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17 March 2010

Answer to Question #283 Submitted to "Ask the Experts"

Category: Radiation Basics — Neutrons

The following question was answered by an expert in the appropriate field:

Where can I find attenuation coefficients for fast neutrons (2.5 MeV) going through steel?

We think you mean cross sections, not attenuation coefficients, since a 2.5 MeV neutron interacts primarily through scattering (elastic, primarily, and inelastic). Scattering removes the neutron from the "fast" category/group but does not eliminate it. As a first approximation for fast neutrons we use the so-called "removal cross section" in an exponential attenuation expression to calculate the attenuation of fast neutrons. Again, keep in mind that "attenuation" or "removal" means removal from the fast group. Anyway, the value used for the removal cross section in steel (Fe) is 1.98 b. The macroscopic cross section is S=0.168/cm. The mean free path l=5.94 cm (the inverse of S). I hope this helps. Nicholas Tsoulfanidis, Ph.D. University of Missouri - Rolla

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