Background: Calibrating the detection efficiency accurately is the key process to the disc filter sample activity detected by High Purity Germanium (HPGe) gamma spectrometer

and the diameter of the disc filter source is varied with the specification of the sampler filter. Purpose: To study further the relation between detection efficiency and the source size

the HPGe detector peak efficiency (relative detective efficiency is 60%) was performed for disc sources with diameters of ?24mm

?80mm and ?90mm

respectively. Methods: The efficiency and the solid angle for disc source with diameter ranging ?40–160mm were simulated using the MCNP and the Geant4. Results: The result shows that the detecting efficiency of three sources is consistent

and the efficiency gradually decreases with the increasing size of the disc source with diameter greater than ?90mm. Conclusion: The efficiency for disc sources with diameter less than ?90mm is directly calibrated by the standard point sources

while for disc sources with diameter larger than ?90mm which is needed to be calibrated by the standard disc sources. Abstract