Experimental studies using a large-volume sodium iodide detector system show how the potassium, uranium, and thorium gamma-ray spectra vary with altitude above the ground. The shapes of the spectra were derived from measurements on large radioactive concrete calibration slabs using plywood sheets to simulate the absorption of the gamma radiation in the air. A mathematical analysis of the data showed that within the range covered by the experiment (0–112 m of air) the spectra of each of the three radioelements are made up essentially of two spectral components, whose proportions vary with the amount of absorbing material between the source and the detector. With this knowledge, it is shown how all the basic information relating to source–detector distance and source concentration may be extracted from the airborne gamma-ray spectrum.

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