The layout is here

Hi Leon,

It appears to be associated with the number of bounces rays make in the pipe. It’s always an integer number. In the attached file I used a Cylinder Pipe and a Source point. I show a detector with the rings you noticed. I also saved two ray files, one with rays which made two bounces, and one with rays which made 6 bounces. The ray files were saved using filter string X_HIT(2,n) where n in the number of bounces. I did not include the ray files in the ZAR because they are huge.

David: 

Thank you for the reply.