Gamma emitters with short half-lives enable safe external detection

Steps:

• Identify detection needs: Tracer must reach detector 20 cm away, requiring high penetration through body and air.
• Eliminate alpha emitters: Alpha particles have low penetration, stopped by skin or short distances.
• Select gamma emitters: Gamma rays penetrate deeply, allowing external detection.
• Choose half-life: Few hours balances activity duration with minimal patient exposure.

Why C is correct:

• Gamma rays have high penetrating power (definition: electromagnetic radiation with energy >100 keV, travels meters in air), enabling detection 20 cm away; short half-life limits dose per ALARA principle.

Why the others are wrong:

• A: Alpha particles lack penetration (stopped within mm in air), undetectable externally.
• B: Alpha particles can't reach detector; long half-life prolongs unnecessary exposure.
• D: Long half-life causes extended radiation dose without benefit for short exam.

Final answer: C