☙

roentgen or röntgen

A unit describing exposure to x-radiation or gamma rays. Abbreviation, R (formerly r). One roentgen = 2.58 × 10⁻⁴ coulombs per kilogram.

The roentgen was defined by the 1937 Radiological Congress (Chicago) as the amount of x-radiation or gamma radiation that produces ionization equal to 1 electrostatic unit of charge, either negative or positive, in 1 cubic centimeter of dry air at 0°C and at standard atmospheric pressure. In some later definitions the cubic centimeter of air was replaced by simply “0.001293 grams of air.”¹

At a meeting of the BIPM’s working group for the measurement of X- and gamma-rays in 1965, the roentgen was redefined as 2.58 × 10⁻⁴ coulomb per kilogram of air, and the symbol as R.²

Although the roentgen has been discontinued in favor of expressing such measurements in coulombs per kilogram, the CIPM has sanctioned its temporarily continued use.

portrait of Roentgen

The roentgen is named for the German physicist Wilhelm Conrad Röntgen (1845 – 1923), the discoverer of x-rays.

1. National Research Council.
A Glossary of Terms in Nuclear Science and Technology.
New York: American Society of Mechanical Engineers, 1955.
Page 147.

2. J. Terrien.
News from the Bureau International des Poids et Mesures.
Metrologia, volume 1, no. 3 (1965). Page 134.