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Section: 16 | Radiation Safety Units |
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John R. Rumble, ed., CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), CRC Press/Taylor & Francis, Boca Raton, FL.
If a specific table is cited, use the format: "Physical Constants of Organic Compounds," in CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.

RADIATION SAFETY UNITS

Thomas J. Bruno and Paris D. N. Svoronos

Ionizing radiation, consisting of x-rays, gamma rays, alpha particles, beta particles, and neutrons, is measured and quantified in units of the radioactivity source and dose (Refs. 1-4). The radioactivity measures the strength of a source in terms of events of emission per second. Dose is a measure of the energy that is actually absorbed into matter.

References

  1. Radiation - Quantities and Units of Ionizing Radiation, Canadian Centre for Occupational Health and Safety, OSH Answer List Series, 2008.
  2. Radioactivity Units, Health Physics Society, <www.hps.org/>, 2013.
  3. Furr, A. K., CRC Handbook of Laboratory Safety, Fifth Edition, CRC Press, Boca Raton, FL, 2000. [https://doi.org/10.1201/9780367801755]
  4. Bruno, T. J., and Svoronos, P. D. N., CRC Handbook of Basic Tables for Chemical Analysis – Data-Driven Methods and Interpretation, Fourth Edition, CRC Press/Taylor & Francis, Boca Raton, FL, 2021. [https://doi.org/10.1201/b22281]
  5. Higley, K. A., Kocher, D.C., Real, A. G. , and Chambers, D. B., Ann. ICRP, 41, 233, 2012. <doi:10.1016/j.icsp.2012.06.014>. [https://doi.org/10.1016/j.icrp.2012.06.014]

Radioactivity

In the SI system, the bequerel (Bq) has replaced the curie (Ci) as the accepted unit of radioactivity (or simply activity). One Bq is one event of radiation emission (such as a disintegration) per second. It is related to the older unit by the following equations.

1 Ci = 3.7 × 1010 Bq

1 Ci = 37 GBq = 37 000 MBq

The following chart provides a practical guide between the two units:

chart showing the above

Class A radionuclides: 0.3 Bq/cm2 = 8.1 pCi/cm2
Class B radionuclides: 3 Bq/cm2 = 81 pCi/cm2
Class C radionuclides: 30 Bq/cm2 = 810 pCi/cm2

Energy

For ionizing radiation, the energy is measured in electron volts (eV), which is related to other energy quantities by:

1 eV = 1.602 176 634 × 10−19 J (exact)

Dose

The older unit of dose, which is defined as the energy that is actually absorbed, is the radiation absorbed dose (RAD). The RAD was defined as the dose that would cause 0.01 J to be absorbed in 1 kg of matter (or 100 ergs per gram). The modern SI unit is the Gray (Gy):

100 RAD = 1 Gy

Equivalent Dose

The committed dose, or more properly the committed dose equivalent, HT,50,  is the total dose accumulated over a 50-year period after ingestion or inhalation. The equivalent dose (also called the dose equivalent or biological dose) describes the effect of radiation on human tissue rather than the physical effects of the radiation alone. This quantity is expressed in Sieverts (Sv), and is found by multiplying the absorbed dose, in grays, by a dimensionless quality factor Q (which depends on the radiation type), and by another dimensionless factor N (the tissue weighting factor). Q is also called the Relative Biological Effectiveness (RBE). The factor N depends upon the part of the body irradiated, the time and volume over which the dose was spread, and the species of the subject.

The currently accepted, approximate Q factors are provided in Table 1.

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TABLE 1. Relative Biological Effectiveness Factors (Q) for Different Radiation Types



Radiation typeQ
X-rays1
Gamma rays1
Beta particles1
Thermal neutrons (<10 keV)5
Fast neutrons (10 keV to 100 keV)10
Fast neutrons (100 keV to 2 MeV)20
Fast neutrons (2 MeV to 20 MeV)10
Fast neutrons (>20 MeV)5
Protons (>2 MeV)5
Alpha particles20
Other atomic nuclei20


The currently accepted values for the tissue-weighting factor N for human body parts are provided in Table 2.

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TABLE 2. Tissue-Weighting Factors N for Body Parts



Body partN
Continued on next page...
Gonads0.20
Bone marrow0.12
Colon0.12
Lung0.12
Stomach0.12
Bladder0.05
Brain0.05
Breast0.05
Kidney0.05
Liver0.05
Muscle0.05
Esophagus0.05
Pancreas0.05
Small intestine0.05
Spleen0.05
Thyroid0.05
Uterus0.05
Bone surface0.01
Skin*0.01

  • *The weighting factor for skin implies a whole body exposure.


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