Time resolved liquid scintillation counting: (TR-LSC™) for environmental 3h analysis

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Courtesy of PerkinElmer, Inc.

Environmental analysis of many naturally occurring and man-made, beta emitting radionuclides, including 3H, 14C, 90Sr/90Y, 241Pu, and 234Th is a significant analytical challenge due to the extremely low activity levels encountered. Under these circumstances, reducing instrument background is essential for improving analytical sensitivity.

Time-Resolved Liquid Scintillation Counting (TR-LSC), a feature available on PerkinElmer Tri-Carb Liquid Scintillation Analyzers, is a form of pulse shape analysis that has proven to be an effective tool in such analyses. TR-LSC increases counting sensitivity by reducing the non-quenchable component of instrument background that is caused by the interaction of high energy cosmic radiation with the sample vial and phtotomultiplier (PMT) glass face. Other non-quenchable background interference including PMT after-pulsing, and natural radiation in the PMT and vial construction materials are also reduced by TR-LSC. Approximately 68% of the observed total background is non-quenchable and therefore can be greatly reduced by this technique1.

Non-quenchable background events can be distinguished from true beta events since they are characterized by a series of low amplitude after-pulses that follow the initial prompt pulse1. True scintillation pulses have fewer after-pulses associated with them. Figure 1 shows a 3-D graphical representation of a background pulse (A) and 3H beta pulse (B). Note that the background pulse has more prominent after-pulse characteristics that extend over time (z axis). TR-LSC is designed to reduce background by evaluating each event for the presence of these afterpulses, and when a series of after-pulses is detected, TR-LSC characterizes the event as background and rejects it. The efficacy of this rejection can be increased when a bismuth germanium oxide (BGO) detector guard is used in conjunction with TR-LSC electronics. The BGO guard completely surrounds the sample vial and acts as a cosmic guard to further increase after-pulse rejection and thus reduce background.

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