Experimental potassium, uranium, and thorium gamma-ray spectra covering a range of equivalent aircraft altitudes from 0 to 112 m were derived from measurements on large radioactive concrete calibration pads using plywood sheets to simulate the absorption effects of the air. A mathematical analysis of all three radioelement spectra showed they are composed of two basic spectral components which are added in different proportions dependent on the altitude of the aircraft above the ground. Above an energy of 0.662 MeV, the two-component spectral model represented accurately the observed gamma-ray spectrum from the ground. Below this particular energy, cesium-137 from atomic weapons fallout was found to contribute significantly to the airborne spectra.A theoretical analysis of the calculated spectrum from typical crustal material showed that by multispectral fitting above the cesium-137 energy, the thorium and uranium concentration errors could be reduced by approximately 25 percent compared to the standard three-window method. Similar increases in accuracy were found experimentally by analyzing a series of 1 s airborne data recorded over a uniformly radioactive test strip.The two-component model was also used to investigate the effect of changing the windows on the standard three-window method. For typical crustal material, no significant increase in uranium accuracy was achieved by changing the windows from those recommended by the International Atomic Energy Agency.

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