Use of 214Pb photopeaks for radon removal:utilising current airborne gamma-ray spectrometer technology and data processing

Abstract

Removal of radiometric anomalies due to airborne radon is one of the most demanding steps in airborne gamma-ray spectrometry data processing. The contribution from airborne radon gas at the usual survey altitude is often several times that emanating from uranium ground sources. If left uncorrected, the radon background component can corrupt the whole chain of radiometric processing and preclude the production of seamless uranium and ternary data images. The concept of the use of 214Pb photopeaks for radon removal has been known for many years (Grasty, 1982), however until now it has not been introduced into regular data processing practice. The main obstacle in the implementation of this concept has been the fact that 214Pb photopeaks are found in the low energy part of the spectrum where there is a high degree of noise caused by scattered radiation. However, advances in modern instrumentation and in multichannel processing techniques allow the utilisation of these 214Pb photopeaks for radon removal in daily practice. The use of low-energy photopeaks is now possible following the release of a new generation of spectrometers, which produce improved spectrum linearity and high resolution in 512 channels for each crystal in the sensor array, The spectral ratio method suggested by Minty (1992, 1998), based on the 609 keV and 1765 keV photopeaks of 214Bi, has been to date the most accepted technique for radon removal. However, it has an inherent difficulty when processing data with a high 137Cs contribution, as is widespread in the Northern Hemisphere. In addition, this method suffers from the inability to resolve in a satisfactory way the overlap of the 208Tl photopeaks at 511 keV and 583 keV with the 214Bi 609 keV photopeak, in cases where there is strong thorium radiation. During recent surveys totalling 290 000 line kilometres, flown for the Geological Survey of Namibia, a Pico Envirotec GRS10 Spectrometer was used to collect 512 channels of raw data per second from each of eight NaI(Tl) crystal sensors. Observation of the 214Pb photopeaks at 295 keV and 352 keV, instead of the more usual 214Bi 609 keV photopeak, allowed successful radon removal.