Abstract: Utilizing the AlphaGUARD emanometer and AquaKIT accessories which are often used as international calibration reference devices, we carried out a calibration experiment without radon source to two scintillation chambers of FD-125 emanometer. By changing the system volume, the readings of AlphaGUARD radon measurement instrument were kept in less than 1000 Bq/m³, 1000~2000 Bq/m³, et al, higher than 10000 Bq/m³ and total six different ranges. When the radon concentration were higher than 1000 Bq/m³, the deviations of K values are all less than 5% relative to the average of all K values, and the K values relative to that using radon source were less than 5 %. When the radon concentration were less than 1000 Bq/m³, the K values deviations were all more than 5% relative to the average of all K values. Therefore, the basic condition of the calibration without radon source was the radon concentration in the system needed higher than 1000 Bq/m³. According to the water vapor balance of radon in the closed system, the lowest water radon concentration could be required. This study not only verified the calibration with standard instrument can replace the calibration with radon source, but also determined the conditions needed for the calibration of standard instrument. This would lay the theoretical foundation for establishment and extension of method procedures.

Key words: FD-125 emanometer, AlphaGUARD emanometer, Scintillation chamber, Calibration with standard instrument