Röntgen


 * For Röntgen radiation see X-ray.
 *  For other uses of röntgen or roentgen, see Roentgen (disambiguation)

The röntgen or roentgen (symbol R) is a unit of measurement for ionizing radiation (such as X-ray and gamma rays), and uses the units charge divided by unit weight (C/kg). It is named after the German physicist Wilhelm Röntgen. One R is the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge in 1 cm³ of air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08&times;109 ion pairs.

In SI units, 1 R = 2.58&times;10&minus;4 C/kg. A dose of 500 R in 5 hours is lethal for humans. In a standard atmosphere (air density ~1.293 kg/m³) and using an air ionisation energy of 36.16 J/C, we have 1 R ≈ 9.330 mGy, or 1 Gy ≈ 107.2 R. Continued use of this non-SI unit is "strongly discouraged" by the National Institute of Standards and Technology.

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 The rem is now superseded by the sievert (see the latter for a list of Q values).

Background exposure in röntgens
Natural background exposure varies widely: from 17 μR/h in some regions to 1.1 mR/h in others (1.5 to 100 mSv/a ), with urban areas typically having higher exposure rates than rural ones. The world's extreme is reached in Ramsar, Iran, where there is at least one house receiving an average individual lifetime dose of natural radiation of 17,000 mSv (240 times more than the current ICRP limit. . The world’s average individual lifetime dose due to natural background radiation is about 16 R (160 mSv; 30 μR/h if you assume an average life expectancy of 60 years).

References in Popular Culture

 * In the movie Back to the Future, after Marty has arrived in 1955 and is stopped at the site of Lyon Estates, he glances at the time readout and then hears an alarm. He looks at the plutonium chamber gauge, which is measured in roentgens.