This project evaluated the performance of a Large Area Plastic Scintillation/Global Positioning System (LAPS/GPS) to determine its reliability and cost effectiveness as a radiation-surveying tool. The LAPS/GPS features a lap top computer that logs and correlates real-time beta and gamma radiation data to latitude and longitude coordinates and that generates a graphical representation of the near surface radionuclide concentrations. The software also applies radionuclide-specific correction factors, which turns the LAPS from a qualitative tool into a quantitative analytical instrument capable of converting count rates into units of pCi/g. The LAPS can detect beta particles greater than 300 kiloelectron volt (keV) and gamma photons greater than 40 keV. This system reduces the number of soil samples collected for offsite analysis, guides soil excavation activities, and provides a rapid survey of shallow beta and gamma radionuclide concentrations relevant to applicable cleanup levels.
The system was tested at three Department of Defense (DoD) sites with known radioactive contamination consisting of depleted uranium (DU), DU radioisotopes U 234, U 235, and U 238, and Thorium 232 (Th-232) previously determined by real-time high purity germanium detectors, in-situ gamma spectroscopy, and conventional laboratory analyses. The LAPS/GPS calibrated minimum detectable activity (MDA) of 5 pCi/g, which was well below cleanup goals that ranged from 35 to hundreds of pCi/g. Statistically, good agreement was observed between the LAPS and the high purity germanium detector.
This system is easily deployed by a crew of two people, can be mounted on a four-wheel drive vehicle or a hand drawn cart, and can survey a 12 to 25-acre site in one day depending upon site conditions. The system requires little maintenance and typical repairs are made within one day. A three-day LAPS/GPS survey over a 60-acre site with verification samples analyzed offsite by gamma spectroscopy would cost approximately $17,900. This is approximately 25% of the typical radiation site survey costs estimated at $74,200, assuming the LAPS/GPS survey resulted in a 90% sample analysis reduction. The LAPS/GPS survey can also provide a graphical representation of the site data within 3 to 4 days as compared to several weeks with conventional means. The LAPS/GPS can be purchased for $21,000.
The LAPS/GPS is a survey tool that does not eliminate the need for conventional soil sampling and analysis. The system can only survey surface contamination and must be calibrated for each anticipated radioactive isotope. Detection limits are influenced by the distance from the LAPS to the ground (e.g., several inches) and the travel velocity (e.g., 2 to 4 miles per hour). (Project Completed - 2000)