Radon Exhalation Measurement
The exhalation rate of building materials with relatively high Radium concentrations can be determined using the two NED-NTUA Radon chambers, with volumes of 1.8 m3 and 8.5 m3. The volume of the sample, which may also be a structural module, cannot exceed 10% of the total chamber volume. The following important environmental parameters, which affect an exhalation experiment, are monitored in the container through analogue-to-digital converters interfaced to the bus of a PC : pressure changes, using a ±50 mbar differential pressure transducer, temperature and relative humidity. An aerosol generating system is also used to produce particles.
Four methods can be employed for determining the radon concentration in the container environment:
- Placement of active Radon monitors inside the chamber, monitored on-line through an RS-232 connection.
- Grab sampling of 15-20% of the chamber air through a glass fiber filter and alpha-spectroscopic analysis of the radon daughters Po-218 and Po-214 using a PIPS detector set-up.
- Grab sampling of 15-20% of the chamber air through a glass fiber filter and gamma-spectroscopic analysis of the radon daughters Pb-214 and Bi-214 using a Ge detector set-up.
- In-situ radon progeny measurement inside the container, using an Am-241 doped 2×2′′ NaI detector, which records the area under the 609 keV photopeak of Bi-214.
For the calibration of the container a Pylon 2000A - NIST cross-certified source of a nominal Ra-226 activity of 102.8 kBq was used. According to the systematic results already obtained in a clean nitrogen environment, within the range of radon concentrations 0.1-30 kBqm-3, temperature 19-21 °C and relative humidity 30-40%, the collection efficiency, when sampling 160 L of gas through 37 mm glass microfiber submicron filters for 5 minutes, was determined as 10±1.5%. Furthermore, preliminary results show that the collection efficiency is increasing with the aerosol concentration inside the container, up to a maximum of 4 times.
Exhalation Rate Results
A least squares fitting to the radon growth curve of the examined materials was used to calculate their radon exhalation rate; for this purpose each one of the specimens was enclosed in the container for a period of about 20 days. The estimated effective decay constants did not significantly differ from the decay constant of Rn-222; the total uncertainty associated with these calculations was about 25%. According to the results obtained the cement specimens with Ra-226 concentrations in the range 100-140 Bq/kg (Louizi 94, Louizi 94c). present exhalation rates between 15-20 µBq /kg s. Brick specimens pulverised to less than 90 µm with Ra-226 concentrations in the range 28-48 Bq/kg present exhalation rates between 3-10 µBq / kg s. Furthermore, two typical greek structural modules, a clay brick wall and a concrete slab, of an exhaling area of 1m², constructed from raw materials with the highest Ra-226 concentrations detected, were also tested. The exhalation rate of the wall, with calculated value of Ra-226 content equal to 40 Bq/kg, was 2 mBq/m² s while that of the slab with Ra-226 content equal to 26 Bq/kg, was 3 mBq/m² s. Assuming a typical Greek room (4×4×3.5m) constructed using materials with the above exhalation rates, and an air-exchange rate of 0.5/h the maximum radon concentration is assessed equal to 34 Bq/m3, which is much lower than the 150 Bq/m3 recommended action level.