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C.2 Aspects to consider at the selection of field survey and laboratory equipment

Choose reliable instruments that are suited to the physical and environmental conditions at the site and capable of detecting the radiations of concern to the appropriate minimum detectable concentration (MDC). During survey design, it is generally considered good practice to select a measurement system with an MDC between 10-50% of the DCGL. Sometimes this goal may not be achievable based on site-specific conditions (e.g., best available technology, cost restrictions).

The minimum detectable concentration is calculated based on a hypothesis test for individual measurements (see Section 3.3.7), and results below the minimum detectable concentration are variable and lead to a high value for σ (σ is defined as the standard deviation of the measurements in the survey unit) of the measured values in the survey unit or reference area. This high value for σ can be accounted for using the statistical tests described in Section 3 for the final status survey, but a large number of measurements are needed to account for the variability.

During scoping and characterization surveys, low MDCs help in the identification of areas that can be classified as non-impacted or Class 3 areas. These decisions are usually based on fewer numbers of samples, and each measurement is evaluated individually. Using an optimistic estimation of the MDC (see Section 3.11) for these surveys may result in the mis-classification of a survey unit and cleaning up an uncontaminated area or performing a final status survey in a contaminated area. Selecting a measurement technique with a well defined MDC or a conservative estimate of the MDC ensures the usefulness of the data for making decisions for planning the final status survey. For these reasons, EURSSEM recommends that a realistic or conservative estimate of the MDC be used instead of an optimistic estimate. A conservative estimate of the MDC uses reasonably conservative values for parameters with a high level of uncertainty, and results in a MDC value that is higher than a non-conservative or optimistic estimate.

The instrument should be calibrated for the radiations and energies of interest at the site. This calibration should be traceable to an accepted standards organization such as national institutes or qualified calibration services. Routine operational checks of instrument performance should be conducted to assure that the check source response is maintained within acceptable ranges and that any changes in instrument background are not attributable to contamination of the detector. If the radionuclide contaminants cannot be detected at desired levels by direct measurement (see Section 3.3.7), the portion of the survey dealing with measurements at discrete locations should be designed to rely primarily on sampling and laboratory analysis.

Assuming the contaminants can be detected, either directly or by measuring a key-nuclide or surrogate radio-nuclide in the mixture, the next decision point depends on whether the radionuclide being measured is present in background. Gross measurement methods will likely be more appropriate for measuring surface contamination in structures, scanning for locations of elevated activity, and determining exposure rates. Nuclide-specific measurement techniques, such as gamma spectrometry, provide a marked increase in detection sensitivity over gross measurements because of their ability to screen out contributions from other sources.
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Figure B.1 Flow diagram for selection of field survey instrumentation for direct measurements and analysis of samples
Figure C.1 Flow diagram for selection of field survey instrumentation for direct measurements and analysis of samples

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Figure C.1 illustrates the sequence of steps in determining if direct measurement techniques can be applied at a particular site, or if laboratory analysis is more appropriate. Scanning surveys are typically performed at all sites. The selection of appropriate instruments for scanning, direct measurement, and sampling and analysis should be survey specific.