Experimental methods for determining the counting efficiency of instruments that measure radon progeny concentration are proposed and described in this paper. For determining the counting efficiencies of
218Po and
214Po, alpha particles from radon progeny were first sampled from a radon-tank and deposited on the surface of a filter. These alpha particles were then alternately counted by two instruments; a standard instrument, S, with known counting efficiency, and an instrument, G, with unknown counting efficiency. Using the proporsed methods, the relative counting efficiency of
218Po and
214Po using instrument G was determined by comparing the indicated counting efficiency with the known counting efficiency of instrument S. In addition, measured radon progeny concentration data are shown that were calculated from the counting efficiencies obtained with this calibration procedure. These results show that it is important to accurately determine counting efficiencies because a small difference between the counting efficiencies of
218Po and
214Po may give relatively large differences between indicated concentrations of
218Po,
214Pb and
214Bi.
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