b'FeatureAirborne radiometrics over Maralinga - call for a new survey Modern systems The \x1cying height was 100 m and the line spacing was 1500m. The direction was east west. The airspeed was 70m/s. AGSO (1992), WGC (1992), Fugro (2005) and Thomson (2018)4windowed channels were recorded. 256 channels were surveyed with modern spectrometers and processed the datarecorded every 100 s (summed over the previous 100 s).with computer gridding and contouring followed by image processing. These systems and methods were \x1erst employed by anProcessingAustralian Company, Austirex, in Iran in 1976 when they installed a Geometrics GR-8000 gamma ray spectrometer combined withWhen the data processors began the routine removal of radiation a Geometrics 50 litre main pack detector and 8 litre upwards- from radon gas, they discovered an elevation of the background looking detector (Richardson 2021). 256 channels were acquired,total count window in the order of 150% over Maralinga. and the data were computer contoured and image processed.Suspecting the in\x1cuence of Cs-137 (photopeak 662keV) on the Later Austirex became a subsidiary of World Geoscience, whoradon daughter photopeak Bi-214 (609 keV) (Figure 4) they used their processing package on the Maralinga data. determined to test for Cs-137. Brian Minty and Ross Brodie devised a simple method of isolating man-made elements from In 1976 EG&G bought Geometrics Inc and formed EG&Gnaturally occurring elements. They \x1erst created a pseudo total Geometrics which they held until 1997 when they sold it tocount window by exposing the sensors of the spectrometer to OYO of Japan. Therefore EG&G, through their subsidiary EG&Gclean radioactive sources of potassium, uranium and thorium. Geometrics, were manufacturers of modern spectrometers atThis was done on the ground. The sensors were partially shielded the time of the Maralinga survey but chose to use the Redar IV.with wood panels to simulate the attenuation of response At the time EG&G/Energy Measurements Inc. were managers ofto be expected from an air column of 100 m, which was the the US governments nuclear test facility in Nevada and were\x1cying height of the aircraft. The three spectra were normalised experienced in mapping radioisotopes (Briener 2024) with observed counts from the survey and then summed. The Modern spectrometer systems have generally similarpseudo total count obtained thereby was subtracted from the architectures, employing eight large (4196 mL) hexagonalobserved total count to yield man-made isotopes.Sodium Iodide (thallium activated) crystal detectors making 33.6 litres in total. The systems are continually stabilised byResultslocking on to naturally occurring photo peaks. The outputThe procedure was successful in isolating Cs-137 (Figure 11). of each sodium iodide crystal is computer controlled withTheplume of Cs-137 was estimated to be about 10 km long. automatic gain correction. Each block of four crystals has itsNeither americium, cobalt 60 or europium 152 was observed own Data Processing Unit (Figure 10).(Minty et al. 1994).The AGSO (Geoscience Australia) 1992 airborne survey The World Geoscience (WGC) 1992 airborne survey Equipment In 1992 WGC was in the vicinity Maralinga area and decided to \x1cy the test range on their own initiative.AGSO collected data from 300-3000 keV in 256 channels restricted to four windows WGC equipment The spectrometer was an Exploranium GR800 with 8 NaI(Tl)WGC collected data from 300-3000 keV in 256 channels. The 4.2 litre detectors making 33.6 litres in all. The equipment wasspectrometer was an Exploranium GR800. A data acquisition mounted inside a Beechcraft Twin Otter aircraft. Each detectorsystem interrogated two crystal packs each containing four had its own photomultiplier.SurveyingIn 1992 AGSO was \x1cying the Maurice 1:250 000 sheet in South Australia as part of the coverage of Australia. Figure 11. AGSO 1992 survey: Perspective image of stacked pro\x1dles of the total count in part of the Maurice 1: 250 000 sheet showing the observed data in the top image and the extracted Cs-137 image in the bottom image. The Figure 10. Block diagram of a modern RSX-4 spectrometer with 33.6 litres ofplume from the Maralinga site extends to the northeast for an estimated 10 km NaI (Tl) crystal sensor (Thomson Aviation 2020). (Mintyet al. 1994).52 PREVIEWAPRIL 2024'