roentgen

The roentgen (R, also röntgen) is a unit of measurement for exposure to ionizing radiation (such as X-ray and gamma rays), and is named after the German physicist Wilhelm Röntgen. Adopted in 1928, 1 R is the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge (esu) in 1 cm³ of dry air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08×10⁹ ion pairs.

Until 2006, the roentgen was accepted for use with the SI system but its value is expressed in terms of the SI units charge divided by unit mass (C/kg) rather than as in the original definition. Although its use was allowable under the SI system, it is not itself an SI unit and its continued use is "strongly discouraged" by the National Institute of Standards and Technology style guide for NIST authors.

The roentgen was occasionally used to measure exposure to radiation in forms other than X-rays or gamma rays. To adjust for the different impact of different forms of radiation on biological matter, "roentgen equivalent man" or rem was also in use. Exposure in rems is equal to the exposure in roentgens multiplied by the Q value, a constant describing the type of radiation. The rem is now superseded by the sievert.