The Röntgen equivalent physical or rep (symbol rep) is a legacy unit of

absorbed dose Absorbed dose is a dose quantity which represents the specific energy (energy per unit mass) deposited by ionizing radiation in living matter. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protecti ...

first introduced by

Herbert Parker Herbert or Herb Parker may refer to: Politicians * Herbert Parker (American politician) (1856–1939), Massachusetts Attorney General *John Parker (Labour politician) Herbert John Harvey Parker (15 July 1906 – 24 November 1987) was a British ...

in 1945 to replace an improper application of the roentgen unit to biological tissue. It is the absorbed energetic dose before the biological efficiency of the radiation is factored in. The rep has variously been defined as 83 or 93

erg The erg is a unit of energy equal to 10−7joules (100Nano-, nJ). It is not an SI unit, instead originating from the centimetre–gram–second system of units (CGS). Its name is derived from (), a Greek language, Greek word meaning 'work' or ' ...

s per gram of tissue (8.3/9.3

mGy The gray (symbol: Gy) is the unit of ionizing radiation dose in the International System of Units (SI), defined as the absorption of one joule of radiation energy per kilogram of matter. It is used as a unit of the radiation quantity absorbed d ...

) or per cm³ of tissue. At the time, this was thought to be the amount of energy deposited by 1 roentgen. Improved measurements have since found that one roentgen of air

kerma Kerma was the capital city of the Kerma culture, which was founded in present-day Sudan before 3500 BC. Kerma is one of the largest archaeological sites in ancient Nubia. It has produced decades of extensive excavations and research, including t ...

deposits 8.77 mGy in dry air, or 9.6 mGy in soft tissue, but the rep was defined as a fixed number of ergs per unit gram. A 1952 handbook from the US National Bureau of Standards affirms that "The numerical coefficient of the rep has been deliberately changed to 93, instead of the earlier 83, to agree with L. H. Gray's 'energy-unit'." Gray's 'energy unit' was " one roentgen of hard gamma resulted in about 93 ergs per gram energy absorption in water". The lower range value of 83.8 ergs was the value in air corresponding to wet tissue. The rep was commonly used until the 1960s, but was gradually displaced by the rad starting in 1954 and later the

gray Grey (more frequent in British English) or gray (more frequent in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning that it has no chroma. It is the color of a cloud-covered s ...

starting in 1977.

References

See also