Photoelectric absorption coefficient

Wavelength of Primary Beam, A Exciting Potential, V Scattering Coefficient, a Photoelectric Absorption Coefficient, T cr/r... 

Fig. 1.6. Schematic diagram showing the variation of the photoelectric absorption coefficient as a function of energy for a t5rpical target element...

ABSORPTION EDGE The sharp changes in photoelectric absorption coefficient as a function of energy as the energy of the gamma-rays decreases, related to the energy of electrons within their shells. 

The values of r and a in Table 4-2 show that photoelectric absorption generally makes the greater contribution to the mass absorption coefficient. Absorption leading to scattering, which will be discussed in the next section, gains in relative importance as atomic number Z and wavelength X decrease. 

Between absorption edges, the photoelectric (true) mass absorption coefficient r can be expressed as the following approximate empirical function of Z and X ... 

The alternating component of the applied voltage causes a continuous variation in the efficiency of the x-ray excitation (Step II) during each cycle of operation. A polychromatic x-ray beam has a different total absorption coefficient and a different ratio of photoelectric to scattering absorption coefficient for each wavelength (Step V). It is very difficult to take account of these factors exactly. Fortunately, reasonable estimates will suffice in the making of the calculations for Table 4-4 and Figure 4-15. The efficiency has been taken as that for the root-mean-... 

Absorption Coefficient—Fractional absorption of the energy of an unscattered beam of x- or gamma-radiation per unit thickness (linear absorption coefficient), per unit mass (mass absorption coefficient), or per atom (atomic absorption coefficient) of absorber, due to transfer of energy to the absorber. The total absorption coefficient is the sum of individual energy absorption processes (see Compton Effect, Photoelectric Effect, and Pair Production). 

The photoelectric mass absorption coefficients of these polymegs and of poly(methyl methacrylate) PMMA at 8.3 X and 13.3 + A are shown in Table II. These coefficients were calculated using relation (8) and the data listed in Table I. 

Table I. Photoelectric Mass Absorption Coefficients of Halogen Atoms and of Carbon, Oxygen, Hydrogen at 8.34 A (A1 Kali2 emission line) and 13.34 A (Cu Lai. o emission line) (6)...

Table II. Photoelectric Mass Absorption Coefficients of the Halogenated Polymethacrylates and of PMMA...

Far from the Laue condition the absorption shows the normal photoelectric absorption, as would be measured (with allowance for density) in a liquid or gas of the same atomic species. Close to the Laue condition, the absorption is quantified by the imaginary parts of the susceptibilities, leading to imaginary components of the wavevectors. These imaginary components are always normal to the crystal surface and hence the planes of constant attenuation are parallel to the surface. The attenuation coefficient (n) normal to the surface is given by (n)=-4 lm(K o) (4.28)... 

Figure 5 X-ray mass attenuation coefficients for aluminum as a function of photon energy. At low energies, photoelectric absorption predominates. At higher energy, incoherent (Compton) scatter becomes almost the exclusive contributing mode. Eventually, pair production dominates at very high energies (above 10 MeV).

Fig. 2-2, Room-temperature absorption coefficients and cross sections of NO by photoelectric detection A = 590-960 A absorption coefficients to base e for 1 atm of gas at 273°K (from Metzger and Cook309 with permission).

The requirements of high-energy resolution are well met by Si and Ge. The photoelectric effect increases with Z to Z, where Z is the atomic number of the substance, and the linear absorption coefficient for lOOkeV y rays in Ge is higher than in Si by a factor of about 40. Therefore, Ge crystals are better suited for measuring y radiation. Semiconductors with still higher atomic numbers, such as CdTe and Hgl2, have been investigated with respect to their suitability as detector materials, but they are not commonly used. 

Fig. 5.11. Linear absorption coefficients vs. gamma-ray energy for Si and Ge (PE = photoelectric, C =...

FIGURE 4b. Absorption coefficients of CO by photoelectric detection with an instrumental band width of 0.05 nm. Absorption coefficient is to the base e for 1 atm of gas at 2730k (from Huffman et al. with permission). 

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