Thoron progeny activity

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). 

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 4 and 5 show significant differences in thoron progeny activity plated-out on cotton cloth and emery paper (Figure 4), and cotton cloth and Fiberglas filters (Figure 5). Differences between the other pair of materials were less pronounced or difficult to ascertain (<7 ).

Table I Thoron progeny activity plated-out on different materials...

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

The thoron progeny Working Levels for calculation of activity distributions were calculated as ... 

Experiments were conducted in a large (-26 m3) radon/thoron test facility (RTTF) designed for calibration purposes and simulation studies (Bigu, 1984). A number of different materials were exposed in the RTTF to a radon/radon progeny or thoron/thoron progeny atmosphere. Exposure of the materials was carried out under laboratory-controlled conditions of radiation level, aerosol concentration, air moisture content and temperature. The materials used were in the form of circular discs of the same thickness (-0.5 mm) and diameter (-25 mm), and they were placed at different locations on the walls of the RTTF at about 1.6 m above the floor. Other samples were placed on horizontal trays. Samples (discs) of different materials were arranged in sets of 3 to 4 they were placed very close to one another to ensure exposure under identical conditions. Exposure time was at least 24 hours to ensure surface activity equilibrium, or near equilibrium, conditions. 

Bq/m3) thoron progeny Working Level, WL(Tn) 0.15-14 WL. (The square brackets are used here to denote activity concentration.)... 

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). 

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.

Figure 3 Alpha activity versus time from thoron progeny plated-out on several materials.

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. 

II and III). This result was expected because of the well known adsorptive properties of carbon for radon and thoron. However, the activated carbon data on Tables II and III is in sharp disagreement with previous, and numerous, data obtained in the large RTTF (see Table I). This topic is under investigation. It should be noted that there are substantial quantitative differences between the behaviour of the radon and thoron progeny relative to activated carbon. 

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). 

Cheng, Y. S. Yu, C. P. "Hi, K. W. Intercon parison of Activity Size Distributions of Thoron Progeny by Alpha and Gamma Counting Mediods. Health Phys. 1994,66,72-79. 

Cheng, Y.S., Yu, C.C., Tu, K.W. (1994). Intercomparison of activity size distributions of thoron progeny by alpha-and gamma-counting methods. Health Phys. 66, 72-79. 

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