12VAC5-481-240. Units of exposure and dose.
A. As used in this chapter, the unit of exposure is the coulomb per kilogram (C/kg) of air and the units of radiation dose are gray (Gy), rad, rem, and sievert. (See 12VAC5-481-10 for definitions.) One roentgen is equal to 2.58E-4 coulomb per kilogram of air.
1. Gray (Gy) is the SI unit of absorbed dose. One gray is equal to an absorbed dose of 1 joule per kilogram (100 rad).
2. Rad is the special unit of absorbed dose. One rad is equal to an absorbed dose of 100 ergs per gram or 0.01 joule per kilogram (0.01 Gy).
3. Rem is the special unit of any of the quantities expressed as dose equivalent. The dose equivalent in rem is equal to the absorbed dose in rad multiplied by the quality factor (1 rem = 0.01 Sv).
4. Sievert is the SI unit of any of the quantities as dose equivalent. The dose equivalent is equal to the absorbed dose in gray multiplied by the quality factor (1 Sv = 100 rem).
B. As used in this chapter, the quality factors for converting absorbed dose to dose equivalent are shown in Quality Factors and Absorbed Dose Equivalencies table in this subsection.
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| Quality Factors and Absorbed Dose Equivalencies | ||
| Type of Radiation | Quality factor (Q) | Absorbed dose equal to a unit dose equivalenta | |
| X, gamma, or beta | 1 | 1 | |
| Alpha particles, multiple-charged particles, fission fragments and heavy particles of unknown charge | 20 | 0.05 | |
| Neutrons of unknown energy | 10 | 0.1 | |
| High energy protons | 10 | 0.1 | |
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| aAbsorbed dose in rad equal to 1 rem or the absorbed dose in gray equal to 1 sievert. | ||
C. If it is more convenient to measure the neutron fluence rate than to determine the neutron dose equivalent rate in rems per hour or sieverts per hour, as provided in subsection B of this section, 1 rem (0.01 Sv) of neutron radiation of unknown energies may, for purposes of this chapter, be assumed to result from a total fluence of 25 million neutrons per square centimeter incident upon the body. If sufficient information exists to estimate the approximate energy of the neutrons, the licensee may use the fluence rate per unit dose equivalent of the approximate Q value from the Mean Quality Factors, Q, and Fluence per Unit Dose Equivalent for Monoenergetic Neutrons table in this subsection to convert a measured tissue dose in rads to dose equivalent in rems.
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| Mean Quality Factors, Q, and Fluence per Unit Dose Equivalent for Monoenergetic Neutrons | |||
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| Neutron energy (MeV) | Quality factor (Q)a | Fluence per unit dose equivalent (neutrons cm-2 rem-1)b |
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| (thermal) | 2.5 x 10-8 | 2 | 980 x 106 |
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| 1 x 10-7 | 2 | 980 x 106 | |
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| 1 x 10-6 | 2 | 810 x 106 | |
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| 1 x 10-5 | 2 | 810 x 106 | |
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| 1 x 10-4 | 2 | 840 x 106 | |
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| 1 x 10-3 | 2 | 980 x 106 | |
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| 1 x 10-2 | 2.5 | 1010 x 106 | |
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| 1 x 10-1 | 7.5 | 170 x 106 | |
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| 5 x 10-1 | 11 | 39 x 106 | |
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| 1 | 11 | 27 x 106 | |
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| 2.5 | 9 | 29 x 106 | |
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| 5 | 8 | 23 x 106 | |
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| 7 | 7 | 24 x 106 | |
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| 10 | 6.5 | 24 x 106 | |
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| 14 | 7.5 | 17 x 106 | |
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| 20 | 8 | 16 x 106 | |
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| 40 | 7 | 14 x 106 | |
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| 60 | 5.5 | 16 x 106 | |
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| 1 x 102 | 4 | 20 x 106 | |
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| 2x102 | 3.5 | 19x106 | |
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| 3x102 | 3.5 | 16x106 | |
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| 4x102 | 3.5 | 14x106 | |
| aValue of quality factor (Q) at the point where the dose equivalent is maximum in a 30-cm diameter cylinder tissue-equivalent phantom. bMonoenergetic neutrons incident normally on a 30-cm diameter cylinder tissue-equivalent phantom. | ||||
Statutory Authority
§ 32.1-229 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 22, Issue 25, eff. September 20, 2006; amended, Virginia Register Volume 24, Issue 18, eff. June 12, 2008; Volume 32, Issue 24, eff. August 25, 2016.