The (specific) Gamma-ray Constant is a useful numerical quantity which can be used to predict exposure in terms of Equivalent Dose per unit activity per unit distance for gamma emitters. As the term is a constant, exposure (in terms of dose rate) will vary proportionally with distance and activity values, making approximate calculations (especially from Point Sources) quite easy.

For example, the gamma-ray constant for Caesium-137 is 76 micro Sv/h per GBq at 1m from an unshielded point source. From this we can see that:

• the dose rate will be 152 micro Sv/h at 1m from a 2GBq Cs-137 source.
• the dose rate will be 19 micro Sv/h at 2m from a 1GBq Cs-137 source (using the inverse square law).

It is important to note that the gamma-ray constant is specific to a particular radionuclide.