Elements photon attenuation coefficients

Photon energy (keV) Absorber element Mass attenuation coefficient (m kg " ) Density (kgm- ) Thickness for 1000-fold attenuation (mm)... 

Bulk chemical composition was determined by X-Ray Fluorescence (XRF) and carbon-sulphur analyses to (i) define the best sample preparation methodology for Po analysis by alpha spectrometry and (ii) estimate the matrix attenuation effect of 46.5 keV gamma photon emissions from Pb by gamma spectrometry using theoretical elemental mass attenuation coefficients from the literature [4],... 

Figure 4.23 shows the individual coefficients as well as the total mass attenuation coefficient for lead, as a function of photon energy. The total mass attenuation coefficient shows a minimum because as E increases, r decreases, k increases, and a does not change appreciably. However, the minimum of p, does not fall at the same energy for all elements. For lead, p, shows a minimum at .y 3.5 MeV for aluminum, the minimum is at 20 MeV and for Nal, the minimum is at 5 MeV. 

This table gives mass attenuation coefficients for photons for all elements at energies between 1 keV (soft x-rays) and 1 GeV (hard gamma rays). The mass attenuation coefficient i describes the attenuation of radiation as it passes through matter by the relation... 

The mass absorption coefBdent// plays a very important role in quantitative XRF analysis. Both the exciting primary radiation and the fluorescence radiation are attenuated in the sample. To relate the observed fluorescence intensity to the concentration, this attenuation must be taken into account As illustrated in Fig. 11.1, the absorption of radiation in matter is the cumulative effect of several types of photon—matter interaction processes that take place in parallel. Accordingly, in the X-ray range the mass attenuation coefficient of element i can be expressed as ... 

The relative values of absorption and transmission of adjacent elemental ray paths in an x-ray beam provide the contrast in a projected image. The associated interaction of the x-rays with the material body can be defined in terms of collision cross sections that describe the probability of particular events occurring along the ray path. Here, the principal effects, for x-ray energies below 1 MeV, are identified in Sec. 26.2 as the photoelectron event and the Compton scattering event. These effects are combined to define a linear attenuation coefficient pj(x) that describes the variation of photon intensity along the ray path x according to the exponential relation = o... 

The variation of these terms with x-ray energy is usefully demonstrated for water (Fig. 26.7). With reference to the overall attenuation coefficient [Eq. (26.19)], and to the associated cross section [Eq. (26.16)], for each element there is a photon energy for which iif = pf. Values of these energies can be used to indicate the region in which either process is dominant (Fig. 26.8). 

As already mentioned, the total attenuation coefficient is the sum of the attenuation coefficients for each interaction process (eqn [1]). The percentage contribution of each process to the total attenuation changes with photon energy and depends on the atomic number of a single element absorber or the so-called effective atomic number in case of complex materials. 

The m values for a tissue vary in the range 4-5 for the photoelectric effect 2-3 for the coherent (Rayleigh) scattering 1 for the incoherent (Compton) scattering. K E) is a function of photon energy for each partial process. For a specific interaction the mass attenuation coefficient for the individual element is given by... 

Mass attenuation coefficients for light elements from 1 keV to 10 MeV photon energy... 

As the mass attennation coefficient is an atomic constant only depending on the energy of the photons varions experiments have been performed in the past to determine oq. For most chemical elements the mass attennation coefficient can be found in literature (e.g. [15]). For mixtures and componnds the mass attenuation coefficient (xt can be obtained according to simple additivity ... 

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