Annual Report for 2010/11 and Forward Programme - Sellafield Ltd

The results have shown that in the presence of approximately uniform natural andanthropogenic background radiation fields typical for the sand and gravel beaches, “rapid”AGS surveys would have a detection limit for superficial 137 Cs sources of 5-10 MBq, andmore detailed “low and slow” surveys would have detection limits of 200-300 kBq. Detectionlimits for sources at 10cm burial are a factor of 4 greater. “Low and slow” surveys havedetection limits for superficial 241 Am of 1-2 MBq. The patches of activity simulated heregenerate almost identical flight line profiles as point sources of similar total activity. Atground level the smaller field of view compared with airborne systems assists suchdiscrimination for activity patches of the dimensions simulated here, although the sameambiguity would exist for smaller patches of dispersed activity. The use of collimation in theairborne system to reduce the field of view could be considered. A moderately dense patternof patches (eg: >25% of the area) approximates a uniform distribution of activity. In thissituation, detection limits for a uniform 137 Cs background of 50 Bq kg -1 are increased by asimilar factor to changing from a medium to high natural activity background.Reducing ground clearance would increase full energy peak count rates, by an order ofmagnitude at 5m compared to 15m. Thus surveys at these ground clearances wouldsignificantly increase the significance of full-energy peak counts from sources in comparisonwith the 15m specification. To achieve a detection limit for 137 Cs sources at 10cm depth of10 5 Bq in uniform background radiation fields it is estimated that it would be necessary toreduce ground speed to below 5 knots (2 m s -1 ) and ground clearance to 5-10m. A dataprocessing methodology that utilises the scattered part of the spectra (>80% of the total countrate from buried sources) might potentially further improve detection efficiency, and providean estimate of source burial depth. Detector collimation may allow better discriminationbetween point sources and small distributed patches of activity.The analysis here has related to detection of sources in single measurements, or summedconsecutive measurements treated as single measurements. In survey design and dataanalysis, observations are combined from multiple survey lines. Methods which utiliseinterpolation across several observations on multiple survey lines have the potential to lowerdetection limits compared to approaches that utilise information within single survey lines.However such analysis would be retrospective, which may have implications forimplementation in source searches.This work has confirmed that airborne methods are capable of providing useful levels ofsensitivity, with survey rates which can not be practically achieved at ground level. Airbornemethods can demonstrate the absence of strong sources within large areas whereanthropogenic background distributions are approximately uniform, and identify areas wherepatchy anthropogenic distributions would require further ground based investigations toconfirm the absence of sources. AGS has an important role in identifying where valuableground-based resources should be focussed. It is suggested that practical work be initiated toverify the theoretical detection limits presented here and that consideration be given todeveloping the AGS role in future programmes. In addition to validating the sensitivity anddetection limits demonstrated in this theoretical study, this would provide an opportunity toevaluate the practical limits for operating helicopters at low speed and ground clearance overpublic beaches while complying with safety and regulatory requirements.ii