Portable calibration sources for airborne gamma-ray spectrometers

Abstract

Standard radioactive sources of known element abundances are essential for the calibration of airborne gamma-ray spectrometers. The conventional approach to calibration has been the determination of 3-channel stripping ratios and sensitivity constants from large airport calibration pads, and a well calibrated test strip, respectively. However, calibration pads are expensive to build and suitable test strips are difficult to find and expensive to calibrate. Also, both pads and test strips are subject to variations in radiation output due to variable moisture content and radon loss, and this can lead to considerable calibration errors. This paper describes an alternative approach to the calibration of airborne gamma-ray spectrometers. We have built a set of portable calibration sources, and have used these to simulate the response of an airborne spectrometer to infinite sources over a range of altitudes. The calibration has produced conventional 3-channel stripping ratios and sensitivity constants as a function of altitude, as well as good quality, pure multichannel spectra which are fundamental to any research into improved processing techniques. The sources are inexpensive, and since they are completely sealed, they do not suffer from variations in radiation output.