I just got off the phone with Bicron and they evidently just gave you the efficiency of neutron proton recoils in the plastic. ~38%. Approximately 22% of those are so weak that no PMT would ever see them (few EV). There is also only about 20% of the recoil strong enough to squeak through detection at the 500kev level. (which is where the background is reduced.)
As you discriminate out the garbage, the efficiency for actual neutron induced recoils plunges to below 3% This is where most earth base gamma and lower end cosmics start to disappear. from the BC-400.
I have stated before that the BC-720 is BC 400!!! There is no difference in the plastic at all!!! The BC720 just pumps the proton recoils detected to huge levels over any possible gamma signature.
The engineer agreed that with good discrimination which would effectively elimenate most normal gamma energies in a standard environment and with no device made gammas present in the neutron field, that the BC-400 could be nearly 1% efficient in a 2X2 crystal and beat out the BC-720 by a factor of 2, but the background count in the BC-400 would still exceed that of the BC-720 by a factor of 10 to 50! (normal un-discriminated BC-400 has a background to neutron detection ratio of about 1000 times that of the BC-720. He noted that if the BC-720 activates a PMT you can rest assured that it was a neutron event. With the BC-400 and good discrimination you might still be in doubt.
We both agreed that with a good calibrated neutron source of the energy you are looking at, half way decent calibration to within about 15-20% for real neutrons could be relied upon.
The real test might be to use a totally undiscriminated BC-400 near a fusor in the following manner.
1. Take a 100 second background count 2. With air in the fusor at full operating potential take another 100 second count. If it is up you are throwing stuff into the crystal due to some process which ain't fusion. 3. Do a full fledged fusor run with D2 at the same voltage and curent as in test #2. If the count is way up then you are counting a gang of proton recoils due to fusion.
A good discrimination level would be to find the point in discrimination where there is zero differential between the full power air run and the no operation, background count. Then, boost this level by another 25% as a safety zone. Finally send the thing off for calibration against a known source. This might allow up to a 5% net efficiency.
I have a lot of BC 400 here and have several detectors made up and the stuff is noisy as hell! Joe was getting thousands of counts per minute - background!! Compare this to a BF3 tube in moderator's 1 count eevry 1.8 minutes - background
Richad Hull.
Created on Tuesday, May 08, 2001 2:49 PM EDT by Richard Hull