Performance-Specifications-for-Instrumentation-Systems

MS-PC 2015: Performance Specifications for Instrumentation Systems Designed to Measure Radon Gas in Air Page 4 of 20 4 Definitions & Abbreviations Continued Lower Limit of Detection (LLD CT ), Counting Technology Methods – the smallest net count rate at which there is 95% confidence that a signal above background is detected. For the purpose of this standard, and for devices that rely on counting technology, the following equation is used (Currie, 1968): LLD CT = [2.71/t s + 3.29(R b /t b + R b /t s ) ½ ] where LLD CT = Lower Limit of Detection (cpm) for counting technology methods t s = Sample counting time (min) R b = Background or blank count rate (cpm) t b = Background or blank counting time (min) For continuous radon monitors, the sample counting time is the time spent making a radon measurement, and the background count rate and counting time are determined when measuring an atmosphere free of radon, such as nitrogen or aged air. For charcoal devices, the sample counting time is the time spent counting the sample in the laboratory. The blank count rate and blank counting time are determined from counting a blank sample in the laboratory. For alpha-track detectors, the parameters are defined as follows: t s = Area of sample scanned (mm 2 ) R b = Blank sample track density (tracks/mm 2 ) t b = Area of blank sample scanned (mm 2 ) Lower Limit of Detection, Noncounting Technology Methods (LLD NCT ) – the smallest measure at which there is a specified degree of confidence that a signal above background is detected. For consistency with the LLD CT for radioactive decay counting methods, a value of k=3.29, or 3.3, is used for purposes of this standard. This results in an LLD above which there is at least a 95% confidence that the measured value is not due to background. This LLD NCT value is calculated by the following equation: LLD NCT = ! + ! where ! is the mean of the blank measures, ! is the standard deviation of the blank measures, and k is a numerical factor chosen according to the confidence desired (IUPAC Compendium of Chemical Terminology, 2013; ISO 11843). Manufacturer – individual or organization engaged in the design, manufacture or sale of commercial instrumentation. May – signifies an acceptable method or good practice. Measurement Method – the combination of air sample collection system design, detector technology and analysis procedure, including software, used in the instrumentation to make radon measurements. Minimum Detectable Concentration (MDC) – the lowest concentration that is detectable above background with 95% confidence. This concentration is derived from the LLD by applying the same factors that are used to convert the sample net count rate to radon concentration or integrated concentration. For a CRM, the LLD is divided by the calibration factor to obtain the MDC. For charcoal devices, the net count rate may be divided by factors that take into account such parameters as the adsorbed moisture, the duration of the exposure, the system counting calibration factor, and radioactive decay.