2010)
(_ Feedback: Re: RE: Optically conected volumes   (Gumplinger Peter - 25 Mar, 2010)
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