The measurement of U, Th and K concentrations in building materials

A shielded high purity germanium detector has been used to measure the abundance of U, Th and K in aggregate, sand and cement. The measurements were performed in order to demonstrate that the materials contained less than 10 ppm U and less than 20 ppm Th and were therefore suitable for the construction of a low background shielding enclosure. A particular feature of this work is the simplicity of the absolute efficiency calibration. In brief the relative U and Th contents of each sample were derived by first determining the relative detection efficiency curve for each sample using the known relative gamma-ray emission probabilities of the U- and Th-decay series lines from the radionuclides naturally present. To obtain the true activities, the relative efficiency curves for each sample were scaled so that they passed through the absolute efficiency value measurement at 1461 keV using a sample containing KCl bought as an analytical reagent and requiring no special radiological handling precautions. A small experimentally estimated correction was applied for the difference in self absorption between the KCl and building materials due to the differences in material density. The 40K content of the unknown samples was obtained directly from the density adjusted KCl calibration. The measurements are believed to be accurate to approximately +/- 5% absolute.