In reply to Jones Beene's message of Sun, 9 Oct 2005 07:16:24 -0700: Hi, [snip] >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.
Even at 3837 barns in pure Boron, the mean free path for the neutron is 19.9 microns / 19.7% = 101 microns. This is considerably larger than the stopping distance of alpha particles in a solid (usually on the order of 10-20 microns, and these are comparatively low energy alphas - so the stopping distance will be on the short side). In a Boron compound, the distance between Boron nuclei will be even larger, hence the mean free path greater. This implies as far as I can tell, that even an optimally configured strip may not detect up to 90% of the available neutrons. If any of the neutrons have a higher energy, they will probably not be detected at all. Neutrons with much less than thermal energy which might have an improved chance of being captured, will unfortunately be thermalized after very few collisions ( 1 on average), hence have essentially no range, and consequently don't "exist". This picture however changes appreciably if pure B10 is substituted for natural Boron. (MFP determined be dividing the atomic volume by the cross section). [snip] Regards, Robin van Spaandonk In a town full of candlestick makers, everyone lives in the light, In a town full of thieves, there is only one candle, and everyone lives in the night.
