The 222Rn Reference Measurement System developed at ENEA

A 222Rn Reference Measurement System was recently developed at ENEA. The system is based on a radon transfer and counting system calibrated by a 226Ra standard liquid solution. The reference counting instrument is a cylindrical electrostatic cell with a Si detector. It is used for the detection of the electrostatically collected 218Po ions produced in the decay of 222Rn. The activity of 222Rn gas in a spherical glass bulb can be measured in the Reference Measurement System by a particular procedure that does not require volume determinations or any assumption of total radon removal from the liquid 226Ra solution. The combined uncertainty estimated for the calibration of 222Rn sources as in a typical gas transfer system is about 1% (one standard deviation). The Reference Measurement System and the operational procedure adopted by ENEA are described together with a short outline of the transfer instrument based on a NaI(Tl) well-type detector used for calibration purposes. (orig.)