TAILIEUCHUNG - Ebook An introduction to x- ray physics, optics, and applications: Part 2
Part 2 book “An introduction to x- ray physics, optics, and applications” has contents: Compton scattering, photoelectric absorption, absorption spectroscopy, imaging, and detection, refractive and reflective optics, single-crystal and three- dimensional diffraction, diffraction optics, and other contents. | PART III X-RAY INTERACTIONS WITH MATTER 9 PHOTOELECTRIC ABSORPTION, ABSORPTION SPECTROSCOPY, IMAGING, AND DETECTION Absorption coefficients When a beam of light or x rays is incident on a material, photons can be absorbed, causing electrons to be emitted. From the point of view of the electron, this process is called photoelectric emission. From the point of view of the photon, it is termed photoelectric absorption. This knock-out of an electron is the same process discussed in section on x-ray fluorescence, although in that case the emphasis was on what FIGURE 9-1. Absorption of happened next if the emitted electron came from a core level. a photon can be regarded Absorption can be regarded as causing a fluctuation of the as the fluctuation of the electron energy up to a virtual level, as shown in Figure 9-1. electron energy up to a For an electron to be emitted, the incoming photon must “virtual” electron level. have an energy greater than the binding energy of the electron, but it is not necessary that the incoming photon energy match a difference in electron energy levels, as is the case for the emitted photon in x-ray fluorescence, illustrated in Figure 4-3. The rate at which photons will be absorbed by a single atom is given in terms of a cross section σab for absorption, Γab, 1 = σabΨ , (9-1) where Γab,1 is the absorption rate for a single atom, and Ψ is the photon intensity, the number of photons per area per second in the beam. For comparison, the rate Γ at which photons would hit a target of area A placed in the beam is Γ = AΨ , (9-2) so that the cross section can be seen to have units of area. Cross sections are described in units of barns, where 1 barn = 10−24 cm2. (The unit is a reference to an old saying referring to someone with poor aim as not being able to hit “the broad side of a barn.” A barn is a large cross section in nuclear physics.) 140 ■ Chapter 9 A sense of the dependence of the cross section on photon energy and .
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