This geometry is used as the “gamma catcher.” For the plots I've posted here, generally I will not be using this geometry (as neutron capture is difficult to detect experimentally using this setup; we have switched to the NaI geometry).

This is the PMT/plastic scintillator “optics” geometry I refer to in the pages here. The blue/red object is the 2“PMT (with the red face being the 2 mm thick photocathode). The gray object is the PTFE-wrapped scintillator, while the green fiber (which doesn't protrude in the actual experiment but does here for the sake of visualization) is the WLS fiber.