Long-Term Drift Analysis for Gamma Spectroscopy

Los Alamos National Laboratory has developed a nondestructive assay instrument for continuous, unattended measurement of the enrichment of UF6 gas flowing in process pipes. The most recent generation of instrumentation uses commercially available 3-inch by ½-inch NaI detectors and digital multichannel analyzers (MCAs). While monitoring these instruments, we found that the decay-corrected net count rate from a transmission check source was not stable over periods of weeks. Manually looking for correlated trends in analysis and hardware values for many runs (~150) and for long periods of time (~30 days) is very time consuming and error prone. However, we have developed software that greatly reduces both the person-time and the frequency of errors by allowing the user to easily and quickly examine the value of many parameters over long periods of time. We have achieved this by placing many (~100) parameters in the header of each spectrum file and creating a program that will allow the user to select any of these parameters to extract and plot as a function of time. The parameters in the header include hardware values such as gain-stabilizer correction, high voltage, and amplifier shaping parameters; values used for analysis such as region-of-interest designations, background count rate, and correction factors; and calculated values such as gross- and net-peak counts, calculated enrichment, and associated error. We have extended our data-acquisition software to allow it to monitor any hardware parameter available from the MCA. Additional parameters can be monitored and recorded by simply adding the appropriate MCA command to the program’s initialization file with any text editor, such as Microsoft Notepad. The acquisition program can monitor and record a parameter’s value at the end of each run or periodically sample hardware parameters during a run to collect and record statistics on the parameter’s stability during a single run. Additionally, any of the parameters in the spectrum header can be tabulated and exported to a text file for importing into Excel for more detailed analysis. This analysis tool has proven very useful for both quick in-the-field checks on instrument performance and as an indepth analysis tool in the laboratory.