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07 November 2009

Answer to Question #7385 Submitted to "Ask the Experts"

Category: Radiation Basics — Photons

The following question was answered by an expert in the appropriate field:

Do you know of a publication listing the current "gamma exposure constants" for point sources? I am interested in a definitive reference that reflects current scientific research and a best estimate of exposure levels away from a point source of radioactive material.

You can review Question 3269 and its answer on the HPS Ask the Experts Web site as a starting point. It includes reference to what I believe is still the most comprehensive listing of gamma-dose constants that is still in use—that published in 1982 by Oak Ridge National Laboratory (ORNL) as Publication ORNL/RSIC-45/R1, which is available on the ORNL Web site.

The values given in this publication are dose equivalent rates, in mSv h^-1 at one meter from respective point isotropic sources of 1 MBq. To obtain dose equivalent rates in rem h^-1 mCi^-1 at one centimeter, we would multiply the table values by 3.7 x 10⁴, and to obtain rem h^-1 Ci^-1 at one meter we would multiply the table values by 3.7 x 10³. With some manipulation you can convert the dose equivalent values to air kerma or exposure values. (If you want to do this you might want to review and use the original dose equivalent rates per unit gamma fluence rates as a function of gamma energy as discussed in the Oak Ridge presentation.)

There are other lesser compilations that might be useful for specific purposes, and you can investigate some of these on the Internet. For example, a 2005 paper by M.M. Ninkovic, J.J. Raicevic, and F. Androvic in Radiation Protection Dosimetry presents air kerma rate gamma constants for 35 radionuclides used in some medical applications. A recent paper, "The Air-Kerma Rate Constant: Application To Air-Kerma Measurements for Homeland Security" by L. Pibida, R. Minniti, L. Lucas, and S.M. Seltzer, describes the measurement of air kerma rates from sources of ⁵⁷Co, ⁶⁰Co, and ¹³⁷Cs and is available in the Health Physics Journal (Volume 94(2), February 2008, pp 126-133) and can be accessed online if you are a Health Physics Society member.

If you cannot find the gamma constant for the radionuclides you are interested in, you can calculate the values yourself as long as you have the decay data that give the yields and energies of the emitted photons. There is an information sheet available on the ATE Web site that discusses how to calculate exposure rates from point isotropic sources.

Good luck.

George Chabot, PhD, CHP

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