When energetic particles enter a target material, they ionize some of the atoms of the target by breaking bonds. This requires energy, between 2 and 20 electron volts per bond depending upon the material (the electron volt, or eV, is a unit of energy; 1 eV = 1.602 X 10–19 joules). The ability of the radiation to deposit energy per unit mass of the target material is defined as the "radiation dose." It is expressed in grays, or Gy (in honor of Louis Harold Gray, 1905–1965, the British physician who was an authority on the use of radiation in the treatment of cancer). One gray equals 1 joule of deposited energy per kilogram of the target material. An older, similar unit, still in widespread use, is the rad (radiation absorbed dose), which is equal to 100 ergs per gram. One gray equals 100 rads. It is also customary to define the target material (e.g., Gy(Si) in the case of silicon) because different materials have different abilities to absorb energy from a given radiation field.
Another common unit is the roentgen, a unit of exposure, rather than dose. This unit is named for Wilhelm Roentgen (1845–1923), the first Nobel laureate in physics and the discoverer of the x ray. For x rays and gamma rays, which are forms of electromagnetic radiation, exposure is defined as the ability of the radiation to produce ionization charges in air. One roentgen is defined as 1 electrostatic unit, or esu (1/3 X 10–9 coulomb) per cubic centimeter of dry air at standard temperature and pressure. The measurement of gamma-ray or x-ray dose can be performed using an electrometer type of ionization detector, which directly measures this charge. A conversion from exposure to dose is straightforward, however it is material dependent.