Roentgen (unit)

The roentgen or röntgen (/ˈrɛntɡən, -dʒən, ˈrʌnt-/; 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. 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. 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.

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. 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.

The roentgen has its roots in the Villard unit defined in 1908 by the American Roentgen Ray Society as "the quantity of radiation which liberates by ionisation one esu of electricity per cm³ of air under normal conditions of temperature and pressure." Using 1 esu ≈ 3.33564×10⁻¹⁰ C and the air density of ~1.293 kg/m³ at 0 °C and 101 kPa, this converts to 2.58 × 10⁻⁴ C/kg, which is the modern value given by NIST.

1 ⁠esu/cm³⁠ × 3.33564 × 10⁻¹⁰ ⁠C/esu⁠ × 1,000,000 ⁠cm³/m³⁠ ÷ 1.293 ⁠kg/m³⁠ = 2.58 × 10⁻⁴ ⁠C/kg⁠

This definition was used under different names (e, R, and German unit of radiation) for the next 20 years. In the meantime, the French Roentgen was given a different definition which amounted to 0.444 German R.

In 1928, the International Congress of Radiology (ICR) defined the roentgen as "the quantity of X-radiation which, when the secondary electrons are fully utilised and the wall effect of the chamber is avoided, produce in 1 cc of atmospheric air at 0 °C and 76 cm of mercury pressure such a degree of conductivity that 1 esu of charge is measured at saturation current." The stated cubic centimetre (1 cc) of air would have a mass of 1.293 mg at the conditions given, so in 1937 the ICR rewrote this definition in terms of this mass of air instead of volume, temperature and pressure. The 1937 definition was also extended to gamma rays, but later capped at 3 MeV in 1950.