I have successfully detected something, this might sound a little uncertain. But there definitely is detected some kind of radiation or particle energy that causes various levels of light generated in the BC408 scintillation plastic. It is being detected by being attached to the front of the photomultiplier tube.
I have updated the article about the photomultiplier tube with all the details and measurements from this short proof of concept test.
Just the fact that a PMT assembly straight out of a industrial application can be reused for physic experiments after it has been fitted with a scintillation crystal or plastic is a huge leap towards cheaper amateur science in the fields of neutron detection, muon detection and cosmic ray detection.