Thoron measurement

Several papers present reviews of measurement methods or improvements in existing methods. Yamashita et al. (1987) present the description of a portable liquid scintillation system that can be used for thoron (Rn-220) as well as radon (Rn-222) in water samples. Thoron measurements have not been made for houses where radon in water may be a significant source. Such an instrument could be useful in making such determinations as well as in studying geochemical problems as described in this report. A review of measurement methods by Shimo et al. (1987) and of development and calibration of track-etch detectors (Yonehara et al., 1987) are also included. Samuelsson... 

Figure 3 Experimental set-up for optimizing the counting system for thoron measurements.

In an earlier paper, results of measurements of Rn-222 (thoron progeny) have been discussed. In normal areas, the concentration of Rn-220 progeny is usally lower than 2-3 Bq/m. in some houses in a Th-rich area, the Rn-220 progeny concentration was higher than 10 Bqnf 3. (stranden, 1984). 

Khan, A., F. Bandi, C.R. Phillips and P. Duport, Underground Measurements of Aerosol and Radon and Thoron Progeny Activity Distributions, to be published in Proc. 191st American Chemical Society National Meeting, New York, April 13-18 (1986). 

During the transport in the chamber, radon atoms decay to form RaA atoms. The RaA atoms except those diffused to the wall of the chamber are collected on the exit filter. The sample collected on the exit filter is removed and counted. The present method is able to simultaneously measure radon and thoron concentrations by alpha spectroscopic technique (Ikebe et al., 1979). 

The availability and suitability of those devices are briefly descrived as follows In high radon concentration, greater than 10 times outdoor levels, the DSC is suitable because air can simply be sampled in a short time. On the other hand, the ACC is useful for very low concentration but the operation is troublesome. For continuously measurement of the radon concentration over long period, the PFC and the ERM are available. The TF can be used for obtaing the radon and thoron concentrations. The PRM is useful for simultaneously obtaining many radon values at various locations. 

A. M. Bhagwat, and S. D. Soman, Passive Measurement of Radon and Thoron Using TLD of SSNTD on Electrets, Health Physics, 43 399-404... 

Megumi, K. and T. Mamuro, A Method for Measuring Radon and Thoron Exhalation from the Ground, J. Geophysical Red earch, 77 3051-3056 (1972). 

On other hand, we found a good correlation between the results by the present method and those by the Terradex detector. However, the mean value obtained by the Terradex detector were about twice those by the present method. The reasons for this significant difference are unknown and may be due to errors in the calibration experiments and in the conditions during the measurements. In the calibration experiments the effect of existence of thoron in the chambers could be one of the reasons. As regards the condition in the measurements, methods for subtraction of background tracks and deposition of dust onto the bare detector could be candidates. However, we do not have enough data to determine the reasons for some of this difference. 

Underground Measurements of Aerosol and Activity Distributions of Radon and Thoron Progeny... 

Radon concentrations were measured by use of calibrated Lucas scintillation flasks, while radon and thoron daughters and the resulting potential alpha energy concentration (PAECj were determined using filter samples (Thomas, 1972) and a continuous electrostatic precipitator (Andrews et al., 1984). The radon daughter positive... 

This paper deals with the plate-out characteristics of a variety of materials such as metals, plastics, fabrics and powders to the decay products of radon and thoron under laboratory-controlled conditions. In a previous paper, the author reported on measurements on the attachment rate and deposition velocity of radon and thoron decay products (Bigu, 1985). In these experiments, stainless steel discs and filter paper were used. At the time, the assumption was made that the surface a-activity measured was independent of the chemical and physical nature, and conditions, of the surface on which the products were deposited. The present work was partly aimed at verifying this assumption. 

Large Radon/Thoron Test Facility (RTTF). Figures 1 to 5 show the surface a-activity measured on several materials exposed to a radon progeny atmosphere (Figures 1 and 2), and to a thoron progeny atmosphere (Figures 3 to 5). 

Figure 3 shows a significant difference in thoron progeny activity plated-out on laboratory coat cotton cloth samples and Millipore filters (0.8 /un), and emery cloth. Differences between the two latter sample materials are not clear in these measurements. 

Figures 3 to 5 show that the surface thoron progeny a-activity remained fairly constant during the counting period, i.e., about 100-min. This result indicates that the a-activity measured on the surface of the materials was mainly due to Bi-212, and Po-212 in equilibrium with Bi-212, which was in equilibrium with the relatively long-lived, 10.6 hour half-life, Pb-212. Hence, Bi-212 decayed with the half-life of Pb-212.

In order to verify the results reported above, an independent series of measurements in the large RTTF was carried out. Samples were exposed to a thoron progeny atmosphere, and surface gross a-particle activity was measured this time for periods of 30 min. The environmental conditions during this experimental phase were in the fol lowing range. Temperature 24-27°C, relative humidity 40-55%, aerosol concentration 1.2 x 103 - 3.4 x 103 cm-3. Some of the data obtained are reproduced in Table I. 

Tables II (radon progeny) and III (thoron progeny) present some of the data obtained. In these Tables Nj and N2, obtained with the weak reference sources, stand for the gross a-particle count measured 1-min and 40-min after exposure, respectively. The symbols N3 and N4 represent the same as Nj and N2, respectively, but for the strong references sources. Furthermore, in the above Tables Ni 2 = N1 +...

The underlying physical and/or chemical mechanisms responsible for the differences observed between the radon progeny and the thoron progeny as related to different materials are not clearly understood. Finally, it should be pointed out that the main thrust in this paper was to determine differences in surface a-activity measured on different materials with the same geometrical characteristics exposed to identical radioactive atmospheres. The calculation of deposition velocities and attachment rates, although it follows from surface a-activity measurements, was not the intent of this paper. This topic is dealt with elsewhere (Bigu, 1985). 

The same kind of optimization has been performed for the thoron daughters. In the calculations the sampling period was set at 30 min and the first decay time interval is started after the decay of the radon daughters (270 min). For a total measurement time of 16 hours the optimized MMC of Pb-212 and Bi-212 are respectively 0.02 Bq/m and 60 Bq/m (270-370 min, 540-960 min). Better results for Bi-212 are obtained with only one decay time interval and an estimation of the ratio of Pb-212 to Bi-212 out of the removal processes (ventilation and deposition of the attached thoron daughters). The influence of the removal rate on the potential alpha energy concentration is small. For the decay interval (270-960 min) the MMC of Pb-212 is 0.014 Bq/m, assuming the sum of the removal rates to be 0.6+0.5/h. 

Exposure to thorium can be determined by measurement of radioactive thorium and/or daughters in the feces, urine, and expired air. The primary route of excretion of thorium is in the feces following either inhalation or oral exposure. Fecal excretion is essentially complete in a matter of several days (Patrick and Cross 1948 Scott et al. 1952 Sollman and Brown 1907 Wrenn et al. 1981). The measurement of external gamma rays emitted from thorium daughters present in the subject s body and of thoron in the expired air many years following exposure can be used to estimate the body burden of thorium (Conibear 1983). 

In the environment, thorium and its compounds do not degrade or mineralize like many organic compounds, but instead speciate into different chemical compounds and form radioactive decay products. Analytical methods for the quantification of radioactive decay products, such as radium, radon, polonium and lead are available. However, the decay products of thorium are rarely analyzed in environmental samples. Since radon-220 (thoron, a decay product of thorium-232) is a gas, determination of thoron decay products in some environmental samples may be simpler, and their concentrations may be used as an indirect measure of the parent compound in the environment if a secular equilibrium is reached between thorium-232 and all its decay products. There are few analytical methods that will allow quantification of the speciation products formed as a result of environmental interactions of thorium (e.g., formation of complex). A knowledge of the environmental transformation processes of thorium and the compounds formed as a result is important in the understanding of their transport in environmental media. For example, in aquatic media, formation of soluble complexes will increase thorium mobility, whereas formation of insoluble species will enhance its incorporation into the sediment and limit its mobility. 

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