2005 Volume 54 Issue 12 Pages 599-608
The radon measurement with a gas-filled ionization chamber is generally performed at many laboratories in Japan as a standard method. In this paper, the characteristics of the method have been examined as follows. The wall effects against the different filled gases, nitrogen and dry air, have been evaluated with Monte Carlo simulation. The columnar recombination has been also evaluated from two different ways; one is the common evaluation from the relationship between inverse of ionization current and inverse of applied voltage; another is the evaluation from the relationship between the ionization current in the different gas pressure and the wall effect expected in each pressure.Both results have been agreed with each other. As the total, the efficiencies of ionization chamber, volume of which is 1.5 L, are estimated to be 0.02706 Bq.fA-1 for nitrogen gas of 93.3 kPa and 0.02701 Bq.fA-1 for air of 101.3 kPa, and the values are very close to the conversion factor on the literatures within 3% of difference. Furthermore the effect by humidity has been preliminarily estimated, and the effect is almost -3% per 1 % of absolute humidity but it can be easily removed by the suitable dryer. These examinations have proven the method with a gas filled ionization chamber has high accuracy for radon measurement. An 241Am disk source was used with same methods to confirm the results above mentioned.