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:''Not to be confused with roentgen equivalent man or roentgen equivalent physical'' The roentgen (R, also röntgen) is a legacy unit of measurement for the exposure of X-rays and gamma rays up to several megaelectronvolts. It is a measure of the ionization produced in air by X-rays or gamma radiation and it is used because air ionization can be measured directly.〔(Princeton radiation safety guide, appendix E ) (accessed 2014).〕 It is named after the German physicist Wilhelm Röntgen, who discovered X-rays. Originating in 1908, this unit has been redefined and renamed over the years.〔 It was last defined by the US National Institute of Standards and Technology (NIST) in 1998 as 2.58×10−4 C/kg, (1 C/kg = 3876 R,) with a recommendation that the definition be given in every document where the roentgen is used.〔 One roentgen of air kerma (kinetic energy released per unit mass) deposits of absorbed dose in dry air, or in soft tissue. One roentgen (air kerma) of X-rays may deposit anywhere from in bone depending on the beam energy. This tissue-dependent conversion from kerma to absorbed dose is called the F-factor in radiotherapy contexts. The conversion depends on the ionizing energy of a reference medium, which is ambiguous in the latest NIST definition. Even where the reference medium is fully defined, the ionizing energy of the calibration and target mediums are often not precisely known. == History == 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."〔Van Loon, R.; and Van Tiggelen, R., (''Radiation Dosimetry in Medical Exposure: A Short Historical Overview'' ), 2004>〕 Using 1 esu ≈ 3.33564 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 × 3.33564 × 10−10 × 1,000,000 ÷ 1.293 = 2.58 × 10−4 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Roentgen (unit)」の詳細全文を読む スポンサード リンク
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