Chimney Top Calculators

Radiation units explained: gray, rad, sievert, rem and quality factors

US regulations still use rad, rem and curies alongside SI units. Knowing which quantity a number describes matters as much as converting it correctly.

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Study text drafted from public-domain US government sources. Practice questions are reviewed before publication.

From energy to risk

Absorbed dose is the energy deposited per unit mass: 1 gray (Gy) is 1 joule per kilogram, and 1 Gy = 100 rad. Different radiations do different amounts of biological damage for the same energy, so dose equivalent multiplies absorbed dose by a quality factor. NRC regulations (10 CFR 20.1004) use 1 for x-rays, gamma rays and betas, 10 for neutrons of unknown energy and high-energy protons, and 20 for alpha particles and heavy charged particles. Dose equivalent is measured in sieverts (Sv) or rem: 1 Sv = 100 rem, 1 rem = 10 mSv.

Organs also differ in sensitivity, so tissue weighting factors combine organ doses into an effective dose equivalent. NRC limits are expressed as total effective dose equivalent (TEDE): the deep-dose equivalent from external radiation plus the committed effective dose equivalent from intakes. Deep dose is defined at 1 cm depth, shallow (skin) dose at 0.007 cm and lens dose at 0.3 cm.

Older and instrument units

The roentgen measures exposure: ionization produced in air by x-rays or gamma rays only. For gamma fields, readings in mR/h approximate mrem/h in tissue, which is why survey meters in R or rem are often treated as interchangeable for photons — but not for neutrons or alpha. The ICRP uses "radiation weighting factors" in place of quality factors; the dose unit converter on this site uses the ICRP 103 values, not the NRC quality factors.

Activity is a separate quantity: becquerels and curies tell you how many decays happen per second, not how much dose anyone receives. Converting activity to dose rate needs the emissions, geometry and shielding — which is what the gamma dose-rate calculator does.

Sample quiz

Practice questions for this section are being reviewed.

Practice questions for this section are still in review and will appear here once they have been checked.

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Frequently asked questions

What is the difference between a gray and a sievert?
The gray measures absorbed energy per kilogram. The sievert weights that dose for the type of radiation (and, for effective dose, the organs exposed) to reflect biological risk.
How do I convert mrem to mSv?
Divide by 100: 1 mrem = 0.01 mSv, so 500 mrem = 5 mSv.
Are rad and rem the same?
Numerically they are equal for gamma rays, x-rays and betas (quality factor 1), but not for neutrons or alpha particles.
What is TEDE?
Total effective dose equivalent: external deep-dose equivalent plus the committed effective dose equivalent from internal intakes. The NRC annual occupational limit is 5 rem TEDE.

Sources

  1. 10 CFR Part 20, Standards for Protection Against Radiation (U.S. NRC) www.ecfr.gov
  2. U.S. NRC Glossary (NRC Basic References) www.nrc.gov
  3. DOE Handbook: Radiological Control Technician Training, DOE-HDBK-1122-2009

Explanations on this page are original. Cited US government works are in the public domain; they do not endorse this site.