Gray (unit)

The unit was named after British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue.

The measurement of absorbed dose is a complex problem due to scattering and absorption, and many specialist dosimeters are available for these measurements, and can cover applications in 1-D, 2-D and 3-D.[4][5][6] In radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated.

The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute exposure to high levels of ionizing radiation.

A whole-body acute exposure to 5 grays or more of high-energy radiation usually leads to death within 14 days.

Wilhelm Röntgen discovered X-rays on November 8, 1895, and their use spread very quickly for medical diagnostics, particularly broken bones and embedded foreign objects where they were a revolutionary improvement over previous techniques.

This was called the International Commission on Radiation Units and Measurements, or ICRU,[b] and came into being at the Second ICR in Stockholm in 1928, under the chairmanship of Manne Siegbahn.

This unit of radiation exposure was named the roentgen in honour of Wilhelm Röntgen, who had died five years previously.

[14] In 1940, Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with William Valentine Mayneord and the radiobiologist John Read, published a paper in which a new unit of measure, dubbed the gram roentgen (symbol: gr) was proposed, and 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".

[13] In the late 1950s, the CGPM invited the ICRU to join other scientific bodies to work on the development of the International System of Units, or SI.

Notably, the centigray (numerically equivalent to the rad) is still widely used to describe absolute absorbed doses in radiotherapy.