Four radioactive concrete pads, 3 m in diameter and 0.5 m thick, were constructed to serve as sources of potassium, uranium, and thorium gamma radiation in calibration experiments with portable spectrometers and total-count scintillometers. One of the pads contains an admixture of high-grade uranium ore. The radiation output from this pad fluctuates and has a seasonal variation of + or -20 percent. This observation indicates that the uranium ore emanates radon into the concrete pores, and that this radon is subject to the same pumping action from the atmosphere as radon in soil. In dry and warm weather the concrete surface has a low content of gamma-ray emitting radon daughters, because radon is lost from the pad. This situation is reversed when the weather is cold and moist. Therefore a varying, apparent radiometric concentration of uranium must be ascribed to the pad. The apparent uranium concentration correlates the observed radiation output with the stable radiation output produced by the three other pads, and it can at any time be determined from four readings with a total-count scintillometer operated at an energy threshold of 0.4 MeV.