Chimney Top Calculators

ALARA, time, distance and shielding: inverse square law and half-value layers

Dose limits say how much is too much. ALARA says: whatever the limit, keep doses as low as is reasonably achievable. Time, distance and shielding are how.

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

ALARA in the regulations

10 CFR 20.1003 defines ALARA as making every reasonable effort to maintain exposures as far below the dose limits as is practical, considering technology, economics and the benefits involved. Section 20.1101 requires every licensee to run a radiation protection program that uses procedures and engineering controls to achieve ALARA doses, and to review it at least annually. NRC Regulatory Guide 8.10 describes the management commitment behind a good ALARA program.

Time, distance, shielding

Dose is dose rate multiplied by time: twelve minutes in a 50 mrem/h field gives 10 mrem, and a 100 mrem budget at 25 mrem/h allows four hours. Distance is powerful because dose rate from a point source falls with the square of distance: double the distance and the rate drops to a quarter. Close to large or extended sources the fall-off is slower, and scatter from walls and floors adds dose.

Shielding reduces dose rate exponentially. A half-value layer halves it, three HVLs leave one-eighth, and a tenth-value layer (about 3.3 HVLs) leaves one-tenth. Lead is compact because it is dense and has a high atomic number; steel and concrete need more thickness but are cheaper and structural. For neutrons, hydrogen-rich shields come first.

Postings follow the dose rate at 30 cm: a radiation area exceeds 5 mrem in an hour, a high radiation area exceeds 100 mrem (1 mSv) in an hour, and a very high radiation area exceeds 500 rad (5 Gy) in an hour at 1 m.

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Frequently asked questions

What does ALARA stand for?
As low as is reasonably achievable — keeping exposures as far below the dose limits as is practical.
How much does doubling the distance reduce dose?
For a point source, to one-quarter (the inverse square law). Tripling the distance reduces it to one-ninth.
What is the difference between HVL and TVL?
A half-value layer halves the intensity; a tenth-value layer reduces it to one-tenth. One TVL is about 3.3 HVLs.
Does exposure to a gamma source make you radioactive?
No. External gamma exposure deposits energy but does not make you radioactive. Contamination is a different problem.

Sources

  1. 10 CFR Part 20, Standards for Protection Against Radiation (U.S. NRC) www.ecfr.gov
  2. U.S. NRC Regulatory Guide 8.10, Operating Philosophy for Maintaining Occupational and Public Radiation Exposures As Low As Is Reasonably Achievable
  3. DOE Handbook: Radiological Control Technician Training, DOE-HDBK-1122-2009
  4. U.S. NRC, Radiation Basics www.nrc.gov

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