NukeWorker Forum
Reference, Questions and Help => Nuke Q&A => Topic started by: mostlyharmless on Aug 17, 2010, 08:07
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How do you predict neutron rate from a neutron point source? Since neutrons behave differently than gamma, the inverse square law should not apply in the same way. Given a rate at a distance,how do you predict a rate at another distance?
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Hopefully, this will explain it: http://www.hps.org/publicinformation/ate/q6847.html (http://www.hps.org/publicinformation/ate/q6847.html). It only actually works for the Ra-Be source, but explains enough about where the numbers come from and how they work that you should be able to figure out your dose rate.
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The point source equations are based on geometry and not interactions. IN vacuum both gamma and neutrons dose rates would fall off at same rate. Air, of course, does act as a shield for both. Over relatively short distances it will be not be an issue. Use the normal point source equation
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Sorry about the delay. This came up for me because we had a good sized Cf source to deal with recently. When given the numbers the point source equation was referenced. My first reaction was that the point source would underestimate the rate for neutron, my logic that neutron emissions fall off at a slower rate than do gammas. At very close distances, less that a foot or two, it was irrelevant,the rate was very high. But I began to think about distances in the range of fifeteen or twenty feet. The controls for the source should never allow it to be unshielded for enough length of time as to be a hazard so this is really just a what if practice. But you never know. Anyway I got to thinking about how the point source equation applies to similar curie and Mev gamma vs neutron sources, how the equation applies to gammas of different Mevs and how it applies to neutrons of different Mevs. Mixed sources get complicated as the above post indicates. I will do a bit more research. Thank you for your replies. I know that almost anything I ask will receive a reasonable answer from someone on this site. Almost anything. Well sometimes its reasonable.
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I was under the impression that dose rates changed over distance because of the spherical geometry. The "surface" of the sphere was larger as you moved away and smaller as you got closer resulting in a dispersal or concentration of dose over that surface.