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A characteristic constant for a radionuclide, which is defined in terms of an ideal point source, a as <math> \Gamma _\delta =</math> <math>\frac{\iota^2\ \dot{K}_\delta}{A} </math> , where <math> \dot{K}_\delta</math>is the air-kerma rate due to photons (gamma and x rays) of energy greater than δ at a distance l in a vacuum from a point source of the radionuclide of activity A.