In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Re: Thermal neutrons
Date: Mar 29, 10:55 am
Poster: Richard Hull
On Mar 29, 10:55 am, Richard Hull wrote:
Ely is correct in his response below. All of our Neutrons are fast neutrons. I think Scott just mispoke. He has a Polyethylene thermalizing shroud for his He3 tube. It is part of his instrument. In the first image of his system, he is not using it, but in the next major pass, he is using it. The thing is just not efficient enough at our low flux levels. The same goes for the usual BF3 tube type neutron detector.
Here is the deal.........the PE shroud has to first thermalize the fast 2.45mev neuts from the fusor. A large fraction of those entering the shroud will indeed be thermalized, but not all. secondly, the not too dense He3 detector volume must now detect the thermalized neuts. this is a real crap shoot and only a tiny fraction of the properly thermalized neuts entering the tube will be defected. The efficiency is pretty gross.
This is why Scott got the Bicron BC-720 which is a lot more efficient per unit volume of detector.
The BC-720 is a very dense block of plastic and fast neutrons penetrating it create recoil protons which are huge and heavy. When these crash into the ZnS (Ag) phosphor a brilliant flash occurs. Gamma rays entering the plastic will release electrons which will also make the phosphor flash, but the electron energies (kev) are so low for electron mass that the flash is very dim. The crudest form of pulse height discrimination will may an intense and dangerous gamma blast totally invisible!
The old He3 and BF3 tubes are fine for nuclear fission pile use where they have real flux levels, but is a poor choice for low level counting where thermal gradiants can even register as counts! The BC-720 is more sensitive to fast neutrons directly and is totally insensitive to thermal gradients in the normal laboratory range. It is even proof against rather intense 1 roentgen gamma and beta fields. This makes it the item of choice for fast neut detection.
>>Thus it does not look like my fusor is making any thermal neutrons. Either that or my 3He counter is bad (it is brand new).
>I don't think that the fusor should produce any thermal neutrons. They should all be fast with very few happening to fall into the slow category. You need to thermalize the neutrons, as I understand it. Try completely surrounding the He-3 tube with a thick, polyethylene cylinder (2-3 inch thick) or an equivalent in paraffin, water, etc. and see if the count improves dramatically.