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thoron and decay product concentrations in air
459
A comparison of the precision of (quasi-) continuous methods of measuring radon/thoron and decay product concentrations in air R. Rolle Institute of Physical Chemistry, Georg-August University Goettingen, Tamman Street 6, 37077 Goettingen, Germany
Many types of systems are in use for measuring Rn and decay product concentrations in air. A comparison of measurement methods is attempted which should be helpful in selecting optimum, cost-effective methods for specific applications. The concept of effective volume (EV) of a measuring system is used to compare the sensitivities of different measurement systems. Minimal and optimal methods for differentiating individual 222 Rn- and 220 Rn-decay product concentrations are discussed. The optimal methods can be the most cost effective for the end result that is sought. With improved calibration significant new applications, such as determining the air exchange rate from ratios of decay product concentrations, and the Rn source of a room, may become feasible.
1. New direction in radon and decay product monitoring Exposure to radon decay product concentrations constitutes about half of the radiation burden to man and poses a challenge for cost-effective reduction, which includes efficient metrology for a variety of applications. The tracking down of sporadically occurring high-potential exposure enclosed spaces will remain an ongoing task. For some of these spaces remediation may require critical, cost-effective decisions pending the results of (Rn and) decay product measurements. At an enhanced accuracy improved evaluation of the Rn/air-exchange dynamics of enclosed spaces appears feasible. It remains to be seen whether appropriate Rn decay product measurement systems can be applied to air-exchange determinations in rooms with nonelevated decay product concentrations or, possibly with a convenient portable source of Rn. Over the past decades many millions of α-counting measurements have been made for Rn concentration exposure. Most of these measurements were made by passive Rn gas monitoring, where the detection evaluation occurs after an extended exposure period. A mean equilibRADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07054-8
© 2005 Elsevier Ltd. All rights reserved.
460
R. Rolle
rium factor F is normally assumed for conversion to Rn decay product potential alpha energy exposure, the principal parameter related to radiation dose. Far fewer filtration measurements of the decay product concentrations directly have been made, and mostly using (inefficient) short batch measurement procedures. In all these measurements the accuracy seldom reached as low as 20%, or even near that value. An analysis of more efficient measurement systems, commensurate with current technology – Si detectors, miniaturised electronics and computational evaluation – is attempted.
2. Sensitivity of radon and decay product measuring system Many measurements have shown that in all longer measurements (analogous to breathing) the decay products of 220 Rn (thoron) are also present. When measurement accuracy better than 20% is sought for decay product concentrations, then unattached and aerosol-attached concentrations should be evaluated and all members of the two decay chains, listed below, ought to be considered in the evaluations. For practically all measurements decay chain disequilibrium, i.e., saturation calculations are required, but these will not be detailed here. 222
5.5 MeV α
6 MeV α
βγ
3.62 d
3.1 min
26.8 min
Rn −−−−−−→ 218 Po −−−−−→ 214 Pb −−−−−→ 214 Bi/Po βγ, 7.68 MeV α
βγ, 5.3 MeV α
19.9 min
22.3 a
−−−−−−−−−−→ 210 Pb/Bi/Po −−−−−−−−−→ · · · , 220
6.3 MeV α
6.8 MeV α
βγ
βγ, 6&7.68 MeV α
55.6 s
0.14 s
10.6 h
60.55 min
Rn −−−−−−→ 216 Po −−−−−−→ 212 Pb −−−→ 212 Bi/Po/208 Tl −−−−−−−−−−−−→ · · · .
These nuclides generally occur in air at relatively low concentrations and fluctuate widely in time and space. Enhancement of measurement sensitivity and differentiation of the various concentrations is sought for more demanding applications. Advantage is usually taken of the high energy of α-particles and associated low background count in the high-energy region, due to the high absorption and consequently short range of the α-particles. Since β-radiation can also readily be counted with Si α-detectors, its inclusion with the α-measurement improves the accuracy particularly in continuous measurement. The sensitivity of a system measuring air concentrations can be defined as its effective volume air EVx . It is the system’s equivalent air volume effectively at steady state contributing for each nuclide decay (of the specific air concentration considered) 1 count, or 1 count of a collected nuclide’s decay chain member x. This count, furthermore, needs to be differentiated from others not originating from the specific air concentration. For enhancement of the quality of a measurement (reflected by the covariance matrix of the evaluated measurement data), partial spectral differentiation and timevariant equilibrium differentiation of the partial decay chains is employed in measurement procedures, the former generally being the more effective agent. Time-variant differentiation uses noncontinuous sampling, manipulating the (dis)equilibrium of the partial decay chains, so that for a particular sampling/measurement procedure the EV needs to be multiplied by sampling/decay saturation coefficients Ss Sm . For a fixed on/off sampling cycle air EVx × Ss Sm
A comparison of the precision of (quasi-) continuous methods
461
can be considered as the EV of a quasi-continuous system, or similarly for the single cycle of a batch measurement procedure. Since air concentrations vary in time and space (nonsteady state) the (dis)equilibrium of the partial decay chains needs to be evaluated in any case over all selected periods. The EV of Rn gas monitoring systems is presented in [1]. When continuously collecting a particular decay chain member x at efficiency εc and flow rate f from air, e.g., on a filter or diffusion screen, and continuously measuring member y of its partial decay chain at a detection efficiency y εd , then the effective volume is given by airx EVy
= x εc · f · x λ−1 · y εd .
With an efficient Rn decay product instrument (typically ec = 1, εdα ≈ 0.25, εdβ ≈ 0.15, f 50 cm3 s−1 , quasi-continuous Ss Sm 0.1) the following sensitivities are readily achieved: 3 air EVα ≈ 300Po 218 , 3000Pb 214 , 2000Bi 214 , 0.26Po 216 , 70 000Pb 212 , 6000Bi 212 cm . This shows that 216 Po concentrations are, in relation, negligible while even relatively low 212 Pb concentrations make significant contributions with longer measurements; when considering relative EV’s of the lung (f ≈ 300 cm3 s−1 , cancer initiating εd ?) the clearance and transfer mechanisms dominate the biological t1/2 of 212 Pb, somewhat reducing the EV of 212 Pb in the lung. 3. Differentiating 218 Po, 214 Pb, 214 Bi, 212 Pb, 212 Bi concentrations (covariance matrix quality) The simultaneously sampled activity concentrations (and instrument 210 Pb/Bi/Po accumulation) produce α and β signals, partially overlapping in spectral regions of interest (ROIs), that require differentiation. When sampling steady concentrations for less than 1 h the longer-lived 212 Pb and 210 Bi concentrations (and 210 Pb/Bi/Po) can be neglected. Differentiation in (1) α- and (2) β-spectral ROIs suffices and provides good differentiation. This is generally augmented by time-variant equilibrium differentiation (providing additional degrees of freedom for evaluation). With only α measurement, time-variant differentiation of the successive Pb and Bi/Poα is poor due to the shorter half-life of Bi after the longer half-life Pb precursor. When sampling (quasi-) continuously for longer than 1 h then five independent decay product concentrations need to be evaluated. Here the spectrum essentially offers only 1 β- and 3 α-, i.e., 4 ROI’s, so that time-variant differentiation is required and provides additional degrees of freedom for evaluation of the concentrations. Additionally 210 Pb/Bi/Po ought to be taken care of by instrument + collector background measurement.
4. Calibration Principally a measurement is not better than the calibration of the measurement system. System calibration of α-detection efficiencies has conventionally been carried out with (< tertiary) standards of a long-lived α-emitter, such as an 241 Am disc. This is poorly suited for the measurement sample-specific Rn decay product calibration of different α energy and
462
R. Rolle
spectral region-of-interest (ROI). Usually the α-calibration is referenced to (better) established γ-standards. However, of late absolute α-calibration based on fundamental parameter, absorption calculation can be verified to 1σ < ±1% by α-spectral peak shape matching; even a varying filter depth penetration of Rn decay products can be evaluated online. For diffusion-collection wire screens the α-detection efficiencies have to date been scaled approximations, to 1σ ∼ ±7%, of the efficiency fundamental parameter calculation. In preparation is a screen fundamental parameter calculation for α efficiencies with 1σ < ±2%. To date β calibrations of filters and screens were evaluated via time-variant analysis, of suitable α-quantified filter measurements, to 1σ ∼ ±5%. In preparation is an approximated fundamental parameter calculation for β with 1σ < ±5%. Enhanced accuracy of individual decay product concentrations is desired particularly for the evaluation of ratios of the concentrations. For this purpose conventional flow calibration to 1σ < ±5% is deemed adequate, more stringent calibration is, however, available.
5. Simple model of radon source, decay product attachment and air exchange in a room The diagram in Fig. 1 indicates the various rate constants used in simple mathematical modelling of Rn and decay products in a room. The specific attachment-, recoil- and air exchange rate constants significantly influence the ratios of the various concentrations. A better understanding of some of the attachment rates will still be obtained from small chamber experiments. From these the enhanced accuracy of β, α decay product measurement, outlined in the foregoing, makes the evaluation, inter-alia of air exchange and source term, possible and should lead to better decisions on cost effective remediation.
Fig. 1. Transformation rate constants effective in a room with a Rn source and air exchange (short arrows); λu−a – attachment rate to aerosol, λu−w to wall; ra , rw – recoil fractions.
A comparison of the precision of (quasi-) continuous methods
463
6. Conclusions Important metrology applications in Rn radiation protection are locating ‘buildings’ with (potentially) high indoor (Rn) Rn decay product levels and making proper remediating decisions. The first requires cost-effective screening measurements of adequate PAEC precision. The second in critical situations requires cost-effective determination of Rn sources and air exchange rate, evaluated via room modelling with input of decay product measurements of enhanced accuracy. Subsequently remediating options can be modelled more objectively for the most cost-effective reduction of concentrations, and can be reevaluated after (stepwise) remediation. Conventional track-etch, charcoal and electret Rn gas determination, at an assumed gas to potential alpha energy equilibrium factor F , largely fill the screening need. For a remediation contract the conventional Rn decay product determination merely provides fair PAEC information to test before/after situations. There is room for more accurate decay product measurement, as outlined above, for source and air exchange rate modelling in critical remediation situations, and possibly in ‘non-radon-related’ air-exchange determination.
Reference [1] J.P. McLaughlin, S.E. Simopoulos, F. Steinhäusler (Eds.), The Natural Radiation Environment VII, Proc. VIIth International Symposium on the NRE, Rhodes, Greece, 20–24 May 2002, Radioactivity in the Environment, vol. 7, Elsevier, Amsterdam, 2004, this volume.
A comparison of the precision of (quasi-) continuous methods of measuring radon/thoron and decay product concentrations in air R. Rolle Institute of Physical Chemistry, Georg-August University Goettingen, Tamman Street 6, 37077 Goettingen, Germany
Many types of systems are in use for measuring Rn and decay product concentrations in air. A comparison of measurement methods is attempted which should be helpful in selecting optimum, cost-effective methods for specific applications. The concept of effective volume (EV) of a measuring system is used to compare the sensitivities of different measurement systems. Minimal and optimal methods for differentiating individual 222 Rn- and 220 Rn-decay product concentrations are discussed. The optimal methods can be the most cost effective for the end result that is sought. With improved calibration significant new applications, such as determining the air exchange rate from ratios of decay product concentrations, and the Rn source of a room, may become feasible.
1. New direction in radon and decay product monitoring Exposure to radon decay product concentrations constitutes about half of the radiation burden to man and poses a challenge for cost-effective reduction, which includes efficient metrology for a variety of applications. The tracking down of sporadically occurring high-potential exposure enclosed spaces will remain an ongoing task. For some of these spaces remediation may require critical, cost-effective decisions pending the results of (Rn and) decay product measurements. At an enhanced accuracy improved evaluation of the Rn/air-exchange dynamics of enclosed spaces appears feasible. It remains to be seen whether appropriate Rn decay product measurement systems can be applied to air-exchange determinations in rooms with nonelevated decay product concentrations or, possibly with a convenient portable source of Rn. Over the past decades many millions of α-counting measurements have been made for Rn concentration exposure. Most of these measurements were made by passive Rn gas monitoring, where the detection evaluation occurs after an extended exposure period. A mean equilibRADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07054-8
© 2005 Elsevier Ltd. All rights reserved.
460
R. Rolle
rium factor F is normally assumed for conversion to Rn decay product potential alpha energy exposure, the principal parameter related to radiation dose. Far fewer filtration measurements of the decay product concentrations directly have been made, and mostly using (inefficient) short batch measurement procedures. In all these measurements the accuracy seldom reached as low as 20%, or even near that value. An analysis of more efficient measurement systems, commensurate with current technology – Si detectors, miniaturised electronics and computational evaluation – is attempted.
2. Sensitivity of radon and decay product measuring system Many measurements have shown that in all longer measurements (analogous to breathing) the decay products of 220 Rn (thoron) are also present. When measurement accuracy better than 20% is sought for decay product concentrations, then unattached and aerosol-attached concentrations should be evaluated and all members of the two decay chains, listed below, ought to be considered in the evaluations. For practically all measurements decay chain disequilibrium, i.e., saturation calculations are required, but these will not be detailed here. 222
5.5 MeV α
6 MeV α
βγ
3.62 d
3.1 min
26.8 min
Rn −−−−−−→ 218 Po −−−−−→ 214 Pb −−−−−→ 214 Bi/Po βγ, 7.68 MeV α
βγ, 5.3 MeV α
19.9 min
22.3 a
−−−−−−−−−−→ 210 Pb/Bi/Po −−−−−−−−−→ · · · , 220
6.3 MeV α
6.8 MeV α
βγ
βγ, 6&7.68 MeV α
55.6 s
0.14 s
10.6 h
60.55 min
Rn −−−−−−→ 216 Po −−−−−−→ 212 Pb −−−→ 212 Bi/Po/208 Tl −−−−−−−−−−−−→ · · · .
These nuclides generally occur in air at relatively low concentrations and fluctuate widely in time and space. Enhancement of measurement sensitivity and differentiation of the various concentrations is sought for more demanding applications. Advantage is usually taken of the high energy of α-particles and associated low background count in the high-energy region, due to the high absorption and consequently short range of the α-particles. Since β-radiation can also readily be counted with Si α-detectors, its inclusion with the α-measurement improves the accuracy particularly in continuous measurement. The sensitivity of a system measuring air concentrations can be defined as its effective volume air EVx . It is the system’s equivalent air volume effectively at steady state contributing for each nuclide decay (of the specific air concentration considered) 1 count, or 1 count of a collected nuclide’s decay chain member x. This count, furthermore, needs to be differentiated from others not originating from the specific air concentration. For enhancement of the quality of a measurement (reflected by the covariance matrix of the evaluated measurement data), partial spectral differentiation and timevariant equilibrium differentiation of the partial decay chains is employed in measurement procedures, the former generally being the more effective agent. Time-variant differentiation uses noncontinuous sampling, manipulating the (dis)equilibrium of the partial decay chains, so that for a particular sampling/measurement procedure the EV needs to be multiplied by sampling/decay saturation coefficients Ss Sm . For a fixed on/off sampling cycle air EVx × Ss Sm
A comparison of the precision of (quasi-) continuous methods
461
can be considered as the EV of a quasi-continuous system, or similarly for the single cycle of a batch measurement procedure. Since air concentrations vary in time and space (nonsteady state) the (dis)equilibrium of the partial decay chains needs to be evaluated in any case over all selected periods. The EV of Rn gas monitoring systems is presented in [1]. When continuously collecting a particular decay chain member x at efficiency εc and flow rate f from air, e.g., on a filter or diffusion screen, and continuously measuring member y of its partial decay chain at a detection efficiency y εd , then the effective volume is given by airx EVy
= x εc · f · x λ−1 · y εd .
With an efficient Rn decay product instrument (typically ec = 1, εdα ≈ 0.25, εdβ ≈ 0.15, f 50 cm3 s−1 , quasi-continuous Ss Sm 0.1) the following sensitivities are readily achieved: 3 air EVα ≈ 300Po 218 , 3000Pb 214 , 2000Bi 214 , 0.26Po 216 , 70 000Pb 212 , 6000Bi 212 cm . This shows that 216 Po concentrations are, in relation, negligible while even relatively low 212 Pb concentrations make significant contributions with longer measurements; when considering relative EV’s of the lung (f ≈ 300 cm3 s−1 , cancer initiating εd ?) the clearance and transfer mechanisms dominate the biological t1/2 of 212 Pb, somewhat reducing the EV of 212 Pb in the lung. 3. Differentiating 218 Po, 214 Pb, 214 Bi, 212 Pb, 212 Bi concentrations (covariance matrix quality) The simultaneously sampled activity concentrations (and instrument 210 Pb/Bi/Po accumulation) produce α and β signals, partially overlapping in spectral regions of interest (ROIs), that require differentiation. When sampling steady concentrations for less than 1 h the longer-lived 212 Pb and 210 Bi concentrations (and 210 Pb/Bi/Po) can be neglected. Differentiation in (1) α- and (2) β-spectral ROIs suffices and provides good differentiation. This is generally augmented by time-variant equilibrium differentiation (providing additional degrees of freedom for evaluation). With only α measurement, time-variant differentiation of the successive Pb and Bi/Poα is poor due to the shorter half-life of Bi after the longer half-life Pb precursor. When sampling (quasi-) continuously for longer than 1 h then five independent decay product concentrations need to be evaluated. Here the spectrum essentially offers only 1 β- and 3 α-, i.e., 4 ROI’s, so that time-variant differentiation is required and provides additional degrees of freedom for evaluation of the concentrations. Additionally 210 Pb/Bi/Po ought to be taken care of by instrument + collector background measurement.
4. Calibration Principally a measurement is not better than the calibration of the measurement system. System calibration of α-detection efficiencies has conventionally been carried out with (< tertiary) standards of a long-lived α-emitter, such as an 241 Am disc. This is poorly suited for the measurement sample-specific Rn decay product calibration of different α energy and
462
R. Rolle
spectral region-of-interest (ROI). Usually the α-calibration is referenced to (better) established γ-standards. However, of late absolute α-calibration based on fundamental parameter, absorption calculation can be verified to 1σ < ±1% by α-spectral peak shape matching; even a varying filter depth penetration of Rn decay products can be evaluated online. For diffusion-collection wire screens the α-detection efficiencies have to date been scaled approximations, to 1σ ∼ ±7%, of the efficiency fundamental parameter calculation. In preparation is a screen fundamental parameter calculation for α efficiencies with 1σ < ±2%. To date β calibrations of filters and screens were evaluated via time-variant analysis, of suitable α-quantified filter measurements, to 1σ ∼ ±5%. In preparation is an approximated fundamental parameter calculation for β with 1σ < ±5%. Enhanced accuracy of individual decay product concentrations is desired particularly for the evaluation of ratios of the concentrations. For this purpose conventional flow calibration to 1σ < ±5% is deemed adequate, more stringent calibration is, however, available.
5. Simple model of radon source, decay product attachment and air exchange in a room The diagram in Fig. 1 indicates the various rate constants used in simple mathematical modelling of Rn and decay products in a room. The specific attachment-, recoil- and air exchange rate constants significantly influence the ratios of the various concentrations. A better understanding of some of the attachment rates will still be obtained from small chamber experiments. From these the enhanced accuracy of β, α decay product measurement, outlined in the foregoing, makes the evaluation, inter-alia of air exchange and source term, possible and should lead to better decisions on cost effective remediation.
Fig. 1. Transformation rate constants effective in a room with a Rn source and air exchange (short arrows); λu−a – attachment rate to aerosol, λu−w to wall; ra , rw – recoil fractions.
A comparison of the precision of (quasi-) continuous methods
463
6. Conclusions Important metrology applications in Rn radiation protection are locating ‘buildings’ with (potentially) high indoor (Rn) Rn decay product levels and making proper remediating decisions. The first requires cost-effective screening measurements of adequate PAEC precision. The second in critical situations requires cost-effective determination of Rn sources and air exchange rate, evaluated via room modelling with input of decay product measurements of enhanced accuracy. Subsequently remediating options can be modelled more objectively for the most cost-effective reduction of concentrations, and can be reevaluated after (stepwise) remediation. Conventional track-etch, charcoal and electret Rn gas determination, at an assumed gas to potential alpha energy equilibrium factor F , largely fill the screening need. For a remediation contract the conventional Rn decay product determination merely provides fair PAEC information to test before/after situations. There is room for more accurate decay product measurement, as outlined above, for source and air exchange rate modelling in critical remediation situations, and possibly in ‘non-radon-related’ air-exchange determination.
Reference [1] J.P. McLaughlin, S.E. Simopoulos, F. Steinhäusler (Eds.), The Natural Radiation Environment VII, Proc. VIIth International Symposium on the NRE, Rhodes, Greece, 20–24 May 2002, Radioactivity in the Environment, vol. 7, Elsevier, Amsterdam, 2004, this volume.