b'FeatureAirborne radiometrics over Maralinga - call for a new survey germanium detector for this purpose (Figure 5). Each spectrumkeeping the system at constant temperature but the system was reading took 10 minutes and there were 100 such readings. Theinherently noisy. Calibrations were performed before and after Signal Windows are superimposed on one of the spectrums\x1cight using americium 241 and sodium 22 sources.taken at the Taranaki site shown in Figure 6.Survey speci\x1ecationsEG&G aircraft and equipmentThe lines were either 50 m or 100 m apart east-west and the The crystals were assembled in two pods mounted below aheight was 30 m. The airspeed was 30 m/sBritish Wessex helicopter.Processing A military systemGamma emissions from naturally occurring elements of Data was collected from 0 keV to 3026 keV in 256 channels.potassium, uranium and thorium, together with cosmic rays EG&G used their proprietary Redar IV system linked to forty sodium iodide thallium doped (NaI(Tl)) crystal surface area 330cm2 and volume 645 mL making 26 litres of crystal total. Thelarge number of small crystals was used to increasesensitivity.The \x1erst detector array was composed of 39 detectors to achieve adequate sensitivity (Figure 7). A second was composed of only one crystal to be interrogated when the radiation was too high for the \x1erst array. Each detector was 12.7cm diameter x 5.1 cm deep - volume 645 ml.The volume of NaI in the \x1erst array was 39 x 0.645 litres (25.2litres) and the surface area 1.3 m2. The volume of the single crystal detector was 645 ml with a surface area of 330 cm2.The crystals were kept under power 24 hours a day to keep them at constant temperature (changes in temperature can cause gain shift moving a photopeak out of its window.)The small, thin sensors increased the sensitivity by allowing most of the incident photons to escape before they caused Compton scattering, however, practicality demanded that a single photomultiplier must serve nine or ten crystals making it almost impossible to track the drift in gain for each crystal in 39 of the crystals. The fortieth detector had its own photomultiplier. Drift in the gain was somewhat controlled by Figure 5. High purity germanium detector at Taranaki (Tipton etal. 1988).Figure 4. A typical gamma ray spectrum illustrating windows used to measure total count, potassium, uranium and thorium in a geological surveyFigure 6. Gamma ray spectrum from a high purity germanium detector - set at and Cs-137 and Am-241 from nuclear experimentation. Note the positionone metre above the ground at Taranaki bomb site and measured for a period of the photopeak for Cs-137 (0. 662 MeV) partially overlapping the Bi-214 atof 10 minutes. Windows of investigation as selected by EG&G for the processing 0.609MeV. The Am-241 photopeak at 0.0595 MeV is too low in the energyof data from their airborne survey for Am-241, Cs-137 and combined Co-60 and spectrum to be observed by typical survey instruments which are set at higherEu-152 are indicated. Note K-40 peak at 1406.8 KeV is included in this signal energies to capture naturally occurring radiation (Minty et al. 1997). window (Tipton et al. 1988).50 PREVIEWAPRIL 2024'