Gray (unit) - Leading Up To The Gray

Leading Up To The Gray

The adoption of the gray by the 15th CGPM in 1975 as the unit of measure of the absorption of ionising radiation, specific energy absorption and of kerma in 1975 was the cumulation of over half a century of work, both in the understanding of the nature of ionising radiation and in the refinement of measuring techniques.

Wilhelm Röntgen first discovered X-Rays on November 8, 1895 and within a few years they were being used to examine broken bones. One of the earliest techniques of measuring the intensity of X-Rays was to measure their ionisation potential in air. Initially various countries developed their own standards, but in order to promote international cooperation, the First International Congress of Radiology (ICR) which met in London in 1925 proposed a separate body to consider units of measure. This body, the International Commission on Radiation Units and Measurements (ICRU), came into being at the Second ICR in Stockholm in 1928 under the chairmanship Manne Siegbahn and at their first meeting proposed that one unit X-Ray dose should be defined as the quantity of X-rays that would produce one esu of charge in one cubic centimetre of dry air at 0 °C and a standard atmosphere. This unit was named the roentgen in honour of Röntgen who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply to gamma radiation as well as X-Rays. This technique, although appropriate for the technology of the day, had the disadvantage that it was not a direct measure of either the intensity of X-rays or of their absorption, but rather was a measurement of the effect of the X-Rays in a specific circumstance.

In 1940, Gray, who had been studying the effect of neutron damage on human tissue, together with Mayneord and Read published a paper in which a unit of measure, dubbed the "gram roentgen" (symbol: gr) defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation" was proposed. This unit was found to be equivalent to 88 ergs in air. In 1953 the ICRU recommended the rad, equal to 100 erg/g as the new unit of measure of absorbed radiation. The rad was expressed in coherent cgs units.

In the late 1950s the ICRU was invited by the CGPM to join other scientific bodies to work with the International Committee for Weights and Measures (CIPM) in the development of a system of units that could be used consistently over many disciplines. This body, initially known as the "Commission for the System of Units" (renamed in 1964 as the "Consultative Committee for Units") was responsible overseeing the development of the International System of Units (SI). At the same time it was becoming increasingly obvious that the definition of the roentgen was unsound and many calls were made for its redefinition. In 1962 it was redefined. The definition of the roentgen had the advantage over the gray of being simpler to measure, but the gray is independent of the primary ionising radiation

The CCU decided to define the SI unit of absorbed radiation in terms of energy per unit mass, which in MKS units was J/kg. This was confirmed in 1975 by 15th GCPM and the unit was named the "gray" in honour of Hal Gray who had died in 1965. The gray was exactly equal to 100 rad.