Which radioisotope is most commonly used in gamma counting and what is its approximate half-life?

• A. I-125; 60 days
• B. I-131; 8 days
• C. Tc-99m; 6 hours
• D. P-32; 14 days

Answer: (A)

Counting with a gamma counter relies on using a radionuclide that emits gamma photons in a way that detectors can easily pick up, and that has a half-life long enough to allow practical handling and calibration. Iodine-125 fits this well: it emits low-energy gamma rays (about 35 keV) that are efficiently detected by common gamma counters, and it has a half-life of roughly 60 days, so a single source remains usable for weeks to months, making calibration and routine measurements convenient.

In contrast, iodine-131 also emits gamma rays (around 364 keV) but has a much shorter half-life of about 8 days and is chiefly used for therapy and diagnostic imaging, which makes it less ideal as a stable counting source. Technetium-99m is ideal for imaging due to its 140 keV gamma emission and a very short half-life of about 6 hours; this short life makes it impractical as a general counting source that you’d want to reuse. Phosphorus-32 is primarily a beta emitter and provides little to no gamma signal for a gamma counter, so it isn’t suitable for gamma counting either.

So the isotope most commonly used in gamma counting, with a convenient and long enough half-life, is iodine-125 at about 60 days.