At 4:28 AM 10/2/4, Nick Reiter wrote: >2. Any last thoughts on a better homebrew neutron >detection scheme? Someone hinted to me recently that >old style D cell batteries with manganese sulphate in >them made a good neutron detector for use with a GM >counter.
As you probably know, tubes having boron compounds on their inner walls and/or BF3 gas are typically used for neutron detection. They work due to the reaction: 5B10 + 0n1 --> 3Li7 + 2He4 Thus there is one alpha generated per reaction. The cross section of Boron is large, but unfortunately is inversely proportional to neutron velocity. BF3 tubes are thus only good for low energy neutrons. They can be used for high energy neutron counters by placing high density hydrogen materials (e.g. plastic) in front of the tube to slow down the neutrons (moderate them) via collision with the protons, which are effective at moderating because they have roughly the same mass. If you are looking for high energy neutrons then you might get some results by placing a plastic moderator, say about 1/4" thick, in front of the tube and counting recoil protons, but this would not be any good for low flux counting. If you have a thin window on your geiger tube you could try a thin boron coating on the outside of the window with the hope that a useful number of alphas can penetrate. I've heard but can't confirm that placing a silver dollar between the neutron source and the geiger counter window works for some neutron fluxes and energies. Best thing to do is probably to try to locate a BF3 counter or just buy a tube (readily available as parts) and build your own. Calibration is another problem though, I would think. An impromptu idea is to try placing a thin boron compound coating up close to some film. Possibly the boron compound could be sandwiched between two thin layers of plastic. The whole assembly can then be wrapped in foil to prevent exposure to light. If the film exposes during an experiment, then voila! Idividual events might be counted in the film by microscope, though this sounds like a difficult and amateurish procedure. You could use the film transport mechanism of a cheap or disposable camera to expose small sections of a roll at a time. This would require using a lead shield with a hole in it located at the camera's focal plane, i.e. exposure area. The nice thing about this film set-up is most places don't charge for developing "unexposed" film. It is also nice that film gives an integration over time, vs just an immediate counting rate most cheap geiger counters provide. Again, the problem is calibration, or just basic testing, unless you happen to have a Farnsworth fusor handy, or some other neutron source. Regards, Horace Heffner
