Which statement differentiates gross activity from specific activity?

• A. They are identical concepts.
• B. Gross activity is total decays per unit time; specific activity is activity per unit mass or volume.
• C. Specific activity is time-integrated exposure.
• D. Gross activity is activity per mass; specific is total decays per unit time.

Answer: (B)

Activity is the rate at which a source undergoes radioactive decays, measured in decays per second (Becquerels). Gross activity refers to the total decay rate of the entire sample—basically, all the activity coming from all the material combined. Specific activity, on the other hand, takes that same rate and divides it by the amount of material, giving a rate per unit mass or per unit volume (for example, Bq/kg or Bq/L). This normalization lets you compare samples with different masses or densities on an equal footing.

So, the statement that gross activity is the total decays per unit time and specific activity is the activity per unit mass or volume correctly captures the distinction. The other options either treat the concepts as identical, misstate what specific activity represents (time-integrated exposure is not the same as activity per mass/volume), or reverse the definitions.