Roentgen (unit)

Not to be confused with roentgen equivalent man or roentgen equivalent physical.
Roentgen
Display of quartz fiber dosimeter, in units of roentgen.[1]
General information
Unit systemLegacy unit
Unit ofExposure to ionizing radiation
SymbolR
Named afterWilhelm Röntgen
Conversions
1 R in ...... is equal to ...
   SI base units   2.58×10−4 A⋅s/kg

The roentgen or röntgen (/ˈrɛntɡən, -dʒən, ˈrʌnt-/;[2] symbol R) is a legacy unit of measurement for the exposure of X-rays and gamma rays, and is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air (statcoulomb per kilogram). In 1928, it was adopted as the first international measurement quantity for ionizing radiation to be defined for radiation protection, as it was then the most easily replicated method of measuring air ionization by using ion chambers.[3] It is named after the German physicist Wilhelm Röntgen, who discovered X-rays and was awarded the first Nobel Prize in Physics for the discovery.

However, although this was a major step forward in standardising radiation measurement, the roentgen has the disadvantage that it is only a measure of air ionisation, and not a direct measure of radiation absorption in other materials, such as different forms of human tissue. For instance, one roentgen deposits 0.00877 grays (0.877 rads) of absorbed dose in dry air, or 0.0096 Gy (0.96 rad) in soft tissue.[3] One roentgen of X-rays may deposit anywhere from 0.01 to 0.04 Gy (1.0 to 4.0 rad) in bone depending on the beam energy.[4]

As the science of radiation dosimetry developed, it was realised that the ionising effect, and hence tissue damage, was linked to the energy absorbed, not just radiation exposure. Consequently new radiometric units for radiation protection were defined which took this into account. In 1953 the International Commission on Radiation Units and Measurements (ICRU) recommended the rad, equal to 100 erg/g, as the unit of measure of the new radiation quantity absorbed dose. The rad was expressed in coherent cgs units.[5] In 1975 the unit gray was named as the SI unit of absorbed dose. One gray is equal to 1 J/kg (i.e. 100 rad). Additionally, a new quantity, kerma, was defined for air ionisation as the exposure for instrument calibration, and from this the absorbed dose can be calculated using known coefficients for specific target materials. Today, for radiation protection, the modern units, absorbed dose for energy absorption and the equivalent dose (sievert) for stochastic effect, are overwhelmingly used, and the roentgen is rarely used. The International Committee for Weights and Measures (CIPM) has never accepted the use of the roentgen.

The roentgen has been redefined over the years. It was last defined by the U.S.'s National Institute of Standards and Technology (NIST) in 1998 as 2.58×10−4 C/kg, with a recommendation that the definition be given in every document where the roentgen is used.[6]

  1. ^ Frame, Paul (2007-07-25). "Pocket Chambers and Pocket Dosimeters". Health physics historical instrument museum collection. Oak Ridge Associated Universities. Retrieved 2021-10-07.
  2. ^ "Röntgen". Random House Webster's Unabridged Dictionary.
  3. ^ a b "Princeton Radiation Safety Guide, Appendix E: Roentgens, RADs, REMs, and other Units". Archived from the original on 2015-02-22. Retrieved 10 May 2012.
  4. ^ Sprawls, Perry. "Radiation Quantities and Units". The Physical Principles of Medical Imaging, 2nd Ed. Retrieved 10 May 2012.
  5. ^ Guill, JH; Moteff, John (June 1960). "Dosimetry in Europe and the USSR". Third Pacific Area Meeting Papers — Materials in Nuclear Applications. Symposium on Radiation Effects and Dosimetry - Third Pacific Area Meeting American Society for Testing Materials, October 1959, San Francisco, 12–16 October 1959. American Society Technical Publication. Vol. 276. ASTM International. p. 64. LCCN 60014734. Retrieved 2012-05-15.
  6. ^ Cite error: The named reference fedreg63 was invoked but never defined (see the help page).