RESUMO
@#<b>Objective</b> To correct the counting loss of <sup>37</sup>Ar below the activity threshold during the measurement of the absolute activity of the inert radioactive gas <sup>37</sup>Ar using the proportional counter filled with gas. <b>Methods</b> Monte Carlo simulation with Geant4 was performed to establish a proportional counter model and output the energy deposition spectrum of <sup>37</sup>Ar, which were used to simulate and analyze the causes and correction of counting loss. <b>Results</b> The photon detection efficiency was only 38.7% at 60 kPa. The counting loss was mainly caused by the wall effect produced by the photons, which could be reduced by increasing the gas pressure and corrected by extrapolation. The influence of wall effect at 100 kPa was 4.4%, and the deviation between simulation and experiment was < 0.6%. <b>Conclusion</b> A factor could be calculated by Geant4 simulation for the correction of counting loss, thus achieving the accurate measurement of <sup>37</sup>Ar activity by proportional counter.