Re: More PolySci less SciPolitics

[Jones Beene]

Robin > How does the neutron get converted into an alpha particle so that > the film will detect it? Thanks for mentioning this - I meant to add it to the original post. There are any number of ways, but the most reliable is to buy specialized CR-39 film for this, with the boron impregnated - as there is a thin line between "blocking" and "enhancing," and "thin" is the operative word here - so it is a bit of an art. There are companies making specialized CR-39 products for detecting neutrons, and It would be good to do a thorough web search, but here is one such company: http://www.dosimetry.com/prod/cr39.html This company has thermoluminescent dosimeters (TLDs) which utilizes lithium borate and calcium sulfate. This combination responds to neutrons, as well as, beta, gamma and X-ray radiation. A cheaper home-made solution might involve Borax or boric acid, which are inexpensive sources of boron, available in any drug store. Other of the so-called neutron "poisons" such as cadmium compounds also produce alphas but they can be delayed whereas the boron alphas are prompt.   For those who do not know the mechanism involved - this is based on the nuclear reaction that occurs when the stable isotope10B, which is 20% of natural boron, absorbs a low energy 'thermal' neutron (cross-section nearly 4000 barns) to yield a highly energetic helium-4 (i.e.,alpha particle) and 180 degree recoiling Lithium-7 (7Li) ion. The alpha caries away ~1.5 MeV of energy and the lithium ~840 KeV so either of these two will also emit secondary gammas, which can be picked up with a simple GM detector (and that is the poor man's method) but as the alphas themselves are easily stopped by a thick layer, ergo use with normal film is an art. Because of the high cross section, neutrons can be stopped by a thin enough layer but the alpha is not. Speicalized film is probably the best if you want to publish your results, but the borax+GM is simpler, and can be accurate for you own uses if you have first calibrated it using the film.