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Re: Livermore Pu Measurements, Detection Limits andUncertainty
Erik Nielsen wrote:
>>>>>>>>>>
Radiochemistry analytical results are composed of three numbers: the
result, an analytical uncertainty and a Lower Limit of Detection (LLD) or
MDC (Minimum Detectable Concentration).
When interpeting analytical results, the first step is to compare the
result to the LLD/MDC. This is usually calculated at the 95% confidence
level (4.66 constant) that a result greater than the LLD/MDC is real. Once
this decision is made, the uncertainty is used to bracket the true value
within a certain degree of statistical confidence (1 sigma=65%, 1.96
sigma=95%).
This is supposed to be a two part process. The fact that the uncertainty
subracted from the analytical result may be less than the LLD/MDC does not
mean that the activity is not present.
Erik C. Nielsen
<<<<<<<<<<
Erik, and others:
My recollection is that the process of determining if a given analytical result should be considered "real" or "detected" is slightly different from what you described. When one has in hand an analytical result and associated statistical information, the result should be compared to a "critical level." The critical level is set at a value such that if the true activity (or concentration, or counts, etc) in a sample was zero, the probability of falsely concluding that activity is present would be small (perhaps 5%, etc.). A lower limit of detection or limit of detection (LLD/MDC, etc.) is then determined, in advance of measurements, as the activity (or concentration, etc.) in a sample that is expected to be reported as "detected" most of the time (perhaps 95%, or other). That is, a true sample activity at the LLD is very likely to result in a measured activity at least as great as the critical level (so would be considered detected).
The point is that the LLD is determined in advance, and tells you what level of activity is expected to be detected by the measurement sytstem. In contrast, the critical level is what a measurement is compared to to actually determine if that sample is considered detected. The LLD is generally greater than the critical level. Thus, one could have the result that some values that are less than the detection limit are reported as detected results.
Additional information is given in a classic paper on this subject by L.A. Currie (1968, "Limits for Qualitative Detection and Quantitative Determination," in Analytical Chemistry, Volume 40(3), pages 586-593 [I hope this is correct--don't have paper in front of me]). Some discussion is also given in a draft NRC document: NUREG-1507, minimum Detectable Concentrations with Typical Radiation Survey Instruments for Various Contaminants and Field Conditions, published 1995.
I hope I have not muddied the water too much.
Duane.
My opinion, not the agency's.
Duane W. Schmidt, CHP
U.S. Nuclear Regulatory Commission
Office of Nuclear Material Safety and Safeguards
Division of Waste Management
Uranium Recovery Branch
mailto: dws2@nrc.gov
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