I’m trying to setup a simple geometry where a photon beam, initially collimated gets diffracted after passind an aperture. For doing so I’m changing the direction cosine when passing between two regions using the subroutine USRMED. I generate a randomly an angle between 0 and PI every time a photon crosses a boundary and I assign the new direction cosines to the photon (top simulate the diffraction).
I see the photon changing direction but not as expected: the distribution is not homogenous. I am most probably making a very silly mistake or I misunderstood how to use the direction cosines, but it would be great to have some suggestion.
I attach the input file and the usrmed subroutine I am using.
thanks in advance for your help,