Gamma-Ray Attenuation at Energies of Approximately 6 and 8 MeV

Well-collimated beams of thermal-neutron-capture gamma rays from titanium and nickel, having average energies of about 6 and 8 MeV, respectively, were used to measure narrow-beam and total gamma-ray dose attenuation. Slab shields of lead, iron, and concrete were investigated for normal and oblique beam incidence. Total dose measurements were made by traversing an exposure-responsive detector through a plane behind and parallel to the shield. Monte Carlo and moments-method calculations were used to compare analytical and experimental total dose results. Good agreement was found for iron and concrete shields, but experimental results for the lead shield were higher than those predicted by the moments method by a factor of ∼1.2 for 6 MeV and ∼1.5 for 8 MeV. The reason for this disagreement is believed to be primarily bremsstrah-lung produced by energetic secondary electrons slowing down in lead.