PreStepPoint is the same as PostStepPoint in optical process

Hi all,

I am simulating photon optical process with Geant4. The simulation setup is following:

  • Geometry: world filled with water (refractive index 1.3) and a glass ball loacated at the world center (refractive index 1.5). The optical surface of glass ball is set as (dielectrice_dielectric, polished, glisur).
  • Sensitive Detector: The glass ball is set to be a sensitive detector. Track status is set as StopAndKilled to avoid multi-reflection.
  • Primary particles: photons loacated in water and toward to glass ball
  • Physics List: standard EM physics

When observing the photon steps in the sensitive detector, I found there are photon steps that have exactly the same PreStepPoint and PostStepPoint, i.e. same position, direction, polarization.

And all those photons have momentum direction pointing to the opposite direction to the ball center. This is abnormal since they should propagate toward the ball and thus point to the ball center.

If I set the refractive index of glass ball the same as the water, then this strange phenomenon will disappear.

Dose any one know how this can happen? I guess this might be related to the optical surface process but I can not find out the corresponding code. Thanks in advance!

If the momentum is outward, those are reflections, then. At the boundary, you will always have a step with the pre- and post-step points identical. If you look at the volumes, you’ll see that each point is assigned to a different volume, and if you look at the step status, you’ll see fGeomBoundary.

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Thanks for replay, I will check it tomorrow.

Merry Christmas!

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