In scintillation counting, a crystal gamma counter is used to detect gamma-emitting isotopes such as which of the following?

• A. I-125 and I-131
• B. H-3 and C-14
• C. U-235 and Pu-239
• D. Sr-90 and Cs-137

Answer: (A)

A crystal gamma counter is designed to detect gamma photons, not beta or alpha particles. It uses a scintillating crystal (like NaI(Tl)) that emits light when a gamma ray interacts with it; this light is converted to an electrical signal and counted. Only isotopes that emit gamma radiation produce the signals this detector can reliably measure.

Among the options, iodine-125 and iodine-131 are well-known gamma emitters, releasing gamma photons at energies that the scintillation crystal detects efficiently. The other choices are dominated by beta or alpha emissions, which do not produce the characteristic gamma signals the crystal gamma counter is optimized to count. While some isotopes may emit small gamma components, the pair that clearly provides true gamma emissions suitable for this detector is iodine-125 and iodine-131.