What term describes the degree of concentration of radioactive material in a gamma-ray source?

The term that best describes the degree of concentration of radioactive material in a gamma-ray source is specific activity. Specific activity refers to the radioactivity of a material per unit mass, which allows for the comparison of different isotopes or radioactive materials based on how much radiation they emit relative to their mass. It is crucial in applications like radiographic testing, as it helps determine the efficacy and safety of the gamma-ray source being used.

Radioactivity generally refers to the phenomenon of radioactive decay itself, while decay rate is a measure of how quickly unstable isotopes undergo decay, and isotopic concentration deals with the amount of a specific isotope present in a sample but does not quantify the radioactivity per mass. Specific activity ties together the concepts of mass and activity, making it the most precise term in this context.