Photons are absorbed in a solid through a process known as the photoelectric effect whereby a photon interacts with an electron by giving all its energy to the electron. In effect, the photon disappears and energy is transferred to an electron. The photoelectric process is responsible for the absorption of visible-light with energies of 2 to 3 eV as well as for x-rays with energies a thousand times greater than that of visible light.
The absorption of photons in solids depends on the number of electrons that can accept a transfer of energy from the photon. Not all electrons meet this criterion. Some electrons are so tightly bound to their orbital around the atomic nucleus that the photon energy is unable to break the bonds. Other electrons are involved in the bonding between atoms and cannot be freed by the energy of the incident photons. For example, ordinary window glass is transparent to visible photons, but is strongly absorbent to ultraviolet radiation where the photons have energies only a few times greater than that of visible light.
Photon absorption in the visible region of the spectrum depends on the atomic arrangement of the atoms and their bonding. Pure silicon (Si) is strongly absorbing but silicon combined with oxygen is transparent. For the energetic photons in the x-ray regime, photon absorption is much easier to predict and is independent of the details of atomic arrangement. It depends primarily on the electron concentration per unit volume. Since the concentration of atoms per unit volume only differ by factors of 2 or 3 from each other, the electron concentration in two materials can be estimated from the atomic number, Z. Lead (Z=82) absorbs x-rays much more efficiently than aluminum (Z=13) and consequently is used in shielding around x-ray apparatus. X-ray absorption does depend on the energy of the x-rays and decreases with increasing x-ray energy, E. Absorption decreases nearly proportional to the cube of the energy (i.e. absorption proportional to (1/E3).
In spite of the different material dependencies in the description of visible light and x-ray absorption, the mathematical description is the same for the two: the exponential decay law. This decay law is due to the fact that each successive interval of equal thickness of a sample absorbs an equal fraction of the photons passing through it. Each successive interval, however, receives few photons than the preceding interval because the number of photons decreases with depth. For example, if the first layer thickness absorbs one-half of the photons, then only half of the original number of photons are incident on the second layer thickness which absorbs one-half of these, i.e. on quarter of the original number of photons incident on the sample surface.
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