Sample Prepartion  Analysis
TRAC employs an integrated technical methodology that leads to excellent detection levels of many artificial radionuclides potentially released from a nuclear facility into its surroundings. The key elements of TRACís methodology are
In Field Methods
Sample Media: TRAC examines potential sample material along an accessible pathway and selects the most favorable sample medium. Vegetation is TRACís preferred sample medium because of its availability, its ease of collection and processing, and its tendency to bio-accumulate artificial radionuclides (individual radioactive isotopes). Thus, vegetation samples are often good indicators of the presence or absence of radioactivity of concern. Aquatic mosses, kelp, and sorrel are among TRACís favorite sample media. TRAC samples other media to check pathways of particular concern. Examples of such sample media are algae, fish, water, sediment, dust, grass, mulberry tree leaves, and even one sample of road-kill squirrel tails.
TRAC prepares all samples to a standard 35 gram / 130 milliliter geometry, filling a standard PET laboratory bottle for which TRACís in-house spectrometer was specially designed to accept. Vegetation, sediment, and most other sample media are ordinarily oven dried <100 C. Water samples in the range of 10 to 100 liters are quiescently evaporated in a special microwave oven to a paste, on plastic film. TRAC develops special processing procedures for samples of particular interest. For one example, mulberries were made into jam before analysis. For another example, colloids were stripped off sediment grains before analysis of colloidal radioactive wastes sticking to the sediment particles.
TRAC bulks up under-sized samples to the standard geometry with bagged, synthetic cosmetic puffs dampened with distilled water. Laboratory bottles are slowly filled and then wiped to minimize dust. The contents of the filled and sealed laboratory bottles are dry and non-acidified, to minimize release of gases to the detector during analysis. Laboratory bottles are placed in thin plastic bags before loading into the spectrometer, as a final barrier against contamination of the detector. Room air on both sides of the laboratory entrance is filtered to remove dust.
Analysis TRAC specializes in a single analysis, developed over the past 15 years. The heart of this analysis is a custom designed, broad-band, multiply stabilized, efficient, sodium-iodide, deep well detector, situated in a copper-lined lead shield placed in a temperature-controlled housing, located in TRACís temperature-controlled laboratory.
TRAC routinely counts a sample for 23 hours, in a Canberra Genie 2K spectrometer having a window of 8,000 energy channels. The energy calibration of the sample is held invariant during acquisition by fine gain stabilization on the 1461 KeV emission from naturally occurring potassium-40. The source of this stabilization peak is an x-ray shielded capsule of potassium chloride placed underneath the electronics below the sodium-iodide detector.
After the spectrum is counted, it is transformed to constant photopeak width (CPW) of three channels (FWHM), with custom software. This raw acquired spectrum is 165 energy channels wide, with an energy window of 3 to 3,000 KeV.
TRACís spectral acquisition system is conducive to true spectral subtractions. The first spectrum routinely subtracted from a raw acquired sample spectrum is a long-acquisition blank. Blank is subtracted according to the ratio of sample acquisition time to blank acquisition time. The second spectrum subtracted is potassium-40 (in natural potassium chloride). Potassium-40 is subtracted according to the ratio of the sample photopeak counts at 1460 KeV to the reference photopeak counts at that energy.
TRAC maintains reference materials in secular equilibrium for the natural thorium decay chain and the natural uranium decay chain. A substandard for the short-lived (lead-212) portion of the thorium decay chain is collected annually from airborne dust decayed from radon-220 gas. As with the potassium-40 spectral subtraction, the spectra obtained from these reference materials are sequentially subtracted from the sample spectrum according to the ratios of sample photopeak counts to the corresponding photopeak counts in the reference spectra.
After these blank and natural spectra are subtracted from a sample spectrum, that spectrum is checked for photopeaks in other reference spectra obtained from reference materials. TRAC maintains: europium-152, cobalt-60, cesium-137, strontium-90, americium-241, and technetium-99, in that order. (Strontium-90 and technetium-99 are nominally pure beta emitters. However, decays of these two radionuclides involve complex interactions that yield unique broad and narrow band energy spectra. If a sample checks positive for either strontium-90 or technetium-99, the reference spectrum is subtracted to match points.)
TRAC counts samples plausibly containing short-lived, artificial radioactivity as soon as feasible and then recounts them after several days. The second acquired spectrum is then subtracted from the first sample spectrum, yielding a ďdecay spectrumĒ for the sample. This decay spectrum is then treated to the four natural thorium and uranium reference spectra, described two paragraphs above. The processed, decay spectrum is then checked for residual photopeaks and broad band radiation. The photopeak at 478 KeV for natural beryllium-7 (53 day half-life) is usually evident in the decay spectrum. (TRAC sometimes uses this photopeak to date rainwater in the sample.) Short-lived, pure beta emitters, like phosphorus-32 (14 day half-life) are identified by the high energy cut-off of the bremsstrahlung radiation of their beta particles and by calculating their half-lives from re-recounting and re-subtraction of spectra.
Once these analyses are complete, TRAC begins the post-analysis. TRACís goal for the post-analysis is to diminish false positive and false negative results from data reports and final reports and to accurately portray uncertainties in study conclusions.
Post-analysis involves additional quality control, quality assurance, and validation steps that challenge both tentatively positive results and unaccounted residuals. Both tentatively positive and residual photopeak results are checked against TRACís in-house reference library and against other kinds of references, including, the 16th edition of the Lockheed Martin CHART OF THE NUCLIDES, Lederer and Shirleyís 7th edition of the TABLE OF ISOTOPES, and selected data from the Brookhaven National Laboratory. TRAC occasionally recounts samples as part of this procedure.
TRACís data validation involves comparison of radiological results of a set of samples from around one site. When comparing those samples, TRAC takes into consideration the nature of the facility studied, the pathways sampled, and the accumulation or decay factors expected of the sample medium. The final, pre-report check is for realism: Is the report sensible?
TRAC routinely reports all analytical results in units of picocuries per kilogram wet weight (pCi/kg wet). These units have three merits: (1) A single, consistent unit of measure aids the non-expert, concerned citizen in comprehending the results from various sample media. (2) Important bio-accumulation factors are immediately evident as the dimensionless ratios of radioactivity-in-biota divided by radioactivity-in-water. This straight forward comparability aids interpretation of processes affecting radioactivity as it moves along a pathway. (3) Data for both water and biological samples are easily referenced to drinking water standards circulated by the Environmental Protection Agency. This allows an at-a-glance assessment of which radiological results warrant follow-up or are of other concern.
TRAC takes the following steps to assure reliable, reproducible reports of radiological results: (1) TRAC tabulates sample location and sample medium with the radiological data, allowing any interested party to independently replicate TRACís work. (2) TRAC archives samples for at least three years. Archived samples are available to other laboratories to re-analyze. (3) TRAC invites technically interested parties to arrange joint sampling and independent analysis. Such arrangements have been made with the U.S. Department of Energy, the Environmental Protection Agency, and the States of Oregon and Washington, on a case-by-case basis.
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