Older gamma-ray detectors use the Geiger-Mueller counter principle, but have been mostly replaced thallium-doped sodium-iodide (NaI) scintillation detector, which has a higher efficiency. NaI detectors are usually composed of a NaI crystal coupled with a photomultiplier. When gamma ray from formation enters the crystal, it undergoes successive collisions with the atoms of the crystal, resulting in a short flashes of light when the gamma-ray is absorbed. The light is detected by the photomultiplier, which converts the energy into an electric pulse with amplitude proportional to the gamma-ray energy. The number of electric pulses is recorded in counts per seconds (CPS). The higher the gamma-ray count rate, the larger the clay content and vice versa.
Primary calibration of gamma-ray tool is the test pit at the University of Houston. The artificial formation simulate about twice the radioactivity of a shale, which generates 200 API units of gamma radiation.  The detector crystal is affected by hydration and its response changes with time. Consequently, a secondary and a field calibration is achieved with a portable jig carrying a small radioactive source.