A gamma counter is an instrument used to measure gamma radiation in a sample.
A gamma counter uses a scintillation crystal surrounding the sample to detect gamma rays: gamma rays interact with the crystal, are absorbed, and produce light; then the produced light is measured by a light detector, usually a photomultiplier tube. The energy of gamma rays that can be detected is dependent on the thickness of the crystal: thin crystals are suitable to detect low-energy gammas, while high-energy gammas need thicker crystals. Gamma counters are not portable, as they need heavy lead shielding to protect the detector from radiation sources other than the sample (for example, other samples, or background radiation of the laboratory).
Gamma counters are used to measure substances labelled with gamma-emitting isotopes. The most usual applications fall into two main groups:
- Radioimmunoassays, including RIA, IRMA and others; this includes both the use of radioimmunoassays as diagnostics tool, and the quality control of radioimmunoassays.
- Nuclear Medicine assays, such as the Schilling test, Red Cell Mass and Plasma Volume test, GFR (Glomerular Filtration Rate test), and others.