Remember, Joe is counting every thing, and not just neutrons. I was there and he was getting many thousands of counts/minute!!! My neutron counter right next to his scintillator rarely read 200 counts in the same interval. recoiling protons in a bare scintillator create light at efficiencies for fusion neutrons on the order of .05%. In some cases multiple events can occur from a single recoil giving erroneous extra counts. Nonetheless, with a good calibration scource at hand, such a counter can be made to give statistically averaged results with decent resolution.
With the $350 BC-720 scintillator this is vastly improved by about an order of magnitude to .6%, plus, you gain gamma immunity while Joe's system is actually gamma sensitive! (explains the high counts.)
You must use a flat, end on windowed PMT with an S-11 response or something similar in the blue end for all scintillation counting, especially with the BC-720.
Richard Hull
Created on Wednesday, May 02, 2001 3:27 PM EDT by Richard Hull