Evaluating airborne and ground based gamma spectrometry methods for detecting particulate radioactivity in the...

In several places, programmes are in place to locate and recover radioactive particles that have the potential to cause detrimental health effects in any member of the public who may encounter them. A model has been developed to evaluate the use of mobile gamma spectrometry systems within such programmes, with particular emphasis on large volume (16l) NaI(Tl) detectors mounted in low flying helicopters. This model uses a validated Monte Carlo code with assessment of local geochemistry and natural and anthropogenic background radiation concentrations and distributions. The results of the model, applied to the example of particles recovered from beaches in the vicinity of Sellafield, clearly show the ability of rapid airborne surveys conducted at 75 m ground clearance and 120 kph speeds to demonstrate the absence of sources greater than 5 MBq (137)Cs within large areas (10-20 km(2)h(-1)), and identify areas requiring further ground based investigation. Lowering ground clearance for airborne surveys to 15m whilst maintaining speeds covering 1-2 km(2) h(-1) can detect buried (137)Cs sources of 0.5MBq or greater activity. A survey design to detect 100 kBq (137)Cs sources at 10 cm depth has also been defined, requiring surveys at <15m ground clearance and <2 ms(-1) ground speed. The response of airborne systems to the Sellafield particles recovered to date has also been simulated, and the proportion of the existing radiocaesium background in the vicinity of the nuclear site has been established. Finally the rates of area coverage and sensitivities of both airborne and ground based approaches are compared, demonstrating the ability of airborne systems to increase the rate of particle recovery in a cost effective manner. The potential for equipment and methodological developments to improve performance are discussed.