Simultaneous Measurement of Radon and Thoron Exhalation Rate from Soil and Building Materials

The mainly sources for indoor radon concentrations in all types of houses are the soil radon flux under the building (about 80%) and the radon exhalation from building materials (about20%). Our paper presents two methods for simultaneous measurement of this exhalation from soil and building materials: (1) -charcoal adsorption, respectively (2)-Lucas cell method. The both methods are applicable especially in the case of a soil flux enhanced in thoron gas. In the Lucas cell case a short accumulation time was used (10-15 minutes). The thoron concentration under accumulation volume was measured immediately after the sample gas extraction and a regression equation is used for determining equilibrium thoron concentration. In the case of charcoal method the thoron was measured 4 hours after a special degassing of the sample. Using the LUK-3A device for the building materials, the thoron flux was measured only on a special enhanced thorium+radium sample and the results are not presented here. INDEX TERMS Radon and Thoron, Exhalation, Charcoal adsorption, Soil radon INTRODUCTION The radon isotopes 220 Rn (thoron) and 222 Rn (radon) were intensely studied in the last time due to their involvement in the lung cancer risk (Nazaroff and Nero, 1988) and also for geological purposes (Akerblom and Melander, 1996). The main sources of the indoor radon are the soil radon and radon exhalation from building materials (Stranden, 1988). The active charcoal was often utilized for radon measurement especially for indoor radon (George, 1984; Prichard and Marien, 1985). The method of the soil radon and exhalation measurement from the soil and building materials using adsorption in active charcoal was also used (Megumi and Mamuro, 1979; Li, Schery and Turk, 1992; Oberstedt and Vanmarke, 1996) with good results but in this case corrections regarding charcoal humidity related of the break point of charcoal are needed (Scarpitta, 1996). In all these cases the radon adsorbed in charcoal is measured by gamma spectrometry, commonly with NaI(Tl) detectors. For economical and practical reasons, the charcoal canisters must be degassed and re-used of many times. Commonly, the charcoal cleaning is made by degassing the charcoal at 120-140C during 12-14 hours, but in the case of high radon content adsorbed in charcoal, for a complete desorption of radon, 2-3 steps are necessary (Cosma, Van Deynse and Poffijn, 1997). The method for charcoal degassing proposed in this work has the advantage to be more rapid (10-15 minutes) and it produces a much better degassing. On the other hand, the manner of degassing allows the thoron determination from charcoal even in the case when it is measured by low gamma spectrometry using NaI (Tl) detectors. Generally, the measurement methods for thoron determination require either alpha or high gamma resolution spectroscopy. This work presents the method for simultaneous measurement of radon and thoron from soil using charcoal adsorption. Exhalation of radon from some Romanian buildings materials obtained by charcoal adsorption method was verified in several cases by Lucas cell method. In the case of one sample the result was compared with those obtained in the frame of ERRICCA Intercomparison Exercise from Athens in April 1999 (Petropoulos, Anagnostakis and Simopoulos, 1999). Finally, the both radon and thoron exhalation from a special concrete sample made in our laboratory, artificially enhanced in 226 Ra and also in 224 Ra(Th[NO3]2) was made. EXPERIMENTAL METHODS A brass cylindrical vessel of 0,7 L, Fig. 1, was used for charcoal degassing. In the upper part it is connected to a vacuum pump. After filling (250 g of charcoal) and closing using six screws and a lead fitting, the vessel is put into another larger vessel with water. The later is placed on an electric boiling. When reaching the water boiling temperature, the pump is started for 15 minutes. The charcoal is then pulled out and can be used for radon-thoron adsorption in the next exposure. Measuring the charcoal radioactivity after this operation, the control of degassing efficiency is made. After about 3,5 hours from the end of the degassing process, the gamma radioactivity is Fig.1.The brass container for radon degassing Fig.2.The device for radon flux gathering reduced to the background radioactivity of the charcoal. For radon exhalation gathered from soil was used the device from Fig. 2, which consists from a frame box of 0.23 m 2 (37x62 cm), and about 200g degassed charcoal. After exposure (as a rule, 4-12 hours) the charcoal is introduced into the Marinelli vessel and gamma radioactivity is measured over the whole spectrum with an NP424 gamma spectrometer equipped with a large NaI(Tl) detector (76x45 mm) enclosed in a lead castle of 30 mm wall thickness to reduce the background. In the case of thoron absence from the charcoal, the radon exhalation rate Rn can be calculated following equation: