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Detailed introduction
Scintillation materials used to produce good detectors have characteristics such as transparency, large size, and large light output proportional to the gamma ray energy. Few materials have good properties for use in detectors. Commonly used materials are activated [NaI(Tl)] and cesium iodide [CsI(Tl)] crystals, as well as a variety of plastics. CsI(Tl) and plastic have much faster light decay times than NaI(Tl) and are primarily used in timing applications.
The high Z iodine in the NaI(Tl) crystal ensures high efficiency in gamma ray detection. For 137Cs, resolution is typically around 7% for 3" diameter and 3" long crystals, slightly worse for larger sizes. The light decay time constant of NaI(Tl) crystal is about 0.23 μs. A typical charge-sensitive preamplifier converts this into an output voltage pulse with a rise time of approximately 0.5 μs. Fast coincidence measurements cannot achieve short resolution times with plastic scintillator, especially at low gamma ray energies.
The 905 Series has Nal (TI) detectors in various sizes. These detectors are assembled and supplied to ORTEC by several companies specializing in the production of Nal (TI) detectors.
Typical total counting efficiency given in percent for a 1-μCi 137Cs source centered at a 10-cm distance is as follows:
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-rays, Thyroid Counters and Large Plates are available for special order.
