Multi-parametric approach towards the assessment of radon and thoron progenyexposures Rosaline Mishra, B. K. Sapra, and Y. S. Mayya Citation: Review of Scientific Instruments 85, 022105 (2014); doi: 10.1063/1.4865165 View online: http://dx.doi.org/10.1063/1.4865165 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Approach to precise indoor characterization of multi-junction CPV cells using reference component cells AIP Conf. Proc. 1556, 133 (2013); 10.1063/1.4822216 An indoor radon survey of the X-ray rooms of Mexico City hospitals AIP Conf. Proc. 1544, 86 (2013); 10.1063/1.4813464 Indoor Radon Measurement in Van AIP Conf. Proc. 899, 736 (2007); 10.1063/1.2733477 Indoor Radon Measurement In The City Of Edirne, Turkey AIP Conf. Proc. 899, 395 (2007); 10.1063/1.2733203 Simple, discriminative measurement technique for radon and thoron concentrations with a single scintillation cell Rev. Sci. Instrum. 73, 69 (2002); 10.1063/1.1416121
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 022105 (2014)
Multi-parametric approach towards the assessment of radon and thoron progeny exposures Rosaline Mishra,1,a) B. K. Sapra,1 and Y. S. Mayya2 1 2
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Indian Institute of Technology, Mumbai, India
(Received 14 November 2013; accepted 4 December 2013; published online 20 February 2014) Conventionally, the dosimetry is carried out using radon and thoron gas concentration measurements and doses have been assigned using assumed equilibrium factors for the progeny species, which is inadequate pertaining to the variations in equilibrium factors and possibly due to significant thoron. In fact, since the true exposures depend upon the intricate mechanisms of progeny deposition in the lung, therefore an integrated approach for the assessment of progeny is essential. In this context, the recently developed deposition based progeny concentration measurement techniques (DTPS: Direct Thoron progeny sensors and DRPS: Direct Radon progeny sensors) appear to be best suited for radiological risk assessments both among occupational workers and general study populations. DTPS and DRPS consist of aluminized mylar mounted LR115 type passive detectors, which essentially detects the alpha particles emitted from the deposited progeny atoms on the detector surface. It gives direct measure of progeny activity concentrations in air. DTPS has a lower limit of detection limit of 0.1 Bq/m3 whereas that for DRPS is 1 Bq/m3 , hence are perfectly suitable for indoor environments. These DTPS and DRPS can be capped with 200-mesh type wire-screen to measure the coarse fraction of the progeny concentration and the corresponding coarse fraction deposition velocities as well as the time integrated fine fraction. DTPS and DRPS can also be lodged in an integrated sampler wherein the wire-mesh and filter-paper are arranged in an array in flow-mode, to measure the fine and coarse fraction concentration separately and simultaneously. The details are further discussed in the paper. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865165] INTRODUCTION
The most crucial part of epidemiological studies pertains to the assignment of doses to study subjects, be it occupational workers or general populations. In the case of gamma radiations, measurement of personal doses for occupational workers, and at least environmental exposure levels (if not personal doses) for residential populations, can be achieved fairly accurately by TLD (thermo luminescent detector) techniques (Driscoll et al., 1983). The situation in the case of radon exposures is quite complex and as yet there exist no methods comparable to TLD for gamma radiations for measuring personal doses. The well recognized reason is that radiation doses originate from the concentrations of airborne progeny species and not by the radon or thoron atoms; and progeny concentrations, are extremely difficult to monitor on a time integrated basis using passive detectors. Besides, the deposition of the progeny atoms in the lung depends on their size distribution and hence it is necessary to go beyond the gross concentrations for true dose estimations. Thus, while the gross Potential Alpha Energy Concentration (PAEC) is only a physical measure of air exposure, convenient for regulatory purposes, it does not uniquely determine radiological risks. In order to circumvent this difficulty, an important hypothesis, justifying the use of radon gas concentration itself as index of exposure, was used in most epidemiological studies. This is called the self-compensating theory (see, for a) Author to whom correspondence should be addressed. Electronic
addresses: [email protected] and [email protected] 0034-6748/2014/85(2)/022105/8/$30.00
example, Bochicchio et al., 2005). In this, it is assumed that the actual dose to the lung, is essentially a function of the radon gas concentration, even if the progeny concentrations varied due to varying equilibrium factors. The reason is attributed to the role of unattached fraction to lung dose. Consider two houses having the same radon concentrations, but different progeny concentrations because of differences in the aerosol concentrations. The house with higher aerosol concentration will have higher total progeny concentration. However, this will not lead to a proportional increase in the lung deposition of decay products since in this house the unattached fraction concentration will be smaller, which have an overwhelmingly higher deposition fraction as compared to attached fraction. Hence, overall deposition due to inhalation of the decay products will roughly remain the same for subjects in both the houses. However, the above arguments are at best qualitative and have certain short comings. As shown by Nikezic and Yu (2005), these can lead to a factor of 2 uncertainties in true exposures within the varying aerosol framework. If we consider varying ventilation rates as the cause of differing equilibrium factors, the theory is likely to be far more in error. For example, consider two houses having the same radon concentrations but different ventilation rates. The house with higher ventilation rate will have lower PAEC. Going by the above theory, the increase in the unattached fractions here will just about compensate the decreased PAEC. However, two exclusive processes based on well-known observations, will render the relative proportion of unattached fraction, uncertain. First is the injection of aerosols from the outside. A
85, 022105-1
© 2014 AIP Publishing LLC
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022105-2
Mishra, Sapra, and Mayya
house having higher air exchange rate is likely to bring in more nano-aerosol particles from outside (where the particle concentrations are at least a factor two higher than indoor) which would tend to keep down the unattached fraction to the original level, thereby offsetting the selfcompensation effect. Second, with increased air-exchange rate, the turbulence level (denoted by the friction velocity in the air) will increase thereby increasing plate-out and decreasing of total PAEC, beyond that expected from ventilation alone. The overall effect of this is on lung deposition is not fully studied, and at a first glance, due to nonlinearity of turbulence induced wall deposition, it is unlikely to be self-compensating. Another point against the use of gas concentrations as an index of exposure is due to the growing realization on the importance of thoron. It is now being increasingly recognized as an unjustly neglected component in environmental radioactivity studies. However, due to its short half-life, thoron gas is not uniform in the room and hence its measurement is only an indicator of its presence and not a quantitative measure of the progeny concentrations. A careful reflection shows that it is almost impossible to apply self-compensating justification in this case. There is thus a considerable need to develop techniques for obtaining time integrated measures of progeny related inhalation exposures. However, for estimating true doses, we also need to measure unattached and attached components of progeny PAEC. There have been important developments, towards developing techniques for assessing progeny concentrations directly; wherein the deposition property of the progeny species has been exploited to relate the deposited flux to the activity concentration of progeny in the environment. In this regard, mainly two types of deposition monitors are reported in the literature: (a) Deposition rate monitors developed by Zhuo et al. (2000) and (b) Direct Thoron (radon) progeny sensors DTPS(DRPS) (Mishra and Mayya, 2008). With the development of DTPS and DRPS sensors, it appears that we are close to arriving at a solution to the long standing question of personal dosimetry of subjects exposed to radon and thoron decay products, analogous to TLDs for gamma radiations. Different modes of DTPS and DRPS can be used to extract different parameters which are discussed in the sections on Specification of direct. . . , DRPS/DTPS in flow-mode, DRPS/DTPS in capped-mode, and Measurements of long-time. . . .
ηT T =
ηRR =
Rev. Sci. Instrum. 85, 022105 (2014)
SPECIFICATION OF DIRECT PROGENY SENSOR (DTPS AND DRPS)
Direct progeny sensing detector system is based on selectively registering alpha tracks originating from the deposited progeny activity on LR-115 type solid-state nuclear track detectors. The selection of alpha particle energies is achieved by mounting absorbers of suitable thicknesses on the LR-115 detectors. The 220 Rn (thoron) progeny sensing element is made up of LR-115 track detector mounted with aluminized mylar absorber of 50 μm thickness to selectively detect only the 8.78 MeV alpha particles emitted from 212 Po atoms, which are formed from the decay of the activities of 212 Pb and 212 Bi atoms deposited on the mylar surface. Since this is the highest alpha energy in the natural radioactivity decay series, there will be no interference from other alpha emissions. Similarly, the radon progeny sensor has an absorber thickness of 37 μm to detect mainly the alpha particles emitted from 214 Po (7.69 MeV) formed from the eventual decay of 218 Po, 214 Pb, and 214 Bi atoms deposited on it. For making the sensor system compact, the absorber is kept in close contact with the LR-115 detector rather than leaving an intervening air-gap (Fig. 1). The sensitivity factor of these detector elements is expressed as the track density registered for 1-day exposure to an environment containing 1 Bq m−3 of EETC (Equilibrium Equivalent Thoron Concentration) or EERC (Equilibrium Equivalent Radon Concentration). EETC and EERC are the quantities directly related to the potential alpha energy concentration in air and hence to the inhalation dose. EETC is related to the individual 212 Pb and 212 Bi activity concentrations, C2 and C3 , respectively, through the relation (ICRP, 1981), EETC = 0.91C2 + 0.09C3 . EERC is related to the individual 218 Po, 214 Pb, and 214 Bi activity concentrations, C1 , C2 , and C3 , respectively, through the relation (ICRP, 1981), EERC = 0.1C1 + 0.52C2 + 0.38C3 . Two major factors that decide the sensitivity factor are: (a) track registration efficiency, and (b) deposition velocity of the progeny atoms on the absorber surface. The track registration efficiency of DTPS/DRPS correlates the number of deposited progeny atoms to the alpha tracks registered (upon the ultimate decay of the progeny atoms) in LR-115 and is a fixed intrinsic property of the absorber-detector combination expressed as
Track density (Tr/cm2 ) recorded on the sensor upon complete decay of 212 Pb , Density of 212 Pb +212 Bi atoms (atoms/cm2 ) deposited on the absorber surface
Track density (Tr/cm2 ) recorded on the sensor upon complete decay of 214 Pb , Density of 218 Po +214 Pb +214 Bi atoms (atoms/cm2 ) deposited on the absorber surface
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022105-3
Mishra, Sapra, and Mayya
Rev. Sci. Instrum. 85, 022105 (2014)
where ηTT is the efficiency of thoron progeny registration on the DTPS, and ηRR is that for radon progeny registration on DRPS. The mean track registration efficiency for DTPS was measured as: ηTT = 0.083 ± 0.004 per alpha particle emitted from thoron progeny and ηTR = 0.002 ± 0.001 per alpha particle emitted from radon progeny. Similarly, for DRPS: ηRR = 0.07 ± 0.005 for radon progeny and ηRT = 0.01 ± 0.0004 for thoron progeny (Mishra et al., 2009a). The track registration efficiencies were measured by electrodepositing a known amount of the radon and thoron decay product activities, separately, on the aluminized surface. The track densities registered on the LR-115 detectors after the complete decay of all the deposited atoms were determined using the standard track etching protocols followed by counting with a spark counter.
Ve =
The second factor is the effective deposition velocities that combine the contributions from the fine (unattached) and the coarse (attached) fractions of the individual progeny species. Due to their high diffusivities and ability to stick to surfaces, the freshly formed decay products soon attach to existing aerosol particles, thereby giving rise to a continuous activity size distribution. This distribution is broadly classified into two groups, namely, the fine fraction (∼2 nm diameter) and the coarse fraction (∼100 nm). Since the responses of the sensor are directly proportional to the total number of progeny atoms on the surface, these can yield information only on the effective deposition velocities corresponding to the total progeny atom concentrations present in the atmosphere. It is expressed as
Total atom deposition flux (atoms/cm2 /s) of progeny species on the sensor . Total atom concentration (atoms/cm3 ) in the atmosphere
In the case of 222 Rn progeny species, due to their comparable half lives, the situation is little more complex. As in the earlier case, the effective deposition velocity may be expressed in terms of the atom concentrations (n1 , n2 , n3 ) of 218 Po, 214 Pb, 214 Bi, respectively, as Ve (radon progeny) =
V¯1 n1 + V¯2 n2 + V¯3 n3 , n1 + n2 + n3
V1 = p1 Vf + (1 − p1 )Vc , V2 = p2 Vf + (1 − p2 )Vc , V3 = p3 Vf + (1 − p3 )Vc , where V1 , V2 , and V3 are the corresponding average deposition velocities weighted with respect to their respective fine (p1 , p2 , p3 ) and coarse (1 − p1 , 1 − p2 , 1 − p3 ) activity fractions p2 Vf + (1 − p2 )Vc n2 + p3 Vf + (1 − p3 )Vc n3 Ve (thoron progeny) = , n2 + n3
where n2 and n3 denote the atom concentrations, and p2 and p3 denote the fine fractions, of 212 Pb and 212 Bi in the atmosphere, respectively, and Vf and Vc be the deposition velocities of the fine and coarse fractions. The measurements of deposition velocity were carried out both under controlled conditions and real indoor environments. The controlled condition experiments were aimed at validating the progeny deposition models. For thoron progeny, experiments were carried out in a test house (22.5 m3 , controlled conditions) and in indoor environments of dwellings (natural environment). In the test-house, a steady thoron concentration in the room was measured to be about
FIG. 1. Diagram of the detector element.
11 kBq m−3 , using standard double filter method. Around 62 DTPS detectors were exposed inside the room in 3 different orientations; viz., face-up, face-down, and vertical to study the orientation effect on the particle deposition indoors. Exposures were carried out for 1–5 days. Airborne activity concentrations were estimated by gross filter-paper sampling (flow rate 16 l min−1 ) and alpha counting methods at regular intervals to obtain representative air concentrations during the period of exposure. An orientation-wise breakup of the 62 measurements yields deposition velocities as 0.08 ± 0.02 m h−1 for face-up orientation, 0.061 ± 0.009 m h−1 for face-down orientation, and 0.055 ± 0.009 m h−1 for vertical orientation. The particle concentration measured using Scanning mobility particle sizer was obtained as 29 864 cm−3 . The friction velocity estimated from fan model was 2 cm s−1 , the attachment rate was estimated as 1.38 × 10−2 s−1 , and the activity median particle diameter was 0.116 μm. These input parameters were used for calculating the deposition velocities by Lai-Nazaroff Model, which were comparable to the experimentally obtained deposition velocities.
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022105-4
Mishra, Sapra, and Mayya
Rev. Sci. Instrum. 85, 022105 (2014)
For radon progeny, controlled condition experiments were restricted to a 0.5 m3 chamber, wherein the radon concentration was maintained as 3 kBq m−3 . The chamber was completely closed and hence there was no air exchange. Sixteen Direct Radon Progeny Sensor (DRPS) detectors were exposed inside the radon chamber, in 3 orientations. Air activity was measured using gross filter-paper sampling. An orientation-wise break of the 222 Rn progeny deposition velocities in the chamber yield values of 2.38 ± 0.84 m h−1 for face-up orientation, 2.36 ± 0.84 m h−1 for face-down, and 2.39 ± 0.835 m h−1 for vertical orientation. The particle concentration in the chamber was measured as 894 cm−3 , the friction velocity was estimated as 6.25 cm s−1 , the attachment rate was estimated as 1.49 × 10−3 s−1 , and the activity median particle diameter was 0.148 μm. Using these input parameters in Lai-Nazaroff Model, the theoretical deposition velocities were found to be comparable with the experimentally obtained deposition velocities. The radon progeny deposition velocities extrapolated for the testhouse conditions were obtained as: 0.14 m h−1 for faceup, 0.13 m h−1 for vertical, and 0.13 m h−1 for face-down orientation. Natural condition measurements were carried out in 15 dwellings, selected from different floors (storey) and the detectors were placed in face-up orientation at a height of about 1 m from the ground in the living-room for a period of 90 days. Regular filter paper sampling, covering both night and day time conditions, was carried out for estimating representative values of progeny concentrations in each of the dwellings. In mixed-field exposure situation, one requires all the four registration efficiencies for extracting the deposited radon and thoron progeny atom densities from the tracks counted on the two detectors using two simultaneous equations. Essentially, the thoron progeny atom fluxes are uniquely determined from the tracks on DTPS since it is almost insensitive radon progeny species. In contrast, the DRPS has non-negligible response from thoron progeny and for obtaining radon progeny fluxes, one is required to subtract the tracks contributed from thoron progeny as estimated from DTPS, using the following equation: onlyRnP
T racksDRP S
T otal = T racksDRP S −
progeny particle sizes in indoors using high-volume cascade impactor for sampling followed by gamma counting and observed that the average AMAD of 214 Pb was 170 nm, where as that for 212 Pb was 200 nm. The average AMADs for 214 Pb and 212 Pb were 380 and 390 nm in outdoors. Reineking et al. (1992) using a low-pressure cascade impactor AMAD of 214 Pb and 212 Pb being 199 and 217 nm, respectively, for indoor air, and 386 and 330 nm for outdoor air. Papastcfanou and Bondietti (1987) have reported that in the outdoors using impactors, they measured the mean AMAD values as 160 nm for 214 Pb and 130 nm for 212 Pb. By using the individual track registration efficiency and progeny deposition velocities averaged for indoor environment, a sensitivity factor of 0.94 ± 0.027 Tr cm−2 d−1 /EETC (Bq m−3 ) for DTPS and 0.09 ± 0.0036 Tr cm−2 d−1 /EERC (Bq m−3 ) for DRPS was established for indoors. DTPS has a lower limit of detection (LLD) limit of 0.1 Bq/m3 whereas that for DRPS is 1 Bq/m3 , hence are perfectly suitable for indoor environments. In view of its uniqueness, novelty, ease of deployment, and high potential for use in environments other than natural, the DTPS/DRPS detection concept requires careful elucidation of possible variability due to environmental effects. Chief among these is the variation in the sensitivity factors if the environmental aerosol concentrations or ventilation rates are drastically different from typical values. A sensitivity analysis of this quantity with respect of particle concentrations and ventilation rate suggests that, making allowance for a broad range of possible ventilation rates, one may summarily assign an overall uncertainty of about 40% for the presently estimated sensitivity factor. In fact, extending the model computations to extreme aerosol concentration scenarios and ventilation rates varying from 1.0–8 h−1 (Fig. 2) it may be observed that while the sensitivity factor remains almost independent of the ventilation rate for higher aerosol concentration (≥10 000 cm−3 ), it becomes increasingly sensitive to ventilation rate at low aerosol concentrations (
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 022105 (2014)
Multi-parametric approach towards the assessment of radon and thoron progeny exposures Rosaline Mishra,1,a) B. K. Sapra,1 and Y. S. Mayya2 1 2
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Indian Institute of Technology, Mumbai, India
(Received 14 November 2013; accepted 4 December 2013; published online 20 February 2014) Conventionally, the dosimetry is carried out using radon and thoron gas concentration measurements and doses have been assigned using assumed equilibrium factors for the progeny species, which is inadequate pertaining to the variations in equilibrium factors and possibly due to significant thoron. In fact, since the true exposures depend upon the intricate mechanisms of progeny deposition in the lung, therefore an integrated approach for the assessment of progeny is essential. In this context, the recently developed deposition based progeny concentration measurement techniques (DTPS: Direct Thoron progeny sensors and DRPS: Direct Radon progeny sensors) appear to be best suited for radiological risk assessments both among occupational workers and general study populations. DTPS and DRPS consist of aluminized mylar mounted LR115 type passive detectors, which essentially detects the alpha particles emitted from the deposited progeny atoms on the detector surface. It gives direct measure of progeny activity concentrations in air. DTPS has a lower limit of detection limit of 0.1 Bq/m3 whereas that for DRPS is 1 Bq/m3 , hence are perfectly suitable for indoor environments. These DTPS and DRPS can be capped with 200-mesh type wire-screen to measure the coarse fraction of the progeny concentration and the corresponding coarse fraction deposition velocities as well as the time integrated fine fraction. DTPS and DRPS can also be lodged in an integrated sampler wherein the wire-mesh and filter-paper are arranged in an array in flow-mode, to measure the fine and coarse fraction concentration separately and simultaneously. The details are further discussed in the paper. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865165] INTRODUCTION
The most crucial part of epidemiological studies pertains to the assignment of doses to study subjects, be it occupational workers or general populations. In the case of gamma radiations, measurement of personal doses for occupational workers, and at least environmental exposure levels (if not personal doses) for residential populations, can be achieved fairly accurately by TLD (thermo luminescent detector) techniques (Driscoll et al., 1983). The situation in the case of radon exposures is quite complex and as yet there exist no methods comparable to TLD for gamma radiations for measuring personal doses. The well recognized reason is that radiation doses originate from the concentrations of airborne progeny species and not by the radon or thoron atoms; and progeny concentrations, are extremely difficult to monitor on a time integrated basis using passive detectors. Besides, the deposition of the progeny atoms in the lung depends on their size distribution and hence it is necessary to go beyond the gross concentrations for true dose estimations. Thus, while the gross Potential Alpha Energy Concentration (PAEC) is only a physical measure of air exposure, convenient for regulatory purposes, it does not uniquely determine radiological risks. In order to circumvent this difficulty, an important hypothesis, justifying the use of radon gas concentration itself as index of exposure, was used in most epidemiological studies. This is called the self-compensating theory (see, for a) Author to whom correspondence should be addressed. Electronic
addresses: [email protected] and [email protected] 0034-6748/2014/85(2)/022105/8/$30.00
example, Bochicchio et al., 2005). In this, it is assumed that the actual dose to the lung, is essentially a function of the radon gas concentration, even if the progeny concentrations varied due to varying equilibrium factors. The reason is attributed to the role of unattached fraction to lung dose. Consider two houses having the same radon concentrations, but different progeny concentrations because of differences in the aerosol concentrations. The house with higher aerosol concentration will have higher total progeny concentration. However, this will not lead to a proportional increase in the lung deposition of decay products since in this house the unattached fraction concentration will be smaller, which have an overwhelmingly higher deposition fraction as compared to attached fraction. Hence, overall deposition due to inhalation of the decay products will roughly remain the same for subjects in both the houses. However, the above arguments are at best qualitative and have certain short comings. As shown by Nikezic and Yu (2005), these can lead to a factor of 2 uncertainties in true exposures within the varying aerosol framework. If we consider varying ventilation rates as the cause of differing equilibrium factors, the theory is likely to be far more in error. For example, consider two houses having the same radon concentrations but different ventilation rates. The house with higher ventilation rate will have lower PAEC. Going by the above theory, the increase in the unattached fractions here will just about compensate the decreased PAEC. However, two exclusive processes based on well-known observations, will render the relative proportion of unattached fraction, uncertain. First is the injection of aerosols from the outside. A
85, 022105-1
© 2014 AIP Publishing LLC
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022105-2
Mishra, Sapra, and Mayya
house having higher air exchange rate is likely to bring in more nano-aerosol particles from outside (where the particle concentrations are at least a factor two higher than indoor) which would tend to keep down the unattached fraction to the original level, thereby offsetting the selfcompensation effect. Second, with increased air-exchange rate, the turbulence level (denoted by the friction velocity in the air) will increase thereby increasing plate-out and decreasing of total PAEC, beyond that expected from ventilation alone. The overall effect of this is on lung deposition is not fully studied, and at a first glance, due to nonlinearity of turbulence induced wall deposition, it is unlikely to be self-compensating. Another point against the use of gas concentrations as an index of exposure is due to the growing realization on the importance of thoron. It is now being increasingly recognized as an unjustly neglected component in environmental radioactivity studies. However, due to its short half-life, thoron gas is not uniform in the room and hence its measurement is only an indicator of its presence and not a quantitative measure of the progeny concentrations. A careful reflection shows that it is almost impossible to apply self-compensating justification in this case. There is thus a considerable need to develop techniques for obtaining time integrated measures of progeny related inhalation exposures. However, for estimating true doses, we also need to measure unattached and attached components of progeny PAEC. There have been important developments, towards developing techniques for assessing progeny concentrations directly; wherein the deposition property of the progeny species has been exploited to relate the deposited flux to the activity concentration of progeny in the environment. In this regard, mainly two types of deposition monitors are reported in the literature: (a) Deposition rate monitors developed by Zhuo et al. (2000) and (b) Direct Thoron (radon) progeny sensors DTPS(DRPS) (Mishra and Mayya, 2008). With the development of DTPS and DRPS sensors, it appears that we are close to arriving at a solution to the long standing question of personal dosimetry of subjects exposed to radon and thoron decay products, analogous to TLDs for gamma radiations. Different modes of DTPS and DRPS can be used to extract different parameters which are discussed in the sections on Specification of direct. . . , DRPS/DTPS in flow-mode, DRPS/DTPS in capped-mode, and Measurements of long-time. . . .
ηT T =
ηRR =
Rev. Sci. Instrum. 85, 022105 (2014)
SPECIFICATION OF DIRECT PROGENY SENSOR (DTPS AND DRPS)
Direct progeny sensing detector system is based on selectively registering alpha tracks originating from the deposited progeny activity on LR-115 type solid-state nuclear track detectors. The selection of alpha particle energies is achieved by mounting absorbers of suitable thicknesses on the LR-115 detectors. The 220 Rn (thoron) progeny sensing element is made up of LR-115 track detector mounted with aluminized mylar absorber of 50 μm thickness to selectively detect only the 8.78 MeV alpha particles emitted from 212 Po atoms, which are formed from the decay of the activities of 212 Pb and 212 Bi atoms deposited on the mylar surface. Since this is the highest alpha energy in the natural radioactivity decay series, there will be no interference from other alpha emissions. Similarly, the radon progeny sensor has an absorber thickness of 37 μm to detect mainly the alpha particles emitted from 214 Po (7.69 MeV) formed from the eventual decay of 218 Po, 214 Pb, and 214 Bi atoms deposited on it. For making the sensor system compact, the absorber is kept in close contact with the LR-115 detector rather than leaving an intervening air-gap (Fig. 1). The sensitivity factor of these detector elements is expressed as the track density registered for 1-day exposure to an environment containing 1 Bq m−3 of EETC (Equilibrium Equivalent Thoron Concentration) or EERC (Equilibrium Equivalent Radon Concentration). EETC and EERC are the quantities directly related to the potential alpha energy concentration in air and hence to the inhalation dose. EETC is related to the individual 212 Pb and 212 Bi activity concentrations, C2 and C3 , respectively, through the relation (ICRP, 1981), EETC = 0.91C2 + 0.09C3 . EERC is related to the individual 218 Po, 214 Pb, and 214 Bi activity concentrations, C1 , C2 , and C3 , respectively, through the relation (ICRP, 1981), EERC = 0.1C1 + 0.52C2 + 0.38C3 . Two major factors that decide the sensitivity factor are: (a) track registration efficiency, and (b) deposition velocity of the progeny atoms on the absorber surface. The track registration efficiency of DTPS/DRPS correlates the number of deposited progeny atoms to the alpha tracks registered (upon the ultimate decay of the progeny atoms) in LR-115 and is a fixed intrinsic property of the absorber-detector combination expressed as
Track density (Tr/cm2 ) recorded on the sensor upon complete decay of 212 Pb , Density of 212 Pb +212 Bi atoms (atoms/cm2 ) deposited on the absorber surface
Track density (Tr/cm2 ) recorded on the sensor upon complete decay of 214 Pb , Density of 218 Po +214 Pb +214 Bi atoms (atoms/cm2 ) deposited on the absorber surface
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022105-3
Mishra, Sapra, and Mayya
Rev. Sci. Instrum. 85, 022105 (2014)
where ηTT is the efficiency of thoron progeny registration on the DTPS, and ηRR is that for radon progeny registration on DRPS. The mean track registration efficiency for DTPS was measured as: ηTT = 0.083 ± 0.004 per alpha particle emitted from thoron progeny and ηTR = 0.002 ± 0.001 per alpha particle emitted from radon progeny. Similarly, for DRPS: ηRR = 0.07 ± 0.005 for radon progeny and ηRT = 0.01 ± 0.0004 for thoron progeny (Mishra et al., 2009a). The track registration efficiencies were measured by electrodepositing a known amount of the radon and thoron decay product activities, separately, on the aluminized surface. The track densities registered on the LR-115 detectors after the complete decay of all the deposited atoms were determined using the standard track etching protocols followed by counting with a spark counter.
Ve =
The second factor is the effective deposition velocities that combine the contributions from the fine (unattached) and the coarse (attached) fractions of the individual progeny species. Due to their high diffusivities and ability to stick to surfaces, the freshly formed decay products soon attach to existing aerosol particles, thereby giving rise to a continuous activity size distribution. This distribution is broadly classified into two groups, namely, the fine fraction (∼2 nm diameter) and the coarse fraction (∼100 nm). Since the responses of the sensor are directly proportional to the total number of progeny atoms on the surface, these can yield information only on the effective deposition velocities corresponding to the total progeny atom concentrations present in the atmosphere. It is expressed as
Total atom deposition flux (atoms/cm2 /s) of progeny species on the sensor . Total atom concentration (atoms/cm3 ) in the atmosphere
In the case of 222 Rn progeny species, due to their comparable half lives, the situation is little more complex. As in the earlier case, the effective deposition velocity may be expressed in terms of the atom concentrations (n1 , n2 , n3 ) of 218 Po, 214 Pb, 214 Bi, respectively, as Ve (radon progeny) =
V¯1 n1 + V¯2 n2 + V¯3 n3 , n1 + n2 + n3
V1 = p1 Vf + (1 − p1 )Vc , V2 = p2 Vf + (1 − p2 )Vc , V3 = p3 Vf + (1 − p3 )Vc , where V1 , V2 , and V3 are the corresponding average deposition velocities weighted with respect to their respective fine (p1 , p2 , p3 ) and coarse (1 − p1 , 1 − p2 , 1 − p3 ) activity fractions p2 Vf + (1 − p2 )Vc n2 + p3 Vf + (1 − p3 )Vc n3 Ve (thoron progeny) = , n2 + n3
where n2 and n3 denote the atom concentrations, and p2 and p3 denote the fine fractions, of 212 Pb and 212 Bi in the atmosphere, respectively, and Vf and Vc be the deposition velocities of the fine and coarse fractions. The measurements of deposition velocity were carried out both under controlled conditions and real indoor environments. The controlled condition experiments were aimed at validating the progeny deposition models. For thoron progeny, experiments were carried out in a test house (22.5 m3 , controlled conditions) and in indoor environments of dwellings (natural environment). In the test-house, a steady thoron concentration in the room was measured to be about
FIG. 1. Diagram of the detector element.
11 kBq m−3 , using standard double filter method. Around 62 DTPS detectors were exposed inside the room in 3 different orientations; viz., face-up, face-down, and vertical to study the orientation effect on the particle deposition indoors. Exposures were carried out for 1–5 days. Airborne activity concentrations were estimated by gross filter-paper sampling (flow rate 16 l min−1 ) and alpha counting methods at regular intervals to obtain representative air concentrations during the period of exposure. An orientation-wise breakup of the 62 measurements yields deposition velocities as 0.08 ± 0.02 m h−1 for face-up orientation, 0.061 ± 0.009 m h−1 for face-down orientation, and 0.055 ± 0.009 m h−1 for vertical orientation. The particle concentration measured using Scanning mobility particle sizer was obtained as 29 864 cm−3 . The friction velocity estimated from fan model was 2 cm s−1 , the attachment rate was estimated as 1.38 × 10−2 s−1 , and the activity median particle diameter was 0.116 μm. These input parameters were used for calculating the deposition velocities by Lai-Nazaroff Model, which were comparable to the experimentally obtained deposition velocities.
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022105-4
Mishra, Sapra, and Mayya
Rev. Sci. Instrum. 85, 022105 (2014)
For radon progeny, controlled condition experiments were restricted to a 0.5 m3 chamber, wherein the radon concentration was maintained as 3 kBq m−3 . The chamber was completely closed and hence there was no air exchange. Sixteen Direct Radon Progeny Sensor (DRPS) detectors were exposed inside the radon chamber, in 3 orientations. Air activity was measured using gross filter-paper sampling. An orientation-wise break of the 222 Rn progeny deposition velocities in the chamber yield values of 2.38 ± 0.84 m h−1 for face-up orientation, 2.36 ± 0.84 m h−1 for face-down, and 2.39 ± 0.835 m h−1 for vertical orientation. The particle concentration in the chamber was measured as 894 cm−3 , the friction velocity was estimated as 6.25 cm s−1 , the attachment rate was estimated as 1.49 × 10−3 s−1 , and the activity median particle diameter was 0.148 μm. Using these input parameters in Lai-Nazaroff Model, the theoretical deposition velocities were found to be comparable with the experimentally obtained deposition velocities. The radon progeny deposition velocities extrapolated for the testhouse conditions were obtained as: 0.14 m h−1 for faceup, 0.13 m h−1 for vertical, and 0.13 m h−1 for face-down orientation. Natural condition measurements were carried out in 15 dwellings, selected from different floors (storey) and the detectors were placed in face-up orientation at a height of about 1 m from the ground in the living-room for a period of 90 days. Regular filter paper sampling, covering both night and day time conditions, was carried out for estimating representative values of progeny concentrations in each of the dwellings. In mixed-field exposure situation, one requires all the four registration efficiencies for extracting the deposited radon and thoron progeny atom densities from the tracks counted on the two detectors using two simultaneous equations. Essentially, the thoron progeny atom fluxes are uniquely determined from the tracks on DTPS since it is almost insensitive radon progeny species. In contrast, the DRPS has non-negligible response from thoron progeny and for obtaining radon progeny fluxes, one is required to subtract the tracks contributed from thoron progeny as estimated from DTPS, using the following equation: onlyRnP
T racksDRP S
T otal = T racksDRP S −
progeny particle sizes in indoors using high-volume cascade impactor for sampling followed by gamma counting and observed that the average AMAD of 214 Pb was 170 nm, where as that for 212 Pb was 200 nm. The average AMADs for 214 Pb and 212 Pb were 380 and 390 nm in outdoors. Reineking et al. (1992) using a low-pressure cascade impactor AMAD of 214 Pb and 212 Pb being 199 and 217 nm, respectively, for indoor air, and 386 and 330 nm for outdoor air. Papastcfanou and Bondietti (1987) have reported that in the outdoors using impactors, they measured the mean AMAD values as 160 nm for 214 Pb and 130 nm for 212 Pb. By using the individual track registration efficiency and progeny deposition velocities averaged for indoor environment, a sensitivity factor of 0.94 ± 0.027 Tr cm−2 d−1 /EETC (Bq m−3 ) for DTPS and 0.09 ± 0.0036 Tr cm−2 d−1 /EERC (Bq m−3 ) for DRPS was established for indoors. DTPS has a lower limit of detection (LLD) limit of 0.1 Bq/m3 whereas that for DRPS is 1 Bq/m3 , hence are perfectly suitable for indoor environments. In view of its uniqueness, novelty, ease of deployment, and high potential for use in environments other than natural, the DTPS/DRPS detection concept requires careful elucidation of possible variability due to environmental effects. Chief among these is the variation in the sensitivity factors if the environmental aerosol concentrations or ventilation rates are drastically different from typical values. A sensitivity analysis of this quantity with respect of particle concentrations and ventilation rate suggests that, making allowance for a broad range of possible ventilation rates, one may summarily assign an overall uncertainty of about 40% for the presently estimated sensitivity factor. In fact, extending the model computations to extreme aerosol concentration scenarios and ventilation rates varying from 1.0–8 h−1 (Fig. 2) it may be observed that while the sensitivity factor remains almost independent of the ventilation rate for higher aerosol concentration (≥10 000 cm−3 ), it becomes increasingly sensitive to ventilation rate at low aerosol concentrations (
