In x‐ray diffraction spectrometry as well as in the many other scientific, engineering and medical disciplines involving photon radiation, x‐ray attenuation coefficients are required as input data. Within a very few years after the discovery of x‐rays by Röntgen in 1895, the transmission of a narrow (parallel) beam of x‐rays through layers of different materials was measured and quantified with respect to photon (x‐ray) energy and atomic number of the material by Barkla and Sadler in 1907. This quantification is in terms of the mass attenuation (or absorption) coefficient µ/ρ (cm2 g−1) which for monoenergetic photons can be defined as $$ \mu/\rho = x^{-1}\ln(I_{0}/I), $$ or $$ I/I_{0} = \exp[-(\mu/\rho)x] $$ in which x is the mass thickness of the layer in units of g cm−2, I0 is the intensity (e.g. photons cm−2 s−1) of the incident beam, I is the intensity of the transmitted beam (measured with the layer interposed), ρ is the density of the layer in g cm−3 and µ is the linear attenuation (or absorption, µ*) coefficient in cm−1. Current compilations of µ/ρ are derived from theoretical or semi‐empirical values of the most‐probable individual processes according to $$ \mu/\rho = (\sigma_{\bf pe} + \sigma_{\bf incoh} + \sigma_{\bf coh} + \sigma_{\bf pair} + \sigma_{\bf trip})/uA $$ in which σpe is the atomic photoeffect cross‐section, σincoh> and σcoh are the incoherent (Compton) and coherent (Rayleigh) scattering cross‐sections, respectively, σpair and σtrip are the cross‐sections for electron–positron pair production (creation) in the field of the nucleus and in the field of the atomic electrons (‘triplet’ production), respectively, and the factor 1/uA, where u is the atomic mass unit and A is the relative atomic mass of the target element, converts the σi units from cm2 per atom to cm2 g−1. Since σpair and σtrip have thresholds above 1 MeV, these do not affect the XRS photon energy region, nor does the photonuclear cross‐section σph.n. which has threshold of 5 MeV or higher and is not included in current µ/ρ compilations. In this brief and arbitrarily selective review, a historical account is given of the evolving compilations of µ/ρ from Barkla and Sadler’s work at the beginning of this century, up to the NIST and Livermore compilations as this century comes to its close. For the latter compilations of µ/ρ, some rough estimates of the ‘envelope of uncertainty’ in different ranges of photon energy are given. Copyright © 1999 John Wiley & Sons, Ltd.

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   |texte=   Compilation of photon cross‐sections: some historical remarks and current status
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