Röntgen equivalent physical

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The Röntgen equivalent physical or rep (symbol rep) is a unit of absorbed dose first introduced by Herbert Parker in 1945 to replace an improper application of the roentgen unit to biological tissue.[1] It is the absorbed energetic dose before the biological efficiency of the radiation is factored in. The rep has variously been defined as 83 or 93 ergs per gram of tissue (8.3/9.3 mGy)[2] or per cm3 of tissue.[3] At the time, this was thought to be the amount of energy deposited by 1 roentgen.[4] Improved measurements have since found that one roentgen of air kerma deposits 8.77 mGy in dry air, or 9.6 mGy in soft tissue, but the rep was defined as a fixed number of ergs per unit gram.[5] A 1952 handbook from the US National Bureau of Standards affirms that "The numerical coefficient of the rep has been deliberately changed to 93, instead of the earlier 83, to agree with L. H. Gray's 'energy-unit'."[6] It is unclear what was meant by Gray's 'energy unit', since the gray was not defined until the 1970s; perhaps the gram-roentgen he introduced in 1940? The rep was commonly used until the 1960s,[7] but was gradually displaced by the rad starting in 1954 and later the gray starting in 1977.


  1. ^ Cantrill, S.T; H.M. Parker (1945-01-05). "The Tolerance Dose". Argonne National Laboratory: US Atomic Energy Commission. Retrieved 14 May 2012. 
  2. ^ Dunning, John R.; et al. (1957). A Glossary of Terms in Nuclear Science and Technology. American Society of Mechanical Engineers. Retrieved 14 May 2012. 
  3. ^ Bertram, V. A. Low-Beer (1950). The clinical use of radioactive isotopes. Thomas. Retrieved 14 May 2012. 
  4. ^ Anderson, Elda E. (March 1952). "Units of Radiation and Radioactivity" (PDF). Public Health Reports. 67 (3): 293–297. doi:10.2307/4588064. PMC 2030726Freely accessible. PMID 14900367. Retrieved 18 November 2012. 
  5. ^ "APPENDIX E: Roentgens, RADs, REMs, and other Units". Princeton University Radiation Safety Guide. Princeton University. Retrieved 10 May 2012. 
  6. ^ National Bureau of Standards (1952). Radiological Monitoring Methods and Instruments (PDF). 51. US Department of Commerce. Retrieved 16 November 2012. 
  7. ^ US patent 3568311, Elliott J. Lawton, "INSULATING AND SPACING ELECTRICALLY CONDUCTIVE MEMBERS", published 1971-03-09, issued 1971-03-01, assigned to General Electric Company 

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