Application of NIST 222Rn Emanation Standards for Calibrating 222Rn Monitors

The NIST (National Institute of Standards and Technology) has recently made available 222Rn emanation standards. The NIST certified parameters include the 226Ra strength and the emanation coefficient. When such a source is loaded into a leak tight enclosure of a known volume, 222Rn accumulates over time and it is possible to calculate precisely both the 222Rn concentration after any accumulation time and also the time integrated average 222Rn concentration after any given accumulation time. Equations needed for calculations are given. For example, if a 25 Bq (676 pCi) NIST source is loaded into a jar with an air volume of 3.72 litres, the time integrated average 222Rn concentration is 973 Bq.m-3 (26.3 pCi.l-1) after an accumulation time of two days. If the 222Rn detector placed in the jar is non-radon absorbing and a true integrator, it must yield the theoretically predicted results. If not, an appropriate calibration correction factor can be calculated. The paper describes a study involving 34 randomly chosen commercially available E-PERM(r) (Electret-Passive Environmental Radon Monitor) and 17 NIST sources in 17 different calibration (accumulator) jars. The study indicated that E-PERM(r)S give results within about 5% of the predicted results. The study also includes a study of continuous radon monitors. The data obtained show that, in practice, NIST sources can be used to calibrate both passive and active continuous 222Rn monitors. The availability of NIST sources with a precisely known 222Rn emanation characteristic is a major advance in radon metrology. Practical problems in the successful use of these sources are discussed.