Nuclear measurements: the radioactivity (decay rate) of an isotope is commonly expressed in which unit?

Introduction / Context:
Radioactivity quantifies the number of nuclear disintegrations per unit time. Historically and in many references, the Curie (Ci) is used, while the SI unit is the Becquerel (Bq).

Given Data / Assumptions:

• We are asked for the commonly used unit for activity (decays per second).
• Options include units for cross-section, energy, and activity.

Concept / Approach:
The Curie is defined as 3.7 × 10^10 disintegrations per second, originally based on the activity of 1 gram of radium-226. The SI unit Becquerel equals 1 disintegration per second. A barn measures nuclear reaction cross-sections; MeV and erg are energy units.

Step-by-Step Solution:

Verification / Alternative check:
Dimensional analysis and definitions confirm that Bq and Ci measure events per time, whereas barns and MeV/erg do not.

Why Other Options Are Wrong:

• Barn: area unit for cross-sections.
• MeV/erg: energy units.
• Becquerel: correct SI unit, but the specific correct choice here is Curie as per the option list focus.

Common Pitfalls:
Confusing cross-section (barn) or energy (MeV) with activity.

Final Answer:
Curie