Thoron progeny

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). 

Theoretical calculations of unattached fractions of radon or thoron progeny involve four important parameters, namely, 1) the count median diameter of the aerosol, 2) the geometric standard deviation of the particle size distribution, 3) the aerosol concentration, and 4) the age of the air. All of these parameters have a significant effect on the theoretical calculation of the unattached fraction and should be reported with theoretical or experimental values of the unattached fraction. 

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). 

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 ... 

The first sample of the day was counted at 270-285 minutes after sampling in order to allow counting to be completed in the underground working period. Equation (7) was used to calculate the thoron progeny Working Level in this case, with 1(330-345) replaced by 1(270-285). 

Table 11(b). Working Levels of Radon and Thoron Progeny (based on eqns. (6) and (7)... 

Khan, A. and C.R. Phillips, A Simple Two-Count Method for Routine Monitoring of Radon and Thoron Progeny Working Levels in Uranium Mines, Health Phys. 50 381-388 (1986). 

Plate-Out of Radon and Thoron Progeny on Large Surfaces... 

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.)... 

Further plate-out studies were conducted using radon progeny and thoron progeny reference sources, models Rn-190 and Th-190, respectively, manufactured by Pylon Electronic Development (Ottawa), hereafter referred to as Pylon sources, for simplicity. These are small cylindrical containers (<40 cm3 volume) provided with a Ra-226 source or Th-228 source. The containers can be opened at their base and some suitable material can be placed in it for exposure purposes (Vandrish et al., 1984). The Ra-226 and Th-228 sources decay, respectively, to Rn-222 and Rn-220 which in turn, decay into their progeny. In this respect, the above sources can be considered miniature RTTFs quite suitable or plate-out studies, in which air flow pattern effects are minimized. 

The bulk of the data presented here was obtained in a follow-up to a plate-out study reported earlier elsewhere (Bigu and Frattini, 1985). It should be noted that some qualitative and quantitative differences between earlier studies and the most recent follow-up may be observed depending upon experimental conditions. Although the conditions of the surface of the material, environmental conditions and air flow patterns are believed to play a significant role in plate-out phenomena, their effect on the latter, and their relationships, are far from being understood. Furthermore, the chemical nature of the material, and of the radon and thoron progeny, could play a significant role in plate-out phenomena. 

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 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 ).

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.

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

Table III- Thoron progeny plate-out on different materials using Pylon source standards model Th-190...

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. 

The effect of surface electrostatic charge on a material on the attachment of the decay products of radon has been known since pioneering work on atomic structure by Rutherford. Extensive research into this area for the radon and thoron progeny has been conducted in this laboratory for environmental monitoring purposes. Several authors have reported on the effect of electrostatic charge on the collecting characteristics of copper for the radon progeny for exploration purposes (Card and Bell, 1979). 

Pylon Radon (Thoron) Progeny Reference Sources. Plate-out studies using the Pylon reference sources are shown in Tables II and III. In this case the materials (discs) were placed in the reference sources... 

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 +...

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 data presented here suggest some qualitative and/or quantitative differences in behaviour between the radon progeny and thoron progeny relative to some materials, and for some materials relative to either the radon progeny or the thoron progeny. 

Not previously considered in the above reference is a-particle self-absorption phenomena. However, attempts are at present being made to estimate radon and thoron progeny deposited on the surface of materials by y-spectrometry and gross y-count in order to eliminate self-absorption effects. 

Some of the data reported here, and unpublished data, seem to indicate that there might be some qualitative difference between the behaviour of the radon progeny and thoron progeny towards a given material. 

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). 

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