Construction and Characterization of Transportable Calibration Pads for Calibration of Portable and Airborne Gamma Ray Spectrometers in India

Abstract

Ground and airborne gamma ray spectrometers are routinely used in uranium exploration for search of deposits and lithological mapping. These systems are calibrated using standard concrete calibration pads which are constructed with known concentrations of K, U and Th and simulate the infinite source geometry for gamma ray survey instruments. In India Atomic Minerals Directorate for Exploration and Research, a constituent of Department of Atomic Energy created such calibration facility at Nagpur civil Airport in 1984 and maintained it till date. The cost of maintaining these old pads is very high, therefore we have constructed new transportable calibration pads which are much smaller in size (1m x 1m x 30cm) and also transportable to field areas. Five pads namely Background pad, potassium pad, uranium pad, thorium pad and composite pad have been constructed. These are made of concrete spiked with pre determined concentrations of radio-elements uranium, thorium and potassium. These transportable calibration pads are 750kg in weight and are mounted on a trolley. This has eliminated the need for large standard calibration pads, which needed an airport with very high land leasing costs and thereby reducing the maintenance cost significantly. A very distinctive feature of designed transportable pads as compared to pads worldwide, is the achievement of high density and high compressive strength. This high density has resulted in higher sensitivity thereby improving the statistical accuracy of calibration. This has also resulted in reducing the fluctuations in calibration due to moisture in field condition. Another distinctive feature is the achievement of higher potassium concentration > 8.0% in potassium pad. Creation of this new calibration facility has resulted in substantial savings to government exchequer by eliminating the need of the airport. This has enabled calibration in the field areas with easy transportability and thereby reducing the downtime of the survey operations in uranium exploration. This paper describes the design and fabrication of transportable calibration pads in India.