|MadSci Network: Physics|
James, Good followup. The answer is yes and no. Near ground-zero, there will be significant thermal stress and some fires are definitely expected. However the tactical significance of the bomb is seen at much greater range where the radiation passes though targets and "disrupts" the "biologicals" in the targets. This is jargon for kills people and leaves the building standing. So why is that, you ask? Here's a great example from one of my old text books, Basic Nuclear Engineering by Foster and Wright (p146). The old (1928/1937) unit of radiation exposure was the Roentgen (R) defined as the quantity of X- or gamma- radiation interacting such that 0.001293g of air (1cm^3) will contain 1 esu of electrical charge (either sign) or, if you prefer, 2.58x10^-4 coulombs/kg. Thus 1 R after the proper conversion factors equals 87.7 ergs/g of air. What does that mean in terms of something physical? It takes 85.5 ergs to move an wooden #2 pencil 1/16 of an inch. In terms of thermal energy, this is trivial. Now with people, what form the radiation is makes a significant difference in how it affects you. The unit for exposure was the rem (now old due to the use of the Seivert, but since I have a reference handy, you get rem) or the Roentgen equivalent man. Rem represent the biological effect on a person. Various types of radiation have different quality or Q factors. From Introduction to Nuclear Engineering by Lamarsh (p. 404) Q factors for X, gamma, and most beta particles is 1, alpha particles are 10, neutrons range from 2 to 11 (if the energy isn't specified for the neutrons, you assume a Q factor of 10). Ok, so what? Glad you asked. The acute dose LD50-30 for humans is about 450 rem. This means that 50% of the people exposed to that dose will die in 30 days (assuming no medical treatment). When you start talking about the ammount needed to kill people, with the Q factors thrown in, the total thermal energy needed is still trivial. If all the energy were from gammas, the energy equivalent of 450 rem is that of moving a pencil just over 28 inches. Lets assume it's half neutrons, then 450 rem is only moving the same pencil just over 15 inches. All neutrons? Not quite 3 inches. Yikes. Obviously, for some distance around ground zero, the dose will be amazingly high (LD100-0) and will taper off over distance and there will be shielding effects for things like big buildings, and hills. But there is a second little factor involved: secondary radiation. As the neutrons and gammas pass through matter (including air), they produce secondary radiation that adds to the sum of the radiaiton in the area. Gammas tend to make betas or more gammas; neutrons however, will produce just about everything (alpha, beta, gammas, protons, recoil nuclei, and combinations of the above). This ends in the target receiving several hits for the price of one produced by the bomb itself. That is the point of a neutron bomb unlike its cousin the fission/fusion bomb who's purpose is to get hotter than the sun even if only for a little while. Not the most happy thinking there is, but that's what it's all about. As for structural stress, yes there would be effects but in the time frame of a war, these would be trivial as well. You don't care if the building collapses next month, you need to get the data/secret stuff/whatever out of the building this week. I hope that adequately answers your question and thanks for asking! Scott Kniffin Sr. Engineer Radiation Effects and Analysis Group, Code 561.4 NASA GSFC
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