Gamma-Ray Attenuation in the Vicinity of the K Edge in Molybdenum, Tin, Lanthanum, Gadolinium, Tungsten, Lead, and Uranium

The values of gamma-ray buildup factors and attenuation coefficients rise steeply as the source energy decreases near the K edge in heavy materials and discontinuously fall at the K edge. However, the exposure rate attenuation factor, A(E, r) = D(E)B(E, μr) exp(-μr), given as a function of the penetration depth in centimetres, is relatively constant in the vicinity of the K edge. The development of a model that employs 4 K-shell X rays for the source shows that such behavior results from the large contribution of fluorescence radiation to the buildup factor for source photons of energies just above the K edge. In addition, an uncertainty in the extrapolation formula of the K parameter of the geometric progression buildup factor fitting function was removed for the energy range near the K edge.