Answer to Question #932 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Basic dose information, dose quantities, units

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

What is the specific gamma constant and how is it calculated? What makes one greater than another?

A Specific Gamma Ray Dose Constant (SGRDC) is a value for correlating the dose-equivalent rate (per unit of activity) for a radionuclide at a specified distance. For example, the SGRDC (in tissue) for a point source containing ¹³⁷Cs at a radial distance of 1 meter of air is 1 X 10^-4 mSv h^-1 per MBq. Using this value, a 1 MBq point source of ¹³⁷Cs will generate a dose rate of 1 X 10^-4 mSv h^-1 at one meter. These constants are calculated by estimating the energy deposition rate for all significant modes of interaction for gamma and x-ray emissions from a radionuclide. For the most part, the constant for an otherwise fixed geometry will increase with photon energy and gamma-emission probability.

Robert Coleman
Oak Ridge National Laboratory

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