X-RAY PRODUCTION AND SCATTERING
The production and scattering of x rays provide additional examples of the quantum nature of electromagnetic radiation. X rays are produced when rapidly moving electrons that have been accelerated through a potential difference of the order of 103 to 106 V strike a metal target. They were first produced in 1895 by Wilhelm Rontgen (1845-1923), using an apparatus similar in principle to the setup shown in Fig. 4O-22a. Electrons are “boiled off” from the heated cathode by thermionic emission and are accelerated toward the anode (the target) by a large potential difference VAC’ The bulb is evacuated (residual pressure 10-7 atm or less), so the electrons can travel from the cathode to the anode without colliding with air molecules. When VAC is a few thousand volts or more, a very penetrating radiation is emitted from the anode surface. A more modem x-ray device is shown in Fig. 4O-22b.