Which counter type is typically used for gamma-emitting isotopes like I-125 and I-131?

• A. Crystal gamma counter
• B. Liquid beta counter
• C. Gas proportional counter
• D. Scintillation counter

Answer: (A)

Gamma rays are detected most effectively with detectors that can stop photons and convert their energy into a measurable light signal. A crystal gamma counter does exactly that: it uses a dense scintillation crystal, typically sodium iodide doped with thallium (NaI(Tl)), which produces flashes of light when gamma photons interact within the crystal. These light pulses are then sensed by a photomultiplier tube and converted into electrical pulses proportional to the gamma emission rate.

This setup works well for isotopes like I‑125 and I‑131 because it provides high gamma interaction probability and good counting efficiency across their energies (I‑125 around 35 keV and I‑131 at 364 keV). In contrast, a liquid beta counter is optimized for beta particles in liquids, a gas proportional counter detects mostly charged particles with lower efficiency for gamma, and while scintillation counters can count gamma, the crystal-based approach is the standard in clinical labs due to its superior stopping power and energy discrimination for gamma photons.