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

surface. This is a direct consequence of the relatively large field of view of the airbornesystem compared to the dimensions of the patch. Ground based approaches, with significantlysmaller fields of view for the detectors, are better able to distinguish signals from extendedpatches of background activity with these dimensions and discrete particles, although asimilar ambiguity between discrete sources and dispersed activity would still exist for smallerpatches. The use of collimated detectors or self-collimated detector arrays in airborne surveycould be considered to improve the ability of such systems to identify discrete particles inenvironments with patches of enhanced activity.With several patches of activity in a given area, an intermediate situation exists where for lowdensities of patches the problems of distinguishing between a patch of activity and a sourcedominates, and at higher densities of patches the environment approaches one where the 137 Csbackground is uniform.SourceSuperficial137 CsBuried137 CsSuperficial241 Am137 CsBackgroundDetection Limit (MBq)Rapid Survey Low and Slow 4 knots3σ 2σ 3σ 2σ 3σ 2σ0 Bq kg -1 9 5 0.23 0.14 0.050 0.03050 Bq kg -1 10 6 0.28 0.17 0.065 0.0370 Bq kg -1 35 20 1.00 0.60 0.22 0.1450 Bq kg -1 39 23 1.20 0.75 0.29 0.170 Bq kg -1 n/d n/d 1.5 1.0 0.35 0.2250 Bq kg -1 n/d n/d 2.0 1.3 0.45 0.30Table 4.1: Predicted detection limits for 137 Cs and 241 Am for the medium naturalbackground condition and 50Bq kg -1 137 Cs uniform background.4.2 Response to recovered particlesThe March 2000 coastline survey flight has been recreated for the section between St Beesand Duddon, by generating natural series spectra from the simulations conducted in the firstphase of this work scaled by the measured activity concentrations and simulated spectra forthe sources recovered from the beaches. For both 75m and 15m ground clearance data alongthe flight lines of the March 2000 survey, the recovered sources contribute no counts to thespectra. The recovered particles are all below detection limits for the 75m survey, and eventhe cumulative spectrum from several sources is below detection limit. A few (3) of therecovered sources have activities above 200kBq and were recovered from the top 10cm of thebeach. These particles were all recovered from the Sellafield foreshore. These should bedetectable using a slow survey at 15knots or less and 15m ground clearance, especially withclose linespacing. A few more (2-3) may have been detectable under such detailed surveyconditions. The survey flights in March 2000 were too fast, too high and too far apart toobserve them.Residual spectra have been produced by subtracting the simulated natural spectra from themeasured spectra for the March 2000 survey. The count rates within a window around the137 Cs peak in these spectra have been mapped for sections of beach at St Bees, Seascale andDrigg. These show distributions that correspond reasonably well to the 137 Cs activity per unitarea determined in March 2000, although there are some differences in the spatial31