Efficiency calibration and determination of performance characteristics of a large area proportional counter for the field measurement of 238Pu in soils

A large area proportional counter (LAPC) X-ray detector and a method for calibrating the detector system for 238Pu contamination in soils has been developed. The system utilizes the LAPC to measure the L X-ray triplet emitted in the decay of 238Pu at energies of 13.6, 17.13, and 20.29 keV. The LAPC has dimensions of 16.5x70.5x5.9 cm3 (LxWxH) and is filled with a mixture of 90% xenon and 10% methane at a pressure of 0.15 MPa. The detector has a thin (0.025 cm) beryllium window and aluminum support grid with an effective area of 850 cm2. The detector was calibrated for efficiency via semi-empirical methodology consisting of measurements with International Atomic Energy Agency and National Institute of Standards and Technology (NIST) traceable sources combined with fluence rate calculations using Monte Carlo and point-kernel modeling scenarios. The detector efficiency was then empirically measured using NIST traceable soil standards, and the results were compared with the semi-empirical modeled value. Operational performance characteristics including detector field of view, lower limit of detection, and energy resolution of the system were also determined through a series of controlled measurements. The LAPC and associated electronics are mounted on a pushcart with a computer and global positioning system to allow for real-time surveying and mapping of potentially contaminated land areas and detecting 238Pu contamination at concentrations of 1.9 Bq/g of soil. The system, measurement methods, and results are presented and discussed. (author)