A gamma ray interacting with a scintillator produces a pulse of light that is converted to an electric pulse by a photomultiplier tube (PMT). The PMT consists of a photocathode, a focusing electrode, and 10 or more dynodes that multiply the number of electrons striking at each dynode. A chain of resistors typically located in a plug-in tube base assembly biases the anode and dynodes. Complete assemblies including the scintillator and PMT are available.

The properties of a scintillation material required to produce a good detector are transparency, availability in large size, and large light output proportional to gamma-ray energy. Few materials have good properties for detectors. Thallium-activated sodium iodide [NaI(Tl)] and cesium iodide [CsI(Tl)] crystals are commonly used, as well as a wide variety of plastics. CsI(Tl) and plastics have much faster light decay times than NaI(Tl) and are primarily used for timing applications.

The high Z of iodine in NaI(Tl) crystals result in high efficiency for gamma-ray detection. Resolution for a 3-inch diameter by 3-inch length crystal is typically about 7% for 137Cs and slightly worse for larger sizes. The light decay time constant for a NaI(Tl) crystal is about 0.23 µs. Typical charge-sensitive preamplifiers translate this into an output voltage pulse with a rise-time of about 0.5 µs. Fast coincidence measurements cannot achieve the very short resolving times that are possible for plastic scintillators, especially at low gamma-ray energies.

The Model 905 Series offers various sizes of Nal(TI) detectors. These detectors are assembled and supplied to ORTEC by several companies that specialize in the manufacture of Nal(TI) detectors.

Typical, total counting efficiencies given as a percentage for a 1-µCi 137Cs source centered at a distance of 10-cm:
905-1 (1” x 1”): 0.17 @0.5 MeV and 0.09 @2.0 MeV
905-3 (2” x 2”): 0.75 @0.5 MeV and 0.45 @2.0 MeV
905-4 (3” x 3”): 2.00 @0.5 MeV and 1.30 @2.0 MeV

NOTE: Special sizes and configurations such as Wells, Thin-Window X-Ray, Thyroid Counters, and Large Slabs are available on special order.

905-3 NaI Scintillation Detector, 2- x 2-in. crystal, 2-in. tube