This is interesting, for you are taking a gamma sensitive scintillator made secifically for gamma work and trying to make it a non-gamma detector by shielding, This will work only to a point and Joe had it down pretty good with the lead, which shielded most of the pesky X-rays in the 10kv to 50kv region (likely to be created near a working fusor). This quieted the device enough to mask most of the locally produced low energy gammas. It did nothing for 60kv or higher gamma (terrestial NORM radiation),etc., or cosmics. It also demanded little discrimination because the proton recoil quanta were as weak as most gamma scintillations (compton scattering,etc.)
Steel, if calculated to mask up to 40kev gammas will do just as good as lead, for the fast neutrons don't see either as real matter, ziping right through.
Joe did OK with this system, only because he used a local colleges fast neutron source to calibrate from and then it was just a statisitcally averaged tally. This did allow probable accuracies of 20% provided his count rate was an order of magnitude above the base line background. (good hot and working fusor - which he does have)
As a purely qualitative neutron detection device is is as good as any other professional neutron counter.
Again, as stated already, you will count more proton recoils in this system as light outputs than neutrons creating the parent particle. (making the efficiency seem good).
The last viable use of this system was in really old AEC fast neutron counters of the late forties and early fifties. (I have one as a curio in my wax museum of old nuclear instruments) These were used for crude, but effective health warning systems around those early budding piles so popular back then.
There were a lot of micropiles and other "dragon tail tickler" assemblies in huts scattered throughout the deserts of the southwest here, and they needed some simple smaller system to alert technicians and scientists of fast neutron flux dangers. These early simple plastic scintillator units even made it onto the early Nautilus sub until BF3 units got small enough in the late fifties and early sixties to allow a more scientifically accurate accounting of neutron flux.
Richard Hull
Created on Thursday, May 03, 2001 3:10 PM EDT by Richard Hull