In this paper we propose an experimental method for the determination of potassium-40 ( 40 K)
radioactivity in commercial granite samples using x-ray fluorescence (XRF). The method correlates
the total potassium concentration (yield) in samples deduced by XRF analysis with the radioactivity
of the sample due to the 40 K radionuclide. This method can be used in an undergraduate student
laboratory. A brief theoretical background and description of the method, as well as some results
and their interpretation, are presented.