X-Rays

X-rays, discovered by Roentgen in 1895, can be produced in two ways. The first process uses a high-energy electron capable of knocking an electron from an inner energy level of an atom completely out of the atom. This removal of an electron, say, from the K level, immediately creates a hole into which an L-shell electron can fall. As an L-shell electron falls back to the lower-energy K level, the sudden decrease in potential energy is emitted as a photon of electromagnetic energy characteristic of x-rays (atomic radiation). A hole is produced in the L shell, which calls for the transition of another electron from an outer level to the L shell, which produces an additional x-ray. Several x-rays are emitted as electrons cascade down to fill the lower-energy-level vacancies, or holes, until eventually the atom captures an electron from the surrounding region and changes from an ion to a neutral atom in its lowest equilibrium condition. The wavelength of the x-ray depends on the particular energy levels involved, as do the colors in visible light emitted from a given jump. X-rays lie in the wavelength range between I x 10-11 and I x 10-1 m. Another process of generating x-rays (also called cathode rays) is through the sudden braking (deceleration) of high-energy electrons by striking them on a metal target, which results in a conversion of part of the electron's kinetic energy into a quantum of electromagnetic radiation or x-ray. Such a technique is shown schematically in Figure 9-14.