b'FeatureAirborne radiometrics over Maralinga - call for a new survey Figure 7. Block diagram of the Redar IV military system (Tipton et al. 1988).the Compton continuum, background contamination from the aircraft and atmospheric conditions were eliminated byFigure 8. Caesium 137 from EG&G 1987 survey (Tipton et al. 1988).dividing the Signal Window readings by an adjacent higher energy Background Window to determine a constant K (the relationship had been empirically observed to be a constant)The signal response S in counts per second was taken to be asfollows:S=A-KB.Where A is the signal window and B is the Background window.In the absence of radioisotopes, the signal response oscillates around a zero baseline. The presence of an isotope caused the baseline to rise above zero.The raw data were then subjected to two \x1elters. The \x1erst was a simple along line three part moving average. The second involved 24 adjacent readings (6 points along line across four lines) averaged to give just one reading for an area of 200 m byFigure 9. Americium 241 from EG&G 1987 survey. Four contour levels are used, 200m. Due to distortions introduced by this second step theand ground zero sites are annotated (Tipton et al. 1988).results obtained were only used to de\x1ene the outer edges of theplumes.The activity concentrations were then calculated and correctedIn 2002 the Maralinga Technical and Rehabilitation Commission to make them consistent with the values obtained by thedetermined a ratio of 6.91.2 and calculated that in 1985 the carefully conducted ground surveys. The data were thenratio would have been 8.31.4 rather than 7.40.6.presented as hand contours (Tipton et al. 1988). Before the activity ratio is applied the gamma radiation seen on the surface must be multiplied by 1.33 which is the Absorption Results Factor This is the correction to take account of the absorption The contour maps for caesium 137 (Figure 8) and americiumof a portion of the 59.5 keV gamma rays in the soil. By applying 241 (Figure 9) are shown as presented by EG&G in their reportan absorption factor of 1.33 and an Pu-239/Am-241 factor of 8.3 (EG&G 1988). Caesium 137 is a fallout product. Americiumwe can convert airborne Am-241 observations in 1987 to Pu-239 241 plumes are largely the result of plutonium metal ejectedactivity ground concentrations in 2018 (Table 7).explosively in the Minor trials. Cobalt 60 and europium 152 are neutron (or soil) activation products created when the sand directly beneath the nuclear blast is bombarded by neutrons.Table 7.Activity concentrations in kBq/m2 of Am-241 Europium 152 proved impossible to extract due to its manyin1987 and Pu-239 in 2018 at Maralinga.photo-peaks. Cobalt 60 counts were separated by specialAm241 and Pu239 activity concentrations in kBq/m2methods which were not detailed by EG&G and are not shown (Tipton et al. 1988). Am-241 Am-241 Pu-239 Pu-2391987 1987 1987 2018Conversion of americium 241 to plutonium 239 Surface Surface X 1.33 Pu-239/Am-241observed absorption factor X 8.3 activity ratioIn 1989 the Australian Radiation Laboratories determined theA 1.4-3.6 1.86-4.79 15.4-39.8 15-40Pu-239 to Am-241 activity ratio at Maralinga to be 7.40.6. The ratio changes with time because the amount of Am 241B 3.6-10 4.79-13.3 39.8-110.4 40-110increases for over 70 years from its creation in the nuclearC 10-30 13.3-39.9 110.4-331.2 110-331reactor due to its parent Pu-241 (half- life 14.4 years) decaying faster than the daughter Am-241 (half- life 432 years). D 30-100 39.9-133 331.2-1,103.9 331-1,104APRIL 2024PREVIEW 51'