The Standardization of Airborne Gamma Ray Surveys
R. L. Grasty
In the last few years, there has been considerable growth in the use of airborne gamma ray spectrometry. With this growth, there has been an increasing need to standardize the airborne measurements so that they will be independent of survey parameters. Standardized procedures have developed for converting airborne gamma ray measurements to ground concentrations of potassium, uranium and thorium, and air absorbed dose rates due to natural sources of radiation. Concrete calibration pads, typically 30 cm x 30 cm x 1 m, are used to establish the calibration coefficients for the airborne systems and are located in many places throughout the world. The sensitivities of the airborne equipment are determined from flights at different altitude over a calibration range. Ground concentrations are measured on this calibration range with a calibrated portable spectrometer rather than by taking soil samples. One of the main limitations of the airborne gamma ray spectrometric method has been due to variations in radon daughter concentrations in the air. This problem has now largely been overcome by using upward-looking detectors shielded from ground sources of radiation. Another method makes use of the difference between the spectral shape of radon progeny products in the ground and in the air. Recently, it has become common practice to utilize up to 256 channels of spectral information to reduce statistical noise in the airborne measurements.
This abstract was presented at the 33rd Annual Midyear Meeting, "Instrumentation, Measurements, and Electronic Dosimetry", Calibration Session, 1/30/2000 - 2/2/2000, held in Virginia Beach, VA.