Here is an update on this project:
We did some more tests on this neutron detector. It turns out that the efficiency was not that great and most of the background counts were not from neutrons. Burying the detector in Borax didn't remove the background. That means that most of the background was from something other than neutrons (either trace contamination or muons?).
Since then, we solved this and now burying the detector in Borax almost eliminates the background counts. That means we should be able to calibrate the detector on background neutrons. The thermal neutron flux at the reference location is supposed to be ~ 4 n/cm2/hr ( https://ieeexplore.ieee.org/document/1263842 ). At my latitude and altitude, the n flux is 1.6 times reference ( http://www.seutest.com/cgi-bin/FluxCalculator.cgi ). So, all I have to do is measure the difference in background count rate of the bare detector and the borax covered detector and compare it to 6 n/cm2/hr and that's the efficiency.
I'll make a video on that and start a new thread.
This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.