Modern probes use a Scintillation Counter that operates as follows: A scintillation crystal (generally sodium oxide, activated with thallium) emits a luminous photon when it is struck by a gamma ray. This photo-emission is transformed by a photomutiplier tube into a very weak electrical pulse that is amplified to render it measurable. The intensity of each luminous emission is proportional to the energy of the gamma rays that have produced it. The measured electrical pulse height is, therefore proportional to the energy of the incident gamma radiation.

Principle of a Gamma Scintillation Detector

The sensitivity of these counters depends on the size of the scintillation crystal in the detector. The larger the size of the crystal used, the larger the number of pulses detected.Crystal of few Cm³ enables an excellent vertical resolution. On the other hand scintillation counters are very sensitive to temperature variations and often have to be kept in thermos flasks.

Polyprobe Mount Propis