Studies on Pinhole Dosimeter and Conclusions

Transcription

1 Studies on Pinhole Dosimeter and Conclusions Chapter 6 The main objective of the doctoral work was to explore the relation between the congenital malfunctions and environmental high background radiation, if any. However, with academic interest, I undertook an investigation on the new idea of pinhole dosimeters which are being used to replace the conventional membrane type dosimeters. The working principle of membrane dosimeter to discriminate thoron against radon in a mixed environment is based on the time delay produced by a suitable diffusion barrier. In twin cup dosimeter, one compartment has gas diffusion through a glass filter paper to permit entry of thoron and radon preventing their daughter products into the cup volume. The other compartment fixed with the membrane permits entry of radon alone producing sufficient delay for thoron to decay while diffusing through the membrane. 6.1 Thoron Discrimination Using Diffusion Barriers As we deal with radon studies in indoor environment, our focus should also involve a special attention to thoron present together with radon so that the radon risk will be properly evaluated. It is observed that in some cases the concentration of thoron is much larger than that of radon in dwellings especially in the regions with thorium rich soil. Measuring radon and thoron discriminatively is among the major tasks in environmental radiation measurement. Radon and thoron are alpha active, but with different radioactive decay time. The half-life for radon ( 222 Rn) is 3.82 days where as that of thoron ( 220 Rn) is about 55 sec. This difference in half-life is made use in many measurement techniques of radon measurement in a thoron rich environment. In some applications, for the discrimination of radon from a mixed environment of radon and thoron special membranes are used. Such membranes delay the diffusion of the gas and as a result a great percent of the long lived radon will pass through the membrane while virtually all the short lived thoron will decay during its diffusion to the measurement chamber. There are some other applications where longer lived radioactive isotopes of elements can be separated from a mixture of isotopes in passing from one volume to a second. This is done by 167

2 the use of an appropriate intervening barrier by which the shorter lived isotopes decay before reaching the detector (1). Leung et al. (2007) used polyethylene (PE) membrane of different thicknesses, which allows the largest amount of radon gas to diffuse into the diffusion chamber while at the same time the largest amount of thoron gas are screened out. Apart from this, polyethylene membrane prevents the enrichment of alpha particle track density due to the deposition of radon or thoron progeny on the outside surface of the membrane (2). Qiuju Guo et al. in 1995 developed a passive nuclear track detector device with a pair of passive cup monitors for measuring 220 Rn concentration. These monitors have different opening diameters on the bottom. Each opening is covered with a membrane filter of pore size 0.8 µm through which radon or thoron gases in the monitor can be exchanged by diffusion naturally. The cups are hemispherical with one exchange opening of diameter 5mm on the 222 Rn cup and four exchange openings of diameter 20 mm on the 220 Rn cup, respectively (3). The New York University developed 4-Leaf monitor, a passive miniature radon-thoron discriminative device, which can be used as personal or area detector. It utilizes four separate chambers with CR39 SSNTDs for duplicate measurements of radon and total gas (radon plus thoron). Signal difference between the two sets of measurements gives thoron concentration. Each chamber has a conducting foam entrance filter which prevents entry of the decay products of radon and thoron, and an O ring seal to prevent leakage of thoron gas around the diffusion barrier (4). The ideal membrane or barrier should be one that allows radon to pass through without any significant reduction in its concentration while preventing the shorter lived thoron from passing through the measuring volume. The diffusion of the gas through membranes is function of the permeability constant of the membrane material. The permeability constant for different materials like aluminized polycarbonate, aluminized mylar, polycarbonate, polyethylene terephthalate, hydrated cellulose, cellulose nitrate, PVC cling, polyallyldiglycolcarbonate, polyester, makrofol, polyethylene, plexiglas, papers and glass were evaluated by Wafaa Araffa in 2002 (5). In 1997 in order to study thoron and radon diffusion through different filters, Shweikani et al. designed a special chamber which consisted of two equal halves, upper and lower, with 168

3 dimensions of 16 cm diameter and 12 cm height each. Each of these halves had an inlet and outlet pipe for circulating the gas inside the chamber. The top half also had an electronic connection for a semiconductor surface barrier detector (SBD). A filter with a diameter of 18 cm or more could be placed between the two halves. Six different types of filters were used in this study and they are (i) quartz, (ii) glass fibre, (iii) glass fibre coated with Teflon, (iv) normal filter paper, (v) commercial cling film and (vi) ordinary A4 Xerox paper. The results of the study show that thoron gas diffuses through most of the filters, but in varying extents. The study also found commercially available PVC cling film and the quartz micro fibre filter to be the best filters, as they could stop thoron most effectively while also allowing a sufficient amount of radon to diffuse through. Infact, PVC cling film seems to be the best filter as it is very cheap and easily available (6). To separate the contribution from radon and thoron, passive solid state nuclear track detector (SSNTD) type discriminative devices based on different sampling techniques can also be used. In 2004 Eappen and Mayya developed LR-115 SSNTD based cylindrical twin cup dosimeter using membrane for thoron cut-off, which has since then been used widely for measurement of radon and thoron in dwellings. For our present study too the same dosimeter was used. The dosimeter has two opposite facing entrances which use glass fibre filters to cut off entries of decay products. In addition, one entrance uses a cellophane membrane to cut-off thoron transmission and to ensure that only radon enters into the radon chamber. The other entrance i.e., radon plus thoron chamber allows both radon and thoron. The difference in concentration of the two chambers will give the thoron concentration (7). In 2005 for measuring indoor radon/thoron concentrations, Eappen introduced SSNTD based pin-hole dosimeter which is a cylindrical plastic chamber divided into two equal compartments. One of the cups called filter cup allows the entry of both radon and thoron inside by covering the cup with a glass fiber filter and so tracks formed on the film in the filter compartment are due to both radon and thoron gases. The other cup called pinhole cup allows only the entry of radon gas through it. This dosimeter is a modified version of the earlier twin-cup dosimeter system in which the pinhole is designed in size and thickness of cap so as to block thoron, by considering the diffusion length and half- life of thoron (8). In 1994, Doi M et al. developed a pinhole based passive radon thoron discriminative dosimeter. The dosimeter comprises two hemispherical diffusion chambers made of carbonated poly carbonate plastic whose diameters are 110 mm and 70 mm respectively. In 169

4 the dosimeter the pinhole air inlet of size 1 mm is tightly in contact with a glass fiber filter to prevent dust from entering the detector. The air inlet of the detector is also covered with a half-cutted hemispheric wind break of 40 mm height. This is aimed at protecting the glass fiber filter and stabilizing the influence of convectional air flow on the entry rate of radon and thoron into the dosimeter. The experiment done using this radon thoron discriminative dosimeter showed that radon and thoron entry rate into the 110 mm hemisphere chamber were found out to be % and % respectively whereas in the 70 mm hemisphere chamber, they are % and 0.47 % respectively. A short-term study in the concrete cellars in Japan using the aforesaid dosimeter proved that it is comprehensive enough to be used in large scale radon-thoron discriminative surveys (9). In 2014, Amanath et al developed a small and inexpensive passive diffusion chamber for the separate measurement of radon and thoron in soil by using fiber glass filters and 250 µm thick Lexanpolycarbonate detectors (10). 6.2 Gas Diffusion Mechanism Convective flow is generally considered as the primary driving force behind elevated radon levels in homes and buildings due to the reduction of pressure relative to the surrounding soil. Diffusion is also of major importance in this regard and it can at times result in high indoor radon concentrations (11). Radon from domestic water and gas supplies and from building materials can contribute to the indoor radon concentration in a building, although the contribution is very little compared with that from the soil gasses in the ground. Since radon is a gas, it can migrate by mechanism of diffusion and convection through pore spaces in the soil, fractures in the rocks and along with weak zones such as shear, faults, thrust and so on. Radon is continually formed in soil and is released to air where its subsequent decay products are formed. Radon exhalation from soil to the surrounding atmosphere is influenced by various factors such as radon emanating power in the materials, permeability of the soil and underlying rocks, moisture content of the soil etc. When earthquake or landslide occurs, the resultant movement of soil offers more gaps for exhalation of radon gas to surrounding atmosphere. The indoor radon concentration level can also be enhanced by its exhalation from building materials, ventilation pattern, architectural style of building, water, burning gas etc (12). 170

5 There are several factors that affect radon diffusion and transport through different media. For any material medium the porosity, permeability and the diffusion coefficient are the parameters, which can quantify their capability to hinder the flow of radon soil gas. An increase in porosity results in more air space within the material for radon to travel and thereby reduces resistance to radon transport. The permeability of material describes ability to act as a barrier to gas movement when a pressure gradient exists across it and is closely related to the porosity of the material. The radon diffusion coefficient of a material quantifies the capability of radon gas to move through it when a concentration gradient is the driving force. This parameter is proportional to the porosity and permeability of the medium. The diffusion of the radon through the ground is related to the porosity and permeability, which is dependent on grain size distribution, degree of compaction and the water content in the soil. Radon diffusion through material media obeys the following equation, = exp (- ) X (6.1) Where N is the radon concentration at any time t at a distance X from the source, N 0 is the concentration of radon at source, is the decay constant of radon and D is the diffusion coefficient (13). 6.3 Fick s Law of Diffusion The diffusion of gases through solids is governed by the first and second Fick s laws of diffusion. Fick s first law states that the flux goes from the regions of high concentration to regions of lower concentration gradient with a magnitude that is proportional to the concentration gradient. In one dimension, J= D c/ x (6.2) Here J is the diffusion flux which measures the amount of substance that will flow through a small area during a small time interval (mol/ /s). D is the diffusion coefficient ( /s), C is the concentration (amount of substances per unit volume) measured in mol/ and X is the position (length). First law discusses the relation between flux and spatial gradient of concentration. But the density profile (concentration) is not only varying in the spatial dimension, but also in time. That is, C = C(x,t). Determining the time evolution of the density profile enables us to know 171

6 how matter will disperse in space and time. If we consider the following figure, we can write a mass balance for the volume between x and x+dx as, = Inward flux - Outward flux (6.3) = ( - ) (6.4) = A - A (6.5) Now, we can determine the expression for concentration changing with time by dividing both sides by Volume, V as: = [ - ] (6.6) = [ - ] = - J (6.7) But it is clear that = (6.8) Using the above expression, equation (2) becomes, = - (6.9) Then = D (6.10) 172

7 This is the Fick s second law. This leads us to the inference that the time dependence of the concentration profile depends on the second derivative, and effectively, the curvature of the concentration gradient profile. The more severe the curvature, the faster the rate of change. 6.4 Application of Fick s Law to Pinhole Dosimeter As stated earlier, the twin cup dosimeter, developed by Eappen and Mayya, consists of a radon chamber and a radon plus thoron chamber. Radon plus thoron chamber gives the concentration of radon and thoron while radon chamber gives only the radon concentration. The concentration of thoron can be found out by subtracting the track density of radon chamber from that of radon plus thoron chamber. There are cases where this calculation does not always yield the expected results. In some cases it was observed that the track densities of SSNTD detector placed in radon chamber exceeded the track densities of detector placed in radon plus thoron chamber. Consequently, it gives result to a physically unacceptable negative thoron concentration; one possible reason for this could be the different entry rates of radon through two entrances of the dosimeter which may arise from turbulence or air flow in one direction. This ambiguity of different radon entry in two chambers can be removed by developing a twin chamber device having a single entrance (14). The required design can be achieved by replacing the membrane with a pin-hole based discriminating design. 173

8 Figure 6.1: A pinhole based twin cup dosimeter Consider a closed cylindrical chamber having a pinhole of radius r and length d at one face. The assumption is that gas enters the chamber through pin-hole by the process of diffusion. Under the assumption that the transportation of gas is only in one direction and the production of radon inside the media is neglected, the non-steady state differential equation for the diffusion is, = (6.11) Where C(t) is the average radon gas concentration inside the chamber volume at time t, J is the radon activity flux at the hole, A is the area of the hole, V is the volume of the chamber and λ is the radon/ thoron decay constant. The flux J through a pin-hole is related to the difference in the concentration of the gas between the outside and inside air. Using Fick s law of diffusion, this relation can be stated as follows: J = -D J = D (6.12) 174

9 Where C 0 is the radon/ thoron concentration in the outside air at the entry face of the chamber and D is the radon diffusion coefficient in air (hole). Then equation (6.11) will become, = A - c = ( + ) C (6.13) Put = and + = Then = - C (6.14) = dt [ ] =dt = dt On integration we get -log [ ] = k (6.15) Here, K is the constant of integration and can be evaluated from the conditions: at time t = 0, C = 0; therefore, k = -log (6.16) Using the value of K, equation (6.15) becomes, -log ( ) = log ( ) (6.17) The equation 6.17 gives: C(t)= ( ) (1- ) (6.18) If we take, time t =, we have C (t) approximately equal to 95 % ( ) 175

10 The time required by radon or thoron for attaining 95% of its final steady state concentration in the pinhole chamber can be proved as (14), = (6.19) At longer time (say, t =, a steady state is reached having a steady state concentration (C s ) in the pin-hole chamber: = ( ) Now we can define a new quantity, transmission fraction (14), which is the ratio of final steady state concentration of radon or thoron in the pinhole compartment(c S ) to the concentration of radon or thoron at the outside of the chamber at a time t = 0 (C 0 ). Transmission fraction= = Here, we can see that a pinhole with a diameter and hole length 1mm each and chamber volume 150 cm 3, the percentage transmission of radon is 96% and that for thoron is only 4% (14). Therefore, the idea of using pinhole as mode to discriminate thoron is effective. 6.5 Comparative Study between the Membrane and the Pinhole Dosimeters A study of comparison between pinhole and membrane dosimeters was done in order to validate the effectiveness of pinhole dosimeter. 50 twin cup dosimeters equipped with pinhole having a diameter of 1 mm and length 5 mm on one side and membrane on the other side were placed in the mixed field environment of south west coast of Kerala for investigating the effectiveness of thoron discrimination. After 3 months the dosimeters were retrieved from the field and the detectors in the pinhole compartment and membrane compartment were chemically etched. Then the alpha tracks on the detectors were counted by using a spark counter. The results obtained were found to have a very good correlation between the number of tracks produced in pinhole dosimeter and membrane dosimeter so that the pinhole dosimeters can be used as a radon only device. Results from the experiment are given in the table

11 Table 6.1: Track density from the membrane and pinhole compartment Place Track Density Membrane Pinhole Kovilthottam Location Kovilthottam Location Kovilthottam Location Kulangarabhagom Location Kulangarabhagom Location Kulangarabhagom Location Kulangarabhagom Location Karithura Location Karithura Location Puthukkadu Location Palakkadavu Location Kottankulangara Location Kottankulangara Location Puthukkadu Location Puthukkadu Location Payyalakkavu Location Puthukkadu Location Puthukkadu Location Puthukkadu Location Puthukkadu Location Puthukkadu Location Menampalli Location Menampalli Location Mukundapuram Location Mukundapuram Location Pattathanam Location Pattathanam Location Pattathanam Location Puthenchantha Location Mukundapuram Location Kottukadu Location Mukundapuram Location Madappalli Location Padinjattakkara Location Kottakkakam Location Padinjattakkara Location Padinjattakkara Location Padinjattakkara Location

12 Pinhole Tracks Koivila Location Puthensanketham Location Puthensanketham Location Koivila Location Koivila Location Koivila Location Koivila Location Koivila Location Koivila Location Arinalloor South Location Arinalloor South Location There is an excellent correlation (R=0.92) between the tracks obtained from pinhole compartment and membrane compartment. Figure below is a portrayal of the correlation between the membrane tracks and pinhole tracks for a diameter of 1mm and pinhole length 5mm. Correlation Coefficient R = 0.92 Membrane Tracks Figure 6.2: Correlation between pinhole and membrane dosimeters 178

13 6.6 Conclusions A detailed investigation, which was carried out in about four years, in search of a suspected/feared correlation between the natural high background radiation and the congenital malfunctions namely mental retardation and cleft lip/palate seen in the HBRA in Kerala suggest no statistically significant association of either mental retardation or cleft lip/palate with high level natural radiation. The major radiation exposure components namely inhalation and external dose rates were estimated. The state of the art technology was employed for the passive time integrated inhalation dose measurements. External dose measurements were carried out in both active and passive methods and the excellent correlation between the results validate the quality of the data obtained. Major confounding factors such as gestation, mothers chewing habit, mother s blood pressure, mother s anaemia, forceps delivery and the like were considered for the analysis. Along with the major objective, an investigation on the pinhole dosimeters was also carried out. Field measurements strongly indicate the suitability of pinhole dosimeters in place of membrane dosimeters. 6.7 Limitations of the Study No active measurements of inhalation dosimetry were carried out. Regular intercalibration experiments were carried out to validate the data obtained. Apart from the assessments of inhalation and external dosimetry made experimentally, the data and pieces of information collected from the members of the public were obtained orally from either the mother or father of the case or control. The answers against the questionnaire filled up at the time of the field measurements given by the family members may not be accurate. But this does not affect the conclusions. Odds ratios for mental retardation in mothers having blood pressure (OR- 4.38, 95% CI ) and forceps delivery (OR 4.1, 95% CI ) show some significance statistically. Similar is the case for the cases of cleft lip/palate, the mothers with chewing habit (OR 8.5, 95% CI ). These factors have not been studied in detail. 179

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