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18 Radiation for Radionuclide Users counting. Liquid scintillation counting has become an essential tool of research involving radionuclides such as 3 H and 14 C. Counting Efficiency An active radiation detection system can never see 100 percent of the disintegrations occurring in a given radioactive sample. This is due to numerous factors related to both the particular counting system and the specific radionuclides in the sample. The counts per minute (cpm) displayed by the counter must therefore be distinguished from the disintegration rate (dpm) of the sample. The ratio of the count rate (cpm) to the disintegration rate (dpm) expressed as a percent is the efficiency of the counting system. Eq. 6 Efficiencies of a particular radiation counting system for various radionuclides can be determined through calibration of the system with standards of these same radionuclides. Because every counting system will register a certain number of counts from environmental radiation and electronic noise in the counter (referred to as the instrument’s background), a more correct formula is: Eq. 7 Example 2: A sample containing a 14C labeled amino acid is counted in a liquid scintillation counter. The sample count rate is 1200 cpm and the background is 30 cpm. If the counter is 85% efficient for 14C, what is the activity within the sample? γ, χ rays scintillation (light flash) photocathode e - e - Photomultiplier (large pulse) Diagram of a scintillation detector