Radiological False Positives in Environmental Soil and Groundwater Data from Commercial Laboratories
Laboratory identification of radionuclides at environmental concentrations can easily be mistaken, because many energy interferences (coincident or overlapping spectral peaks) are possible. Conventional laboratory quality control measurements are typically not designed to test interferences found in real samples. In order to evaluate the occurrence of radiological false positives in environmental soil and groundwater samples collected at the Savannah River Site, instrument printouts, calibration records, and procedure manuals were examined between 1997 and 2001 at five commercial radiological laboratories. False positives of many radionuclides were found to be routinely reported at all five laboratories; causes vary. Magnitudes were generally between 0.1 and 3 pCi/g in soils, and between 2 and 40 pCi/L in groundwater, within the range of possible concern to regulators. The frequency of false positives varied, but for several nuclides listed below, nearly every detection reported in SRS environmental samples during the study period was judged to be false. Gamma spectroscopy: Low-level false positives of Mn-54, Zr-95, Eu-155, and Np-239 were reported in many soil samples from four laboratories, due to interference from naturally occurring Tl-208, Pb-212, and Ac-228. There were two causes. First, laboratories did not include low abundance (less than 2 per cent) peaks of Tl-208, Pb-212, and Ac-228 in the libraries used by instrument software for interference correction. Second, even when instrument software rejected the identifications, incorrectly identified nuclides were still often reported as detected, because some labs' data management systems were not able to distinguish between software-accepted nuclides and software-rejected nuclides. Magnitudes of these false positives are usually below 1 pCi/g. Alpha spectroscopy: Incomplete chemical separation and breakthrough of natural radionuclides and/or daughters of laboratory tracers generate false positives in soils up to about 5 pCi/g. Examples include natural Th-228 causing false positives of Am-241 and Pu-238; natural U-234 being mistaken for Np-237; and natural Ra-224 generating false positives of Cm-243/244. Peak taildown of tracers or other nuclides into neighboring peak windows can also generate false positives in both soils and groundwater. Examples include taildown of Th-229 tracer into the Th-230 spectral window, and U-234 taildown causing false positives of U-235. Liquid Scintillation: Incomplete chemical separation may generate false positives of virtually any Liquid Scintillation nuclide. High tritium content in Savannah River Site groundwater causes false positives of C-14. Actinides produce false positives of Pm-147. Most false positives in alpha spectroscopy and liquid scintillation counting at commercial labs are due to incomplete separation of the target nuclide from interferors. However, quality control measurements such as matrix spikes and chemical yield are of ten unable to identify cases where interfering nuclides break through into the final preparation. In addition, laboratory personnel often fail to manually interpret sample spectra, and thus do not notice peak interference when it occurs. In fact, some laboratories are unable to produce energy spectra for liquid scintillation or alpha spectroscopy, making recognition of peak interference virtually impossible. Gamma spectroscopy of environmental samples usually does not involve chemical separation, so peak interference may occur in every sample. Interference correction software appears to be ineffective at the concentrations seen in environmental samples. At all labs, careful manual review of printouts appeared to be lacking when the study was performed. Some labs have improved their data review processes since then.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC09-96SR18500
- OSTI ID:
- 821206
- Report Number(s):
- WSRC-MS-2003-00565; TRN: US200409%%18
- Resource Relation:
- Conference: Waste Management '04, Tucson, AZ (US), 02/29/2004--03/04/2004; Other Information: PBD: 2 Feb 2004
- Country of Publication:
- United States
- Language:
- English
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