In the frame of proficiency tests organized by IAEA and IRMM, the specific activity concentration of radium-226 in soil has been measured. The BaSO4 co-precipitation technique has been used. Normally, in this method, the detector efficiency of the alpha spectrometer is determined using a Ra-226 source with known activity. As an alternative to using a Ra-226 standard, we calculated the detector efficiency from the relative solid angle subtended by the detector on the soil samples. The accuracy of this method depends on the uncertainty of geometrical properties and the distribution of activity within the source. An uncertainty budget is provided. The method was applied successfully in the intercomparisons. (C) 2012 Elsevier Ltd. All rights reserved.