What is specific activity and how is it used in dose and source characterization?

• A. Energy per unit mass.
• B. Activity per unit mass or volume; used to compare materials and estimate dose from a given mass of radionuclide.
• C. Total activity of a source regardless of mass.
• D. Activity per unit time per unit area.

Answer: (B)

Specific activity is the amount of radioactivity per unit mass (or per unit volume for a given density). This normalization lets you compare different materials containing radionuclides on a like-for-like basis, regardless of how much material you have. It also lets you estimate dose from a given mass: if you know the specific activity and the mass, you can compute the total activity, and then use appropriate dose conversion factors to estimate the dose.

For example, a material with a specific activity of X Bq/kg means that 1 kg of that material has X Bq of activity. If you have m kg, the total activity is X × m Bq, which is what you’d use to estimate dose for that amount of material.

The other ideas described don’t fit this concept. Energy per unit mass describes energy content rather than activity; total activity ignores how much material you have; and activity per unit time per unit area describes a flux or surface emission rate, not the per-mass normalization used for comparing materials and estimating dose from a given mass.