Units check in nuclear science: which of the following is a unit of radioactivity (historic non-SI unit)?

Introduction / Context:
Radioactivity measures the rate of nuclear disintegrations per unit time. While the SI unit is the becquerel (1 Bq = 1 decay per second), older or non-SI units still appear in literature and exams. Recognising which unit corresponds to activity prevents confusion with length or area units used elsewhere in nuclear engineering.

Given Data / Assumptions:

• Focus is on the physical quantity “activity.”
• Options include various nuclear-related units and constants.

Concept / Approach:
The curie (Ci) is a traditional unit of radioactivity defined as 3.7 * 10^10 decays per second, historically tied to the activity of 1 gram of radium-226. Other listed units are not activity: barn (nuclear cross-sectional area), fermi (length scale ~ 1 fm = 10^-15 m), angstrom (length scale = 10^-10 m).

Step-by-Step Solution:
Identify the quantity: activity = disintegrations per second.Recall the unit: curie (historical), SI unit is becquerel.Select “curie.”

Verification / Alternative check:
Conversion: 1 Ci = 3.7e10 Bq; 1 mCi = 3.7e7 Bq; 1 μCi = 3.7e4 Bq. These are standard in radiological tables.

Why Other Options Are Wrong:
Barn: unit of cross-section (area).Fermi and angstrom: units of length at nuclear and atomic scales, not activity.

Common Pitfalls:
Confusing becquerel (SI) with curie (non-SI) or mixing up barn with activity because both appear in reactor calculations.

Final Answer:
curie