An x-ray is a discrete bundle of electromagnetic energy called a photon. In that regard, it is similar to other forms of electromagnetic energy such as light, infrared, ultraviolet, radio waves, or gamma rays. The associated electromagnetic energy can be thought of as oscillating electric and magnetic fields propagating through space at the speed of light. The various forms of electromagnetic energy differ only in frequency (or wavelength). However, because the energy carried by each photon is proportional to the frequency (the proportionality constant is called Planck's constant), the higher frequency x-ray or gamma ray photons are much more energetic than, for example, light photons and can readily ionize the atoms in materials on which they impinge. The energy of a light photon is of the order of one electron-volt (eV), whereas the average energy of an x-ray photon in a diagnostic x-ray beam is on the order of 30 kiloelectron volts (keV) and its wavelength is smaller than the diameter of an atom (10−8 cm).
In summary, an x-ray beam can be thought of as a swarm of photons traveling at the speed of light, each photon representing a bundle of electromagnetic energy.
Electromagnetic radiation may be produced in a variety of ways. One method is the acceleration or deceleration of electrons. For example, a radio transmitter is merely a source of high-frequency alternating current that causes electrons in an antenna wire to which it is connected to oscillate (accelerate and decelerate), thereby producing radio waves (photons) at the transmitter frequency. In an x-ray tube, electrons boiled off from a hot filament (Figure 2-1) are accelerated toward a tungsten anode by a high voltage on the order of 100 kilovolts (kV). Just before hitting the anode, the electrons will have a kinetic energy in kiloelectron volts equal in magnitude to the kilovoltage (eg, if the voltage across the x-ray tube is 100 kV, the electron energy is 100 keV). When the electrons smash into the tungsten anode, most of them hit other electrons, and their energy is dissipated in the form of heat. In fact, the anode may become white-hot during an x-ray exposure, which is one reason for choosing an anode made of tungsten, with a very high melting point. The electrons penetrate the anode to a depth less than 0.1 mm.
A small fraction of the electrons, however, may have a close encounter with a tungsten nucleus, which, because of its large positive charge, exerts a large attractive force on the electron, giving the electron a hard jerk (acceleration) of sufficient magnitude to produce an x-ray photon. The energy of the x-ray photon, which is derived from the energy of the incident electron, depends ...