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

3.1.2 Survey Line Profiles3.1.2.1 Rapid Large Area SurveySurveys conducted at 200-250ft (60-75m) ground clearance and approximately 70 knots (130kph) ground speed are capable of rapidly surveying large areas. For mapping dispersedactivity it has been shown that line spacings of 250m are usually adequate to locate mostfeatures, and larger line spacings would be acceptable to allow larger areas to be surveyed ina given time (Sanderson et.al. 2001). For rapid survey of large areas for point sources, acloser line spacing would likely be required. For this work, such a survey is defined as 250ft(75m) ground clearance, 70 knots (30 m s -1 ) ground speed and 100m line spacing.Figures 3.6-3.9 show the significance of transects directly over superficial and buried 137 Cssources, and at 50m offset distances, for the modelled uniform 137 Cs backgrounds. Thetransects are for individual 1s measurements, and correspond to the similar figures for naturalbackground in the earlier work.It had been noted in relation to natural backgrounds that increased integration time reducesthe measurement uncertainty, with an integration time that approximates to the time taken tocross the dimensions of the radiation field from the source being optimal. For the 75m groundclearance survey, 90% of the signal is observed within 120m of the source giving an optimalintegration time of 8s at 70 knots. Table 3.1 gives the significance for 9s measurements oversuperficial 137 Cs sources of 1, 10 and 50 MBq in the medium natural activity background, anduniform 137 Cs backgrounds of 12.5, 25 and 50 Bq kg -1 . It can be seen that for a superficial10MBq 137 Cs source, the significance of a single 9s measurement exceeds 2.5, thus it can beconcluded that for rapid surveys at 200-250ft and 70 knot ground speed should readilyobserve 137 Cs sources of 5-10MBq at or near the surface.Source Activity 0 Bq kg -1 12.5 Bq kg -1 25 Bq kg -1 50 Bq kg -11MBq 0.55 0.47 0.42 0.355MBq 2.03 1.85 1.71 1.5010MBq 3.24 3.04 2.88 2.6250MBq 8.17 8.04 7.91 7.68Table 3.1: Significance of single 9s measurements for transects across superficial137 Cs sources, for different 137 Cs background concentrations on the mediumnatural background scenario, under rapid survey conditions.13