FAQ - Fast neutron detector -BC-720 scintillator

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Richard Hull
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FAQ - Fast neutron detector -BC-720 scintillator

Post by Richard Hull » Thu Mar 25, 2021 11:06 pm

I thought that I had made a recent FAQ on this. Anyway, here goes. Bicron sells a very special fast neutron Scintillator in the 2" and 5" sizes. While very expensive $325 and $525 respectively, they do have fabulous gamma immunity. You are required to couple these to a PMT and create all the associated circuitry. The draw back is their horrid and abysmal low neutron detection efficiency. (~0.1% - 0.3%) This is terrible! Plus you get to pay the big bucks and struggle to assemble this neutron detector. It is, nonetheless, a fast only, neutron detector with tremendous gamma noise rejection.

This scintillator relies on proton-recoil at or near the imbedded scintillation rings! Naturally, the scintillator is thin, (~0.5-inch), you do not want to slow the neutrons in the plastic. Also, between the rings, a neutron may scatter, (very unlikely), and hit the phosphor surface and proton recoil giving a flash, also unlikely. Only those fast neutrons reacting and producing a proton recoiling just at the phosphor surfaces will produce a flash, (highly unlikely). As recoiling protons have micron ranges in the plastic even a proton recoil .001" from the scintillator phosphor will not make it even if it is headed straight for the phosphor!! It is readily seen why the BC-720 has terrible efficiency and also why you will need a real blast of neutrons to get any statistically useful detection numbers.

Why even consider this detector?? Well, no cumbersome moderator needed. It is ideal for fusor folks regularly producing in excess of 1 million neutrons per second TIER where a bulky, usually fixed position moderated 3He or BF3 detector is roaring away at 96,000 CPM. The scintillator can be placed very close to the fusor and being quite small, as a finished unit, can be used to study anisotropic emissions due to receiving only directed and not moderated neutrons. In the end, this is a direction sensitive, rather portable neutron detector.

Depending on luck of the draw, a BC720 type scintillator might be cheaper than a 3He detector, provided you are producing a gang of neutrons with your fusor.

I attach a number of photos of these scintillators, PMT and a finished 2" system I used and assembled back in 1999....And yes.....I paid the full list price for my Bicron BC-720s back in 1999!

Richard Hull
BC-720 (6).JPG
The small 2-inch Fast neutron scintillator
BC-720 (5).JPG
The largest Fast neutron detector
BC-720 (16).JPG
A future large Fast neutron scintillation system?? Using the super expensive 5" scintillator by Bicron
BC-720 (14).JPG
My old and first neutron detector using a BC-720 and 2-inch PMT in its housing
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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