page_title: Absorbed dose
section_title: Development of the absorbed dose concept and the gray
hierarchical_section_title: Absorbed dose / Development of the absorbed dose concept and the gray
caption_attribution_description: English: Radiology memorial at St. Georg hospital in Hamburg. Deutsch: Ehrenmal der Radiologie auf dem Gelände des Krankenhauses St. Georg in Hamburg. Dedication&#58; &#32; Den Röntgenologen und Radiologen aller Nationen Ärzten Physikern Chemikern Technikern Laboranten u. Krankenschwestern welche ihr Leben zum Opfer Brachten im Kampfe gegen die Krankheiten ihrer Mitmenschen Sie waren heldenmütige Wegbereiter für eine erfolgreichen und gefahrlose Anwendung der Röntgen- u. Radium-Strahlen in der Heilkunde Unsterblich ist der Toten Tatenruhm Die Deutsche Röntgengesellschaft &#32;&#91;In memory of the Physicians, Physicists, Chemists, Laboratory Technicians and Nurses who sacrificed their lives in the fight against the diseases of their fellow men. They were heroic pioneers for a successful and safe use of the X-ray and Radium rays in medicine. Immortal is the fame of their deeds.&#93;
context_page_description: Absorbed dose is a dose quantity which is the measure of the energy deposited in matter by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection, and radiology. It is also used to directly compare the effect of radiation on inanimate matter such as in radiation hardening.
  The SI unit of measure is the gray, which is defined as one Joule of energy absorbed per kilogram of matter. The older, non-SI CGS unit rad, is sometimes also used, predominantly in the USA.
context_section_description: Wilhelm Röntgen first 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.
  Due to the wide use of X-rays and the growing realisation of the dangers of ionizing radiation, measurement standards became necessary for radiation intensity and various countries developed their own, but using differing definitions and methods. Eventually, in order to promote international standardisation, the first International Congress of Radiology (ICR) meeting in London in 1925, proposed a separate body to consider units of measure. This was called the International Commission on Radiation Units and Measurements, or ICRU, and came into being at the Second ICR in Stockholm in 1928, under the chairmanship of Manne Siegbahn.
  One of the earliest techniques of measuring the intensity of X-rays was to measure their ionising effect in air by means of an air-filled ion chamber. At the first ICRU meeting it was proposed that one unit of 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 1 standard atmosphere of pressure. This unit of radiation exposure was named the roentgen in honour of Wilhelm Röntgen, who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply to gamma radiation. This approach, although a great step forward in standardisation, had the disadvantage of not being a direct measure of the absorption of radiation, and thereby the ionisation effect, in various types of matter including human tissue, and was a measurement only of the effect of the X-rays in a specific circumstance; the ionisation effect in dry air.
  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".  This unit was found to be equivalent to 88 ergs in air, and made the absorbed dose, as it subsequently became known, dependent on the interaction of the radiation with the irradiated material, not just an expression of radiation exposure or intensity, which the roentgen represented. 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 CGPM invited the ICRU to join other scientific bodies to work on the development of the International System of Units, or SI. It was decided to define the SI unit of absorbed radiation as energy deposited per unit mass which is how the rad had been defined, but in MKS units it would be J/kg. This was confirmed in 1975 by the 15th CGPM, and the unit was named the "gray" in honour of Louis Harold Gray, who had died in 1965. The gray was equal to 100 rad, the cgs unit.
caption: The Radiology Martyrs monument, erected 1936 at St. Georg hospital in Hamburg, more names added in 1959.
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