Rad (unit)

Last updated
rad
Unit system CGS units
Unit of Absorbed dose of ionizing radiation
Symbolrad
Conversions
1 rad in ...... is equal to ...
    SI base units    0.01 J⋅kg −1
    SI units    0.01 Gy
    CGS    100 erg/g

The rad is a unit of absorbed radiation dose, defined as 1 rad = 0.01 Gy = 0.01 J/kg. [1] It was originally defined in CGS units in 1953 as the dose causing 100 ergs 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.

Contents

It has been replaced by the gray (Gy) in SI derived units, but is still used in the United States, although this is "strongly discouraged" in Chapter 5.2 of the Guide to the SI, which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. [2] However, the numerically equivalent SI unit, the centigray, is widely used to report absorbed doses within radiotherapy. The roentgen, used to quantify the radiation exposure, may be related to the corresponding absorbed dose by use of the F-factor.

Health effects

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. [3] 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. [4]

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 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- or gamma rays applied uniformly to the whole body, 1 rad of absorbed dose gives 1 rem of effective dose. [5] In other situations, the effective dose in rem might be thirty times higher or thousands of times lower than the absorbed dose in rad.

Dose examples

25 rad: Lowest dose to cause clinically observable blood changes
200 rad: Local dose for onset of erythema in humans
400 rad: Whole body LD50 for acute radiation syndrome in humans
1 krad: Whole body LD100 for acute radiation syndrome in humans [6]
1–20 krad: Typical radiation tolerance of ordinary microchips
4–8 krad: Typical radiotherapy dose, locally applied
10 krad: Fatal whole-body dose in 1964 Wood River Junction criticality accident [7]
1 Mrad: Typical tolerance of radiation-hardened microchips [8]

History

In the 1930s the 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, [9] or anywhere from 1 to more than 4 rad in bone depending on the beam energy. [10] 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" [11] was proposed. This unit was found to be equivalent to 88 ergs in air. It marked a shift towards measurements based on energy rather than charge.

The Röntgen equivalent physical (rep), introduced by Herbert Parker in 1945, [12] was the absorbed energetic dose to tissue before factoring in relative biological effectiveness. The rep has variously been defined as 83 or 93 ergs per gram of tissue (8.3/9.3 mGy) [13] or per cc of tissue. [14]

In 1953 the ICRU recommended the rad, equal to 100 erg/g as a new unit of absorbed radiation, [15] 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. [16] However, the only related CIPM decisions shown in the appendix are with regards to the 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. [17] 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. [18] NIST recommends defining the rad in relation to SI units in every document where this unit is used. [19] Nevertheless, use of the rad remains widespread in the US, where it is still an industry standard. [20] Although the United States Nuclear Regulatory Commission still permits the use of the units curie, rad, and rem alongside SI units, [21] the European Union required that its use for "public health ... purposes" be phased out by 31 December 1985. [22]

The following table shows radiation quantities in SI and non-SI units:

Ionizing radiation related quantities view  ‧  talk  ‧  edit
QuantityUnitSymbolDerivationYear SI equivalent
Activity (A) becquerel Bqs−11974SI unit
curie Ci3.7 × 1010 s−119533.7×1010 Bq
rutherford Rd106 s−119461,000,000 Bq
Exposure (X) coulomb per kilogram C/kgC⋅kg−1 of air1974SI unit
röntgen R esu / 0.001293 g of air19282.58 × 10−4 C/kg
Absorbed dose (D) gray Gy J⋅kg−11974SI unit
erg per gramerg/gerg⋅g−119501.0 × 10−4 Gy
rad rad100 erg⋅g−119530.010 Gy
Equivalent dose (H) sievert SvJ⋅kg−1 × WR 1977SI unit
röntgen equivalent man rem100 erg⋅g−1 × WR 19710.010 Sv
Effective dose (E) sievert SvJ⋅kg−1 × WR × WT 1977SI unit
röntgen equivalent man rem100 erg⋅g−1 × WR × WT 19710.010 Sv

See also

Related Research Articles

<span class="mw-page-title-main">Acute radiation syndrome</span> Health problems caused by exposure to high levels of ionizing radiation

Acute radiation syndrome (ARS), also known as radiation sickness or radiation poisoning, is a collection of health effects that are caused by being exposed to high amounts of ionizing radiation in a short period of time. Symptoms can start within an hour of exposure, and can last for several months. Early symptoms are usually nausea, vomiting and loss of appetite. In the following hours or weeks, initial symptoms may appear to improve, before the development of additional symptoms, after which either recovery or death follow.

<span class="mw-page-title-main">Sievert</span> SI unit of equivalent dose of ionizing radiation

The sievert is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing radiation, which is defined as the probability of causing radiation-induced cancer and genetic damage. The sievert is important in dosimetry and radiation protection. It is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dose measurement and research into the biological effects of radiation.

The becquerel is the unit of radioactivity in the International System of Units (SI). One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. For applications relating to human health this is a small quantity, and SI multiples of the unit are commonly used.

The gray is the unit of ionizing radiation dose in the International System of Units (SI), defined as the absorption of one joule of radiation energy per kilogram of matter.

Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested or inhaled radioactive substances, or externally due to irradiation by sources of radiation.

Equivalent dose is a dose quantity H representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the physical quantity absorbed dose, but also takes into account the biological effectiveness of the radiation, which is dependent on the radiation type and energy. In the SI system of units, the unit of measure is the sievert (Sv).

<span class="mw-page-title-main">Health physics</span>

Health physics, also referred to as the science of radiation protection, is the profession devoted to protecting people and their environment from potential radiation hazards, while making it possible to enjoy the beneficial uses of radiation. Health physicists normally require a four-year bachelor’s degree and qualifying experience that demonstrates a professional knowledge of the theory and application of radiation protection principles and closely related sciences. Health physicists principally work at facilities where radionuclides or other sources of ionizing radiation are used or produced; these include research, industry, education, medical facilities, nuclear power, military, environmental protection, enforcement of government regulations, and decontamination and decommissioning—the combination of education and experience for health physicists depends on the specific field in which the health physicist is engaged.

The roentgen equivalent man (rem) is a CGS unit of equivalent dose, effective dose, and committed dose, which are dose measures used to estimate potential health effects of low levels of ionizing radiation on the human body.

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.

In radiation physics, kerma is an acronym for "kinetic energy released per unit mass", defined as the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation in a sample of matter, divided by the mass of the sample. It is defined by the quotient .

The Röntgen equivalent physical or rep is a legacy unit of absorbed dose first introduced by Herbert Parker in 1945 to replace an improper application of the roentgen unit to biological tissue. 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) or per cm3 of tissue.

The International Commission on Radiological Protection (ICRP) is an independent, international, non-governmental organization, with the mission to protect people, animals, and the environment from the harmful effects of ionising radiation. Its recommendations form the basis of radiological protection policy, regulations, guidelines and practice worldwide.

The International Commission on Radiation Units and Measurements (ICRU) is a standardization body set up in 1925 by the International Congress of Radiology, originally as the X-Ray Unit Committee until 1950. Its objective "is to develop concepts, definitions and recommendations for the use of quantities and their units for ionizing radiation and its interaction with matter, in particular with respect to the biological effects induced by radiation".


In radiobiology, the relative biological effectiveness is the ratio of biological effectiveness of one type of ionizing radiation relative to another, given the same amount of absorbed energy. The RBE is an empirical value that varies depending on the type of ionizing radiation, the energies involved, the biological effects being considered such as cell death, and the oxygen tension of the tissues or so-called oxygen effect.

Committed dose equivalent and Committed effective dose equivalent are dose quantities used in the United States system of radiological protection for irradiation due to an internal source.

<span class="mw-page-title-main">Roentgen (unit)</span> Measurement of radiation exposure

The roentgen or röntgen is a legacy unit of measurement for the exposure of X-rays and gamma rays, and is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air . In 1928, it was adopted as the first international measurement quantity for ionizing radiation to be defined for radiation protection, as it was then the most easily replicated method of measuring air ionization by using ion chambers. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays and was awarded the first Nobel Prize in Physics for the discovery.

Effective dose is a dose quantity in the International Commission on Radiological Protection (ICRP) system of radiological protection.

The committed dose in radiological protection is a measure of the stochastic health risk due to an intake of radioactive material into the human body. Stochastic in this context is defined as the probability of cancer induction and genetic damage, due to low levels of radiation. The SI unit of measure is the sievert.

<span class="mw-page-title-main">Radiation exposure</span> Measure of ionization of air by ionizing radiation

Radiation exposure is a measure of the ionization of air due to ionizing radiation from photons. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. As of 2007, "medical radiation exposure" was defined by the International Commission on Radiological Protection as exposure incurred by people as part of their own medical or dental diagnosis or treatment; by persons, other than those occupationally exposed, knowingly, while voluntarily helping in the support and comfort of patients; and by volunteers in a programme of biomedical research involving their exposure. Common medical tests and treatments involving radiation include X-rays, CT scans, mammography, lung ventilation and perfusion scans, bone scans, cardiac perfusion scan, angiography, radiation therapy, and more. Each type of test carries its own amount of radiation exposure. There are two general categories of adverse health effects caused by radiation exposure: deterministic effects and stochastic effects. Deterministic effects are due to the killing/malfunction of cells following high doses; and stochastic effects involve either cancer development in exposed individuals caused by mutation of somatic cells, or heritable disease in their offspring from mutation of reproductive (germ) cells.

References

  1. ↑ International Bureau of Weights and Measures (2008). United States National Institute of Standards and Technology (ed.). The International System of Units (SI) (PDF). NIST Special Publication 330. Dept. of Commerce, National Institute of Standards and Technology. Retrieved September 1, 2018.
  2. ↑ "NIST Guide to SI Units – ch.5.2 Units temporarily accepted for use with the SI". National Institute of Standards and Technology.
  3. ↑ The Effects of Nuclear Weapons, Revised ed., US DOD 1962, pp. 592–593
  4. ↑ "The 2007 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 103. 37 (2–4). 2007. ISBN   978-0-7020-3048-2 . Retrieved 17 May 2012.
  5. ↑ "Converting rad to rem, Health Physics Society ". Archived from the original on June 26, 2013.
  6. ↑ Anno, GH; Young, RW; Bloom, RM; Mercier, JR (2003). "Dose response relationships for acute ionizing-radiation lethality". Health Physics. 84 (5): 565–575. doi:10.1097/00004032-200305000-00001. PMID   12747475. S2CID   36471776.
  7. ↑ Goans, R E; Wald, N (1 January 2005). "Radiation accidents with multi-organ failure in the United States". British Journal of Radiology: 41–46. doi:10.1259/bjr/27824773.
  8. ↑ Introduction to Radiation-Resistant Semiconductor Devices and Circuits
  9. ↑ "APPENDIX E: Roentgens, RADs, REMs, and other Units". Princeton University Radiation Safety Guide. Princeton University. Retrieved 10 May 2012.
  10. ↑ Sprawls, Perry. "Radiation Quantities and Units". The Physical Principles of Medical Imaging, 2nd Ed. Retrieved 10 May 2012.
  11. ↑ Gupta, S. V. (2009-11-19). "Louis Harold Gray". Units of Measurement: Past, Present and Future : International System of Units. Springer. p. 144. ISBN   978-3-642-00737-8 . Retrieved 2012-05-14.
  12. ↑ Cantrill, S.T; H.M. Parker (1945-01-05). "The Tolerance Dose". Argonne National Laboratory: US Atomic Energy Commission. Archived from the original on November 30, 2012. Retrieved 14 May 2012.{{cite journal}}: Cite journal requires |journal= (help)
  13. ↑ Dunning, John R.; et al. (1957). A Glossary of Terms in Nuclear Science and Technology. American Society of Mechanical Engineers. Retrieved 14 May 2012.
  14. ↑ Bertram, V. A. Low-Beer (1950). The clinical use of radioactive isotopes. Thomas. Retrieved 14 May 2012.
  15. ↑ Guill, JH; Moteff, John (June 1960). "Dosimetry in Europe and the USSR". Third Pacific Area Meeting Papers - Materials in Nuclear Applications - American Society Technical Publication No 276. Symposium on Radiation Effects and Dosimetry - Third Pacific Area Meeting American Society for Testing Materials, October 1959, San Francisco, 12–16 October 1959. Baltimore: ASTM International. p. 64. LCCN   60-14734 . Retrieved 15 May 2012.
  16. ↑ International Bureau of Weights and Measures (1977). United States National Bureau of Standards (ed.). The international system of units (SI). NBS Special Publication 330. Dept. of Commerce, National Bureau of Standards. p.  12 . Retrieved 18 May 2012.
  17. ↑ Le Système international d’unités [The International System of Units](PDF) (in French and English) (9th ed.), International Bureau of Weights and Measures, 2019, ISBN   978-92-822-2272-0
  18. ↑ Lyons, John W. (1990-12-20). "Metric System of Measurement: Interpretation of the International System of Units for the United States". Federal Register. US Office of the Federal Register. 55 (245): 52242–52245.
  19. ↑ Hebner, Robert E. (1998-07-28). "Metric System of Measurement: Interpretation of the International System of Units for the United States" (PDF). Federal Register. US Office of the Federal Register. 63 (144): 40339. Retrieved 9 May 2012.
  20. ↑ Handbook of Radiation Effects, 2nd edition, 2002, Andrew Holmes-Siedle and Len Adams
  21. ↑ 10 CFR 20.1004. US Nuclear Regulatory Commission. 2009.
  22. ↑ The Council of the European Communities (1979-12-21). "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" . Retrieved 19 May 2012.
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