NIST: X-Ray Mass Attenuation Coefficients

X-Ray Mass Attenuation Coefficients

1. Introduction

The mass attenuation coefficient, μ/ρ, and the mass energy-absorption coefficient, μ_en/ρ, are basic quantities used in calculations of the penetration and the energy deposition by photons (x-ray, γ-ray, bremsstrahlung) in biological, shielding and other materials. These coefficients are defined in ICRU Report 33 (1980) and are discussed in Sections 2 and 3 of this work. They have been treated in some detail elsewhere, for example by Berger (1961), Allison (1961), Evans (1968), Hubbell and Berger (1968), Hubbell (1969, 1977, 1982), Storm and Israel (1970), Carlsson (1971), Ribberfors and Carlsson (1985), Cunningham and Johns (1980), Johns and Cunningham (1983), Higgins et al. (1992) and by Seltzer (1993). Compilations of both μ_en/ρ and μ/ρ include those by Plechaty et al. (1978) and by Cullen et al. (1989).

The present compilation is an extension of the recent calculations of Seltzer (1993), and is intended to replace the values of μ/ρ and μ_en/ρ given in Hubbell (1982) which have been widely used as reference data in radiation shielding and dosimetry computations. The present tables differ from those in Hubbell (1982) in the following respects:

Instead of providing results for only 40 selected elements with atomic numbers spanning Z = 1 to 92, now all 92 elements are included.

Instead of using a common energy grid from 1 keV to 20 MeV, without photoelectric absorption edges (K, L₁, etc.), now all edge energies are included and identified, and values of μ/ρ and μ_en/ρ are given just above and below each edge to facilitate accurate interpolation.

Somewhat different values for the atomic photoeffect cross section have been used for Z = 2 to 54. The 1982 compilation was based on the application of renormalization factors given by Scofield (1973) to his calculated values of the cross section. Although Scofield (1973) calculated the cross sections for all Z ≥ 2, he gave renormalization factors only for 2 ≤ Z ≤ 54. An evaluation by Saloman et al. (1988) compared the Scofield theoretical values with the measured-value database, and concluded that overall agreement was better without this renormalization. The present work therefore uses the "un-renormalized" Scofield (1973) theoretical values of the photoeffect cross section for all elements Z ≥ 2. The analytical results used for Z = 1 are the same as those used in the 1982 compilation.

For compounds and mixtures, values for μ/ρ can be obtained by simple additivity, i.e., combining values for the elements according to their proportions by weight. To the extent that values for μ_en/ρ are affected by the radiative losses (bremsstrahlung production, annihilation in flight, etc.) suffered during the course of slowing down in the medium by the electrons and positrons that have been set in motion, simple additivity is no longer adequate. The 1982 compilation ignored such matrix effects (they tend to be small at photon energies below 20 MeV); in the present tables they have been taken into account.

These and other refinements, such as a more complete treatment of the atomic shell vacancy and fluorescence cascades, are discussed briefly in the sections that follow. More details can be found in Seltzer (1993). A similar version of these tables, with text in Japanese, is given in Seltzer and Hubbell (1995).