View source for Rad (radiation unit)

{{Short description|Non-SI unit measuring absorbed dose of ionizing radiation}}
{{redirect-distinguish|Rad (unit)|Rad (angular unit)}}
{{Infobox Unit
| bgcolour =
| name = rad
| image = 
| caption = 
| standard = [[CGS]] units
| quantity = [[Absorbed dose|Absorbed dose of ionizing radiation]]
| symbol = rad
| namedafter = 
| units1 = [[SI derived unit]]s
| inunits1 = 0.01 [[Joule|J]]⋅[[kilogram|kg]]<sup>−1</sup>
| units2 = [[SI units]] 
| inunits2 = 0.01 [[Gray (unit)|Gy]]
| units3 = [[Centimetre–gram–second system of units|CGS]]
| inunits3 = 100 [[erg]]/g
}}
The '''rad''' is a unit of [[absorbed radiation dose]], defined as 1 rad = 0.01 [[gray (unit)|Gy]] = 0.01 J/kg.<ref>{{cite book|author=International Bureau of Weights and Measures|editor=United States National Institute of Standards and Technology|title=The International System of Units (SI)|url=https://www.nist.gov/sites/default/files/documents/2016/12/07/sp330.pdf|access-date=September 1, 2018|series=NIST Special Publication 330|year=2008|publisher=Dept. of Commerce, National Institute of Standards and Technology}}</ref> It was originally defined in [[Centimetre gram second system of units|CGS units]] in 1953 as the dose causing 100 [[erg]]s of energy to be absorbed by one [[gram]] of matter. The material absorbing the radiation can be human tissue, air, water, or any other substance.

It has been replaced by the [[gray (unit)|gray (symbol Gy)]] in [[SI derived unit]]s. The rad is still used in the United States, although this is "strongly discouraged" in Chapter 5.2 of the ''Guide to the SI'', which was written and published by the U.S. [[National Institute of Standards and Technology]].<ref>{{cite web |url=http://physics.nist.gov/Pubs/SP811/sec05.html#5.2 |title=NIST Guide to SI Units – ch.5.2 Units temporarily accepted for use with the SI |publisher=National Institute of Standards and Technology}}</ref> However, the numerically equivalent SI unit [[submultiple]], the centigray (symbol cGy), is widely used to report absorbed doses within radiotherapy. The [[roentgen (unit)|roentgen]], used to quantify the [[Exposure (radiation)|radiation exposure]], may be related to the corresponding absorbed dose by use of the [[F-factor (conversion factor)|F-factor]].

== Health effects ==
{{main article|Acute radiation syndrome}}
A dose of under 100 rad will typically produce no immediate symptoms other than blood changes. A dose of 100 to 200 rad delivered to the entire body in less than a day may cause [[acute radiation syndrome]] (ARS), but is usually not fatal. Doses of 200 to 1,000 rad delivered in a few hours will cause serious illness, with poor prognosis at the upper end of the range. Whole body doses of more than 1,000 rad are almost invariably fatal.<ref name="remeffects">''The Effects of Nuclear Weapons'', Revised ed., US DOD 1962, pp. 592&ndash;593</ref> Therapeutic doses of radiation therapy are often given and tolerated well even at higher doses to treat discrete, well-defined anatomical structures. The same dose given over a longer period of time is less likely to cause ARS. Dose thresholds are about 50% higher for dose rates of 20 rad/h, and even higher for lower dose rates.<ref name="ICRP103">{{cite journal|title=The 2007 Recommendations of the International Commission on Radiological Protection|journal=Annals of the ICRP|year=2007|volume=37|series=ICRP publication 103|issue=2–4|url=http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103|access-date=17 May 2012|isbn=978-0-7020-3048-2}}</ref>

The [[International Commission on Radiological Protection]] maintains a model of health risks as a function of absorbed dose and other factors. That model calculates an [[effective radiation dose]], measured in units of [[Röntgen equivalent man|rem]], which is more representative of the [[stochastic]] risk than the absorbed dose in rad. In most power plant scenarios, where the radiation environment is dominated by [[X-ray|X-]] or [[Gamma ray|gamma]] rays applied uniformly to the whole body, 1 rad of absorbed dose gives 1 rem of effective dose.<ref>{{cite web|url=http://www.hps.org/publicinformation/ate/q3142.html |title=Converting rad to rem, Health Physics Society . |url-status=dead |archive-url=https://web.archive.org/web/20130626085830/http://www.hps.org/publicinformation/ate/q3142.html |archive-date=June 26, 2013 }}</ref> In other situations, the effective dose in rem might be thirty times higher or thousands of times lower than the absorbed dose in rad.

{{anchor|Dose examples}}
{| class=wikitable
|+ Dose examples ([[metric prefix]]ed [[unit multiple]]s)
| align="right" valign="top" | 25&nbsp;rad:&nbsp; || Lowest dose to cause clinically observable blood changes
|-
| align="right" valign="top" | 200&nbsp;rad:&nbsp; || Local dose for onset of [[erythema]] in humans
|-
| align="right" valign="top" | 400&nbsp;rad:&nbsp; || Whole body [[median lethal dose|LD<sub>50</sub>]] for acute radiation syndrome in humans
|-
| align="right" valign="top" | 1&nbsp;krad:&nbsp; || Whole body LD<sub>100</sub> for acute radiation syndrome in humans<ref name=Anno>{{cite journal | last1=Anno  | first1=GH | last2=Young | first2=RW | last3=Bloom | first3=RM | last4=Mercier | first4=JR | year=2003 | title=Dose response relationships for acute ionizing-radiation lethality | journal=Health Physics | volume=84 | issue=5 | pages=565–575 | doi=10.1097/00004032-200305000-00001| pmid=12747475 | s2cid=36471776 }}</ref>
|-
| align="right" valign="top" | {{nowrap|1–20 krad:}}&nbsp; || Typical radiation tolerance of ordinary microchips
|-
| align="right" valign="top" | {{nowrap|4–8 krad:}}&nbsp; || Typical [[radiotherapy]] dose, locally applied
|-
| align="right" valign="top" | 10&nbsp;krad:&nbsp; || Fatal whole-body dose in 1964 [[Wood River Junction, Rhode Island#Criticality accident|Wood River Junction]] criticality accident<ref>{{cite journal|last=Goans|first=R E|author2=Wald, N |title=Radiation accidents with multi-organ failure in the United States|journal=British Journal of Radiology|date=1 January 2005|pages=41–46|doi=10.1259/bjr/27824773}}</ref> 
|-
| align="right" valign="top" | 1&nbsp;Mrad:&nbsp; || Typical tolerance of radiation-hardened microchips<ref>[http://www-physics.lbl.gov/~spieler/radiation_effects/rad_tutor.pdf Introduction to Radiation-Resistant Semiconductor Devices and Circuits]</ref>
|}

{{SI multiples
| unit=rad
| symbol=rad
}}

==History==
In the 1930s the [[roentgen (unit)|roentgen]] was the most commonly used unit of radiation exposure. This unit is obsolete and no longer clearly defined. One roentgen deposits 0.877 rad in dry air, 0.96 rad in soft tissue,<ref>{{cite web|title=APPENDIX E: Roentgens, RADs, REMs, and other Units|url=http://web.princeton.edu/sites/ehs/radsafeguide/rsg_app_e.htm|work=Princeton University Radiation Safety Guide|publisher=Princeton University|access-date=10 May 2012}}</ref> or anywhere from 1 to more than 4 rad in bone depending on the beam energy.<ref>{{cite web|last=Sprawls|first=Perry|title=Radiation Quantities and Units|url=http://www.sprawls.org/ppmi2/RADQU/3RADQU08.gif|work=The Physical Principles of Medical Imaging, 2nd Ed|access-date=10 May 2012}}</ref> These conversions to absorbed energy all depend on the ionizing energy of a standard medium, which is ambiguous in the latest NIST definition. Even where the standard medium is fully defined, the ionizing energy is often not precisely known.

In 1940, British physicist [[Louis Harold Gray]], who had been studying the effect of neutron damage on human tissue, together with [[William Valentine Mayneord]] and John 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"<ref name="Gupta2009">{{cite book|last=Gupta|first=S. V. |title=Units of Measurement: Past, Present and Future : International System of Units|chapter-url=https://books.google.com/books?id=pHiKycrLmEQC&pg=PA144|access-date=2012-05-14|date=2009-11-19|publisher=Springer|isbn=978-3-642-00737-8|page=144|chapter=Louis Harold Gray}}</ref> was proposed. This unit was found to be equivalent to 88&nbsp;ergs in air. It marked a shift towards measurements based on energy rather than charge.

The Röntgen equivalent physical (rep), introduced by [[Herbert Parker (scientist)|Herbert Parker]] in 1945,<ref>{{cite journal|last=Cantrill|first=S.T|author2=H.M. Parker |title=The Tolerance Dose|date=1945-01-05|url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA322447|archive-url=https://web.archive.org/web/20121130121433/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA322447|url-status=dead|archive-date=November 30, 2012|access-date=14 May 2012|publisher=US Atomic Energy Commission|location=Argonne National Laboratory}}</ref> was the absorbed energetic dose to tissue before factoring in [[relative biological effectiveness]]. The rep has variously been defined as 83 or 93 [[erg]]s per gram of tissue (8.3/9.3 [[milligray|mGy]])<ref>{{cite book|last=Dunning|first=John R.|title=A Glossary of Terms in Nuclear Science and Technology|year=1957|publisher=American Society of Mechanical Engineers|url=https://books.google.com/books?id=-zgrAAAAYAAJ&q=83+erg+roentgen&pg=PA146|access-date=14 May 2012|display-authors=etal}}</ref> or per cc of tissue.<ref name="Low-Beer1950">{{cite book|last=Bertram |first=V. A. Low-Beer|title=The clinical use of radioactive isotopes|url=https://books.google.com/books?id=wnTvxg6ZAToC|access-date=14 May 2012|year=1950|publisher=Thomas}}</ref>

In 1953 the ICRU recommended the rad, equal to 100&nbsp;erg/g as a new unit of absorbed radiation,<ref name=GM>{{cite conference
|title = Dosimetry in Europe and the USSR
|first1 = JH
|last1 = Guill
|first2 =John
|last2 = Moteff
|page = 64
|url = https://books.google.com/books?id=czTi4G6-Hq8C&q=roentgen+redefinition&pg=PA63
|conference = Symposium on Radiation Effects and Dosimetry - Third Pacific Area Meeting American Society for Testing Materials, October 1959, San Francisco, 12–16 October 1959
|publisher = ASTM International
|book-title = Third Pacific Area Meeting Papers - Materials in Nuclear Applications - American Society Technical Publication No 276
|lccn = 60-14734
|location = Baltimore
|date = June 1960
|access-date = 15 May 2012}}</ref> but then promoted a switch to the gray in the 1970s.

The [[International Committee for Weights and Measures]] (CIPM) has not accepted the use of the rad. From 1977 to 1998, the US NIST's translations of the SI brochure stated that the CIPM had temporarily accepted the use of the rad (and other radiology units) with SI units since 1969.<ref>{{cite book|author=International Bureau of Weights and Measures|editor=United States National Bureau of Standards|title=The international system of units (SI)|url=https://archive.org/details/bub_gb_YvZNdSdeCnEC|page=[https://archive.org/details/bub_gb_YvZNdSdeCnEC/page/n40 12]|access-date=18 May 2012|series=NBS Special Publication 330|year=1977|publisher=Dept. of Commerce, National Bureau of Standards}}</ref> However, the only related CIPM decisions shown in the appendix are with regards to the [[Curie (unit)|curie]] in 1964 and the [[radian]] (symbol: rad) in 1960. The NIST brochures redefined the rad as 0.01 Gy. The CIPM's current SI brochure excludes the rad from the tables of non-SI units accepted for use with the SI.<ref>{{SIbrochure}}</ref> The US NIST clarified in 1998 that it was providing its own interpretations of the SI system, whereby it accepted the rad for use in the US with the SI, while recognizing that the CIPM did not.<ref>{{cite journal|last=Lyons|first=John W.|title=Metric System of Measurement: Interpretation of the International System of Units for the United States|journal=Federal Register|date=1990-12-20|volume=55|issue=245|pages=52242–52245|publisher=US Office of the Federal Register}}</ref> NIST recommends defining the rad in relation to SI units in every document where this unit is used.<ref name="fedreg63">{{cite journal|last=Hebner|first=Robert E.|title=Metric System of Measurement: Interpretation of the International System of Units for the United States|journal=Federal Register|date=1998-07-28|volume=63|issue=144|page=40339|url=http://www.gpo.gov/fdsys/pkg/FR-1998-07-28/pdf/98-16965.pdf|access-date=9 May 2012|publisher=US Office of the Federal Register}}</ref> Nevertheless, use of the rad remains widespread in the US, where it is still an industry standard.<ref>''Handbook of Radiation Effects'', 2nd edition, 2002, Andrew Holmes-Siedle and Len Adams</ref> Although the United States Nuclear Regulatory Commission still permits the use of the units [[Curie (unit)|curie]], rad, and [[rem (unit)|rem]] alongside SI units,<ref>{{cite book|title=10 CFR 20.1004|year=2009|publisher=US Nuclear Regulatory Commission|url=https://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1004.html}}</ref> the [[European Union]] [[European units of measurement directives|required that its use for "public health ... purposes"]] be phased out by 31 December 1985.<ref>{{cite web 
| url = http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31980L0181:EN:NOT 
| author = The Council of the European Communities 
| title = Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC 
| date=1979-12-21 
| access-date=19 May 2012}}</ref>

==Radiation-related quantities==
The following table shows radiation quantities in SI and non-SI units:
{{Radiation related quantities}}

==See also==

*[[Becquerel]]
*[[Curie (unit)]]
*[[Radiation]]
*[[Gray (unit)]]
*[[Roentgen (unit)]]
*[[Roentgen equivalent man]] (rem)
*[[Sievert]]
*[[Order of magnitude|Order of magnitude (unit)]]

==References==
{{reflist}}

{{systems of measurement}}
{{Radiation protection|state=collapsed}}
{{Ionising radiation related quantities}}

[[Category:Units of radiation dose]]
[[Category:Non-SI metric units]]

[[es:Dosis absorbida#Unidades antiguas]]