|Message: Optically conected volumes||Not Logged In (login)|
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Hi, 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. Thanks!
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