[en] A series of six concrete pads that simulate rock outcrops of known radioactive concentrations have been installed at Risoe National Laboratory. The facility is intended for the calibration of portable counters and spectrometers and is accessible to uranium geologists in Europe and elsewhere. All of the pads are 3 m in diameter and 50 cm thick and contain uranium, thorium, and potassium in various proportions. The concentrations of eU, Th and K with associated standard errors have been determined from the combined use of laboratory assays and radiation monitoring data. The evaluations of instrumental sensitivity factors and stripping ratios are performed with weighted least-squares techniques. Sensitivity factors of grosscounting field scintillometers are supplied in units of radioelement concentration (Ur). In addition to being especially suited to geologic calibrations, the facility may be applied as a source of gamma-ray exposure rates for calibrating environmental survey instruments. (author)