Scintillation Detectors

The scintillation detectors described here are intended to accompany packages of related ORTEC electronics for teaching purposes or similar applications. These selected detector models are supplied to ORTEC by various companies that specialize in that technology.

Scintillation Detectors

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

The properties of a scintillation material required for good detectors are transparency, availability in large size, and large light output proportional to gamma-ray energy. Few materials have good properties for detectors. Thallium-activated Nal and CsI crystals are commonly used, as well as a wide variety of plastics. Both Nal and CsI require a small amount of thallium for proper operation. Nal is the dominant material for gamma detection because it provides much better gamma-ray resolution than cesium iodide (CsI) or plastic. However, CsI and plastic have much faster pulse light decay and find use in timing applications.

Nal Scintillation Detectors (Model 905 Series)

The high Z of iodine in Nal results in high efficiency for gamma-ray detection. A small amount of TI is added in order to activate the crystal, so that the designation is usually Nal(TI) for the crystal. The best resolution achievable is about 7% for the 662-keV gamma ray from ¹³⁷Cs for a 3-in.-diameter by 3-in.-long crystal and slightly worse for smaller or larger sizes.

The light decay time constant in Nal is about 0.23 µs. Typical charge-sensitive preamplifiers translate this into an output pulse rise time of about 0.5 µs. Fast coincidence measurements cannot achieve the very short resolving times that are possible with plastic, especially at low gamma-ray energies. The Model 905 Series offers various sizes of Nal(TI) detectors (see Ordering Information). These detectors are assembled and supplied to ORTEC by several companies that specialize in the manufacture of Nal(TI) detectors.