An international intercomparison of soil gas radon and radon exhalation measurements.

The Environmental Measurements Laboratory hosted the Sixth International Radon Metrology Programme Intercomparison Test and Workshop (IRMP6) from 12-15 June 1995. Thirty participants representing 24 different institutions from 11 countries attended. Laboratory exercises consisted of 220Rn and 222Rn concentration measurements from a source container, and exhalation measurements from a 226Ra-spiked concrete slab and a "normal" concrete slab. Field exercises included soil gas radon measurements and radon exhalation measurements. In this report, we pooled the participants' data and used the ratio of the standard deviation (SD) to the arithmetic mean, expressed as a percentage, to assess participant agreement for each exercise. For the exhalation measurements from the 226Ra-spiked slab, this value is 37%; for soil gas 222Rn, this value is 120%, 36% and 27% for each depth range, 0.4-0.5, 0.6-0.75 and 0.9-1.0 m, respectively; for the surface exhalation measurements, this value is 34%. For the drum 222Rn measurements, the percent SD after removing a linear trend was 13%. These results indicate that sampling errors are greater than instrument errors.

An International Marine-Atmospheric 222Rn Measurement Intercomparison in Bermuda Part II: Results for the Participating Laboratories.

As part of an international measurement intercomparison of instruments used to measure atmospheric 222Rn, four participating laboratories made nearly simultaneous measurements of 222Rn activity concentration in commonly sampled, ambient air over approximately a 2 week period, and three of these four laboratories participated in the measurement comparison of 14 introduced samples with known, but undisclosed ("blind") 222Rn activity concentration. The exercise was conducted in Bermuda in October 1991. The 222Rn activity concentrations in ambient Bermudian air over the course of the intercomparison ranged from a few hundredths of a Bq · m-3 to about 2 Bq · m-3, while the standardized sample additions covered a range from approximately 2.5 Bq · m-3 to 35 Bq · m-3. The overall uncertainty in the latter concentrations was in the general range of 10 %, approximating a 3 standard deviation uncertainty interval. The results of the intercomparison indicated that two of the laboratories were within very good agreement with the standard additions, and almost within expected statistical variations. These same two laboratories, however, at lower ambient concentrations, exhibited a systematic difference with an averaged offset of roughly 0.3 Bq · m-3. The third laboratory participating in the measurement of standardized sample additions was systematically low by about 65 % to 70 %, with respect to the standard addition which was also confirmed in their ambient air concentration measurements. The fourth laboratory, participating in only the ambient measurement part of the intercomparison, was also systematically low by at least 40 % with respect to the first two laboratories.

A method for determining soil gas 220Rn (thoron) concentrations.

A technique has been developed to accurately, precisely, and quickly measure soil gas 220Rn using commercially available grab-sample scintillation detectors. The method requires two counting periods for each sample: a 1-min count as soon as the sample has been drawn into a scintillation cell, followed by a 5- or 10-min count at least 5 min after the soil gas sample has been obtained. The 222Rn concentration is determined from the second count. The counts in the first counting period (1-min) due to 222Rn and its progeny are calculated from the known 222Rn concentration and then subtracted from the total counts obtained in the first counting period. The remaining counts are due to 220Rn and its progeny and are used to calculate the 220Rn concentration. The overall uncertainty when using this method to measure typical soil gas 220Rn and 222Rn concentrations [> approximately 5 kBq m-3 (130 pCi L-1)] was determined to be 19.8% and 10.8% (90% confidence levels), respectively, from analyses of duplicate field measurements. The lowest 220Rn concentrations that can be measured using this technique while maintaining an overall error no greater than about 30% is approximately 500 Bq m-3 (13 pCi L-1), assuming an approximately equal 222Rn concentration. The 220Rn measurement uncertainty at this concentration level decreases to approximately +/- 20% if at least three measurements sampled in series are arithmetically averaged. The uncertainty in the 220Rn measurement at low levels is proportionately affected by the relative concentration of 222Rn present.