Sets of four transportable concrete pads 1 m x 1 m x 30 cm and weighing approximately 675 kg were constructed for calibrating portable gamma-ray spectrometers. Each set consists of a low radioactivity background pad, and three radioelement pads of potassium,
uranium, and thorium which provide almost pure gamma-ray spectra. The potassium pads were manufactured using potassium feldspar and the thorium pads using britholite, a thorium-rich rare earth phosphate. Initial problems of radon loss from the uranium pad were overcome by using a uranium-rich slag
of calcium silicate which was a byproduct from a phosphorus processing plant. Experiments with these small pads have shown that they give the same potassium, uranium, and thorium spectral shapes as the much larger aircraft calibration pads. These small transportable pads can therefore be used for
calibrating large volume airborne systems as well as portable gamma-ray spectrometers.
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