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Having a look at https://www.nist.gov/pml/x-ray-mass-attenuation-coefficients, the introduction states:

  1. 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)

So how valid is this for typical x-rays in the medical range, i.e. 40-150 keV?

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Perfectly valid in that energy range and commonly used technique if you are looking for accuracy within a few percent. If you need better, then you'll need to take into account those more subtle effects

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  • $\begingroup$ Thanks, do you have any source or reasoning on this? $\endgroup$ – Tobias Kienzler Aug 8 '18 at 8:41
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    $\begingroup$ In ICRU 85, page 18, there's a short paragraph that talks about the mass attenuation coefficient for compound materials. " Relationship 4.2.3, which ignores the effects on the cross sections of the molecular, chemical, or crystalline environment of an atom, is justified in most cases, but can occasionally lead to errors, for example, in the interaction of low-energy photons with molecules (Hubbell, 1969) and in the interaction of slow neutrons with molecules, particularly those containing hydrogen (see, e.g., Caswell et al., 1982; Houk and Wilson, 1967; Rauch, 1994)." $\endgroup$ – imabug Aug 8 '18 at 12:03
  • $\begingroup$ Thank you for the reference, now I can accept the answer :) $\endgroup$ – Tobias Kienzler Aug 11 '18 at 18:10

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