Fundamentals of scintillation counting

• Published in: Seminars in nuclear medicine Vol. 3; no. 3; pp. 205 - 223
• Main Author: Cradduck, Trevor D.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.07.1973
• Subjects: Radionuclide Imaging - instrumentation; Scintillation Counting - instrumentation; Spectrometry, Gamma
• ISSN: 0001-2998; 1558-4623
• DOI: 10.1016/S0001-2998(73)80017-0

The operation of a scintillator detector depends upon the detection of fluorescent radiation emitted from the phosphor following absorption of ionizing radiation. In organic phosphors this fluorescence takes place at the molecular level. In in-organic phosphors the fluorescence is dependant upon the crystal structure and impurities have to be present in concentrations of between 0.1% and 0.5% in order to cause irregularities in the crystal lattice. These irregularities act as emission centers for the luminescence. The scintillation is detected by a photomultiplier coupled optically to the scintillator. The function of the photomultipler is to convert light photons into photoelectrons and accelerate these between a number of dynodes in order to cause electron multiplication to take place. The resultant electrical pulse is directly proportional to the amount of energy deposited in the scintillator. It is this latter property that facilitates energy discrimination using scintillation detectors. The operation of sodium-iodide thallium-activate scintillation detectors may be broken down into five distinct stages. Each of these stages has been studied in detail, and it has been established that there are many factors that cause the performance of scintillation detectors to be less than ideal. Very little control can be exercised over some of these factors. On the other hand, improvements in crystal manufacture, crystal packaging, and photomultiplier performance can have profound effect on the overall performance characteristics. Some of these improvements have been mentioned. and others will undoubtedly occur, though it would be presumptuous at this time to suggest what these might be. Despite its deficiencies, the sodium-iodide thallium-activated detector remains one of the most versatile and sensitive detectors at our disposal. Semiconductor detectors are playing an increasing role in nuclear medicine, but the statistics of counting in clinical situations remains the major problem in nuclear medicine. The scintillation detector will continue to play the major role so long as its sensitivity remains so much greater than that of semiconductors. This review deals primarily with the design factors controlling the overall performance of a basic scintillation detector. Other external factors can also affect its day-to-day performance. Every scintillation detector must be supplied with a high voltage (500–1000 V) applied to the photomultiplier to provide the accelerating voltage between dynodes. Most detectors need preamplifiers to provide an impedance match so that the pulse can be driven through a long cable to a relatively low input impedance amplifier. This latter amplifier is expected to provide some degree of pulse shaping and to boost the amplitude of the pulse to a sufficiently large level (say 5–10 V) so that pulse height discrimination may be applied by a single-or multichannel analyzer. The scintillation detector is only one link in the chain and the final result of any counting situation is dependant upon each of the links-not the least of which is the operator. The proper care and maintenance of the associated electronics by the operator is the subject of a separate paper by Harris in this seminar 32