Answer to Question #1591 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

The following question was answered by an expert in the appropriate field:

Q-1: I am surveying for loose ¹⁴C and ¹²⁵I lab contamination using a Packard Tricarb TR2900. How do I calculate if I have a >95 percent confidence level for each wipe/isotope? I will be counting each wipe (approximately 130 wipes) for 2 minutes in 2 channel mode (preset Packard protocol) for ¹⁴C and ¹²⁵I and counting background for 10 minutes.

Q-2: Is there a standard minimum detectable activity (MDA) calculation I can use for liquid scintillation counting (LSC) and portable instrument applications? I will know the efficiencies (for ¹⁴C and ¹²⁵I) for each instrument and for the Packard LSC machine I will be using.

Please refer to Ask the Experts Question/Answer 666 for a discussion of liquid scintillation counting. You also need to review your operation manual for the Packard Tricarb TR2900 in detail. Iodine-125 decays will have monoenergenic conversion electrons and photoelectrons between 23 and 35 kiloelectron volts (keV), while ¹⁴C will have a spectrum of betas up to 156 keV. With ¹⁴C having an average energy of 49 keV, and a fair amount of betas above that, I'd expect the manufacturer to have window settings such that you'd get some fair separation between the two. But you may need a pure standard of each to determine any crossover if both may be on any single wipe (that is, some of the ¹⁴C betas will be in the lower energy channel). Those standards and blanks will also provide the necessary efficiency and background count data to determine disintegrations per unit time per wipe area (for example, dpm/100 cm square) for comparison to acceptable regulatory standards. There are a number of good online informational sources for statistical analysis of counting data (that is, determining standard deviation, 95 percent confidence level, and minimum detectable level given all the above background, efficiency, and count time information). See the Multi-Agency Radiological Laboratory Analytical Protocols Manual (MARLAP) for a wealth of applicable information and the Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM) rev. 1. Textbook references on radiation detection and measurement and, in particular, count statistics are also provided in Ask the Experts Question/Answer 807.

David J. Allard, CHP

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