Answer to Question #10686 Submitted to "Ask the Experts"

I would like your professional opinion with respect to an array of four single–channel analyzers (SCA) with 5 cm x 5 cm NaI(Tl) detectors that have not been calibrated (detector in a fixed geometry) since commissioning eight years ago. The only tests conducted were source checks (with ¹³³Ba) and a ratemeter electronic calibration. The SCA window is approximately 300 keV around the ¹³¹I peak.

While it is true that modern electronics and radiation detection systems are considerably more reliable and stable than some of the older systems, there is still a need for doing reasonable checks and calibrations to ensure that the performance is as expected.

Source checks and electronic calibrations of scaling devices are very helpful in demonstrating proper operability. You do not specify what is involved in the source checks that have been done, but I would recommend that they should include performing counts for a fixed time interval in a fixed geometry to evaluate accumulated counts in the window of interest. These source checks, using the same source(s), should have been performed originally after the initial full calibrations of the SCA/NaI systems. If this was done, then subsequent such checks could be relied upon to demonstrate a reasonable level of performance, based on consistent and reproducible results. The electronic calibrations of the scalers provide a good test of the linearity of the electronic scalers. It is possible, however, that some degradation of the Na(I) detectors may occur over time and some effects may best be seen when the detectors are connected to the SCAs, and counts in the window of interest are accumulated when sources of different activities (or a single source that may be positioned reproducibly at different distances from the respective detector) are used to produce a range of count rates that can be used to investigate the count rate linearity of the complete system. Naturally any degradation or drift that occurs in the electronics would also be noticeable in changes in results that may be associated with voltage changes, changes in threshold settings, changes in electronic gain, appropriate window width, etc.

In some cases in which the SCA/NaI systems are being used only for qualitative or semiquantitative analyses, the simple performance checks and electronic calibrations may be sufficient to demonstrate acceptable performance. If the results of the SCA/NaI units are being used for more exacting purposes, such as meeting operating license requirements, involving such things as evaluation of releases to the environment or contamination of work surfaces, evaluation of thyroid uptakes, etc., then more sophisticated levels of testing/calibration should be considered. In such cases, calibrations that test performance when samples of essentially the same geometry and same gamma ray energies as might be expected in the actual samples to be measured should be performed on a reasonably regular basis; at least an annual assessment using an appropriate standard is a reasonable expectation. If any significant changes are made to the system, such as replacement of a detector or changes or repairs to an SCA, or if an event occurs that might damage the system, the system should be recalibrated.

Following the calibration, a suitable check source should be positioned at a reproducible position with respect to the detector and counted for a reasonable time to accumulate several thousand counts (preferably about 10,000) in the window region of interest. When the system is being used on a routine basis the same check source should be used at least once daily when the system is to be used with the source in the same geometry and counted for the same time as had been done following calibration to evaluate the window response. The results should be recorded and preferably charted, showing the daily results, so that trends or sudden changes in response that might be associated with system degradation can be discerned. 

George Chabot, PhD, CHP