- Email: [email protected]
α-Spectroscopy on CR-39 track detectors for the dosimetry of radon daughters
Environment
Pergamon
International, Vol. 22, Suppl. 1, pp. S595400, 1996 Copyright 01996 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/96 $lS.OO+.OO
PI1SO160-4120(96)00160-2
a-SPECTROSCOPY ON CR-39 TRACK DETECTORS FOR THE DOSIMETRY OF RADON DAUGHTERS P. Mozzo, F. Trotti, A. Temporin, M. Lanciai, F. Predicatori, F. Righetti, and A. Tacconi CRR PMP ULSS 20 Veneto, Policlinico Borgo Roma 37134 Verona, Italy
EI 9510-296 M (Received 7 October 1995; accepted 12 July 1996)
Long-term radon daughter concentration detection is of great interest in lung dose studies. An automatic image analysis system to carry out a-spectroscopy by tracks analysis on CR-39 (ally1 diglycol carbonate) detectors was developed. Identification and separation of 222Rn, 2’sPo, and 2’4Po contributions to the a-tracks in CR-39 exposed inside an open dosimeter were possible using the system. The calibration of the dosimeters with regard to 218Poand 2’4Po was performed through an exposure in the National Radiological Protection Board (U.K.) radon chamber. In four houses, long-term 218Poand 2’4Po concentrations were measured by means of open dosimeters; u2Rn levels were simultaneously assessed with permeation dosimeters. Estimates of the equilibrium factor, F, were made using theoretical equations giving F values as a Iimction of radon daughters to radon gas concentrations ratios. Based on the most sensitive of those relations, F values varying between 0.2 and 0.5 were obtained, in good agreement with data recorded by an active Equilibrium Equivalent Concentration (EEC) detection instrument used for comparison. Copyright ~1996 EISOGW Science od
INTRODUCTION gradient), while the F trend versus normalized 214Po activity alone is less critically dependent, giving the opportunity for more accurate F estimates.
The most common methods for long-term indoor radon exposure estimates are based on passive dosimeters which selectively assess radon gas. As far as dose is concerned, cl-energy released in the respiratory tract by inhaled radon short half-life daughters is of major interest. In recent years, several authors have established new methodologies for time integrated Equilibrium Equivalent Concentration (EEC) analysis. In one of those studies (Planinic and Faj 1990; Faj and Planinic 1991), from the ratio between tracks produced by 222Rn, 218Po, and 214Po cl-particles on a bare solid-state detector, and the tracks of cl-particles generated within a radon selective dosimeter, the equilibrium factor, F, was obtained. From this F value and using the radon gas concentration, the EEC was derived. It has been pointed out (Dorschel and Piesch 1993) that the F functional dependence from 222Rn, 2’8Po, and 214Po activity sum (for unit radon activity) is critical (elevated
Basis Theoretically, the equilibrium factor, F, can be expressed as a function of the ratio between a single radon decay product concentration and the radon concentration or the daughters concentrations sum for unit radon concentration (Planinic and Faj 1990; Faj and Planinic 1991; Dorschel and Piesch 1993). Those relations can be achieved by setting correspondences between ventilation rate and individual concentration ratios (plate out constant and attached daughters atom fractions held fixed), through solution of indoor air radon daughter behaviour equations (Knutson 1988). The following relationships, in particular, are obtained: s595
P. Mozzoet al.
c(*‘aPo)
F =F[
C (222Rn)
F =F[
c(*i4Po)
1
(lb)
C(***Rn)
F =F[
c(*14Po)
+ c(218Po)
C(***Rn)
1
UC)
Curves described by Eqs. la, 1b, and lc are shown in Fig. 1. Due to its steep slope, the curve following Eq. la is not suitable for practical use. The curve described by Eq. 1b shows a slower F increase as the concentration ratio grows compared to what occurs following Eq. 1c, and appears to be the best choice for practical purposes. Preliminary
system setup
A system aimed at separating 222Rn, 2’8Po, and 214Po specific contributions was developed. It is based on the analysis of the geometric parameters of the tracks induced by a-particles coming from the nuclides and detected by CR-39 (ally1 diglycol carbonate) plastic detectors. The CR-39 image analysis system consists of an optical microscope supplied with a telecamera connected to an image digitizer PC board. Self-developed software enables collection and processing of the images as well as control of the motion of the microscope table where the detector lies. For each detector (which has previously undergone standard chemical etching (Ilic 1989)), 289 image frames are scanned, resulting in total areas of 0.85 cm2. In Fig. 2, an image of a tracks from 222Rn, 218Po, and 214Po on an indoor exposed CR-39 is presented. The analysis software bases tracks identification upon the local gradient value existing between the track and background grey level. It not only counts tracks but also measures their area and axes lengths (whose ratio defines the eccentricity “e”). Track eccentricity depends on the a-radiation angle of incidence on the detector. For equal eccentricities, track area is in biunique correspondence with particle energy. More precisely, area and energy are in inverse relation, the former being a function of the radiation specific energy loss (Somogyi 1980).
The opportunity to perform a spectroscopy on CR-39 thus depends on selecting eccentricity windows narrow enough to directly assign an energy value to an area, being sure not to make statistics excessively poor. The window chosen as the most convenient compromise solution, is between e values of 1.O and 1.2 (roughly corresponding to respective incidences of 90’ and 60’ with respect to the detector surface). In this paper, the preliminary results of the application of the above described spectroscopic criteria to the analysis of CR-39 tracks from air 222Rn, 218Po, and 2’4Po a-particles aiming at individual concentrations of these radionuclides are discussed.The equilibrium factor is then derived according to the theoretical model described. MATERIALS
AND METHODS
Four dosimeters for the detection of radon and seven dosimeters for the detection of 218Po and 214PO were exposed in four houses for time periods ranging between 1 and 6 months. The radon dosimeter was made of a plastic shell with two cellulose nitrate (LR 115) opposing detectors inside wrapped in a polyethylene envelope permeable to radon gas but not to its daughters (Tommasino et al. 1986). The same kind of dosimeter, containing CR-39 detectors (Pershore PM500), was chosen for assessing the two polonium levels without the polyethylene envelope barrier (open dosimeter). These measurements were used for F estimates. To check the dosimeters data for each of the houses, mean daily EEC levels were detected three different times with an active instrument (Silena 3s) that analyses air particles in a spectrometry. Passive dosimeter 2’8Po and 214Po detecting efficiency was computed from the calibration data of a group of dosimeters exposed for 10 d inside the National Radiological Protection Board (U.K.) radon chamber to known concentrations of the two isotopes. The efficiencies were, respectively, for 2’4Po and 218Po, (2.95 f 0.27) 10m3and (1.34* 0.21) 10-4tracks cmm2per Bq h rne3. The area (A)-energy (E) calibration curve, carried out for the energy range of interest with regard to radon and its a-emitting daughters is shown in Fig. 3. The curve is described by the following equation:
A = i*(lnE
+b) +c
(2)
u-spectroscopy
s597
on CR-39 track detectors for radon daughters
2
0.7
? 2
0.6
I 2 0.5 S i 5 0.4 wa z
0.3 0.2 0.1 0 CONCENTRATION RATIO
Fig. 1. Equilibrium factor (F) as a tinction of concentration ratios: C( ~‘sPo)/c(%n),
C(2’4Po)/C(2%l),
[C(2’4Po) + c(2’sPo)]/c(wll).
Fig. 2. Image of a CR-39 nuclear track detector, exposed to indoor atmosphere inside an open dosimeter, as captured by the analysis system.
P. Mezzo et al.
S598
1200 1150 NE, 1100 2 9
1050
:: gj 1000 l950 900 850 800 5
6
5.5
6.5
7.5
7
8
ENERGY (MeV)
Fig. 3. Energy calibration curve of the analysis system. Full circles correspond to the energies of interest for the dosimeter. 60 r
2’8Po(po2)
4, ??
0
400
600
800
1000
1200
1400
1600
1800
2000
TRACK AREA (q?)
Fig. 4. Track a rea distributions from u-particles detected by a CR-39 film exposed inside an open dosimeter: pol refers to the plate-out peak of both 214Poand 218Podeposited on the film; po2 is the analogue peak for a-particles emitted by polonium deposited on the opposite film; and, FE is the Full-Energy peak of ***Rn.
a-spectroscopy
s599
on CR-39 track detectors for radon daughters
300
250 F f
200
8 150
_ __._...._......._.. 400
600
800
1000
1200
1400
1600
1800
2000
TRACK AREA (pm*)
Fig. 5. Monte Carlo simulation of the area frequency distribution coming from the detection of a-particles by a CR-39 film exposed inside an open dosimeter.
where, a, b, and c are constants. Full circles in Fig. 3 indicate a-energies of interest for the dosimeter. To check experimental area frequency distributions of the tracks of ***Rn, *l*Po, and *14Po on CR-39 in the open dosimeter, simulations were carried out with Monte Carlo software. RESULTS AND DISCUSSION
In Fig. 4, the area distribution of a CR-39 detector belonging to an open dosimeter exposed in a house is presented. The 880 and 1030 urn* peaks (pol) refer to a-particles, respectively, from *14Po and *l*Po plated out on film. The 1180 pm* peak is formed by alphas from ***Rn close to the detector, while the 940 and 1270 urn* maxima (~02) are respectively related to *14Po and *‘*PO plated out on the CR-39 surface opposite, inside the dosimeter, the detector under consideration. High area values (low energies) cut-off comes from detector thickness reduction in chemical etching. Monte Carlo simulation of the area frequency distribution is given in Fig. 5. To compute the emitters distribution, constant radon concentration during exposure is assumed and the daughter fractions (free, attached, and plated out) were calculated by solving
Jacobi-Porstendorfer equations (Knutson 1988) with dosimeter geometrical parameters (specifically, surface and volume). For the emitted a distribution, dosimeter local geometry together with a limiting angle of incidence on the detector of 60” was used. Based on area spectrum (Fig. 4), both *14Po peaks (po 1, ~02) were selected for quantitative assessments, whilst ‘pol’ peaks only were used for *‘*PO determinations. In Table 1, *14Po, *‘*PO (open dosimeters), and ***Rn (permeation dosimeters) measurement data for four monitored houses are summarized. Single dosimeter concentrations are the average over two detector estimates. In Table 1, EEC levels as recorded by the active device, and averaged over different detection periods during the passive dosimeters exposure time, are also shown. From *14Po, *‘*PO, and ***Rn concentrations detected by means of passive dosimeters, as given in Table 1, estimates of F for the monitored houses were made using Eqs. lb and lc. This data is presented in Table 2. F estimates provided by the passive dosimeters are also compared in Table 2 with values obtained from EEC active instrument assessments. F values computed referring to passive dosimeters as far as *14Po only is concerned are in general agreement with those resulting
S600
P. Mozzo et al.
Table I. ‘14Po and “*PO indoor concentrations measured with passive open dosimeters compared with 222Rn concentration (measured with passive permeation dosimeters) and EEC values given by multiple measurements with an active instrument. Uncertainties are at 68% confidence level. House code
Exposure
Room
214pOa
time
concentration (Bq me3)
(d) Basement
1
Basement
2
218poa
concentration (Bq mT3)
222Rn concentration (Bq m”) 1568=t 19
30
143 5 16
579 f 146
30
123+ 10
602+
178
8.2 f 0.9
56*
11
6.5 f 0.8
27k
11
48.6 jz 1.4
15.8 * 2.1
108 zt 3.0
55.4 f 3.4
86.9 f 2.8
29.2 f 2.8
95 * 21
3
Basement
113
4
Understairs
169
7.8 f 0.9
45*
169
8.1 f 0.9
56 + 12
a)
(Bq m-‘)
102
178
31.6*3.8
EECb
10
Passive dosimeters measurement. b) Active instrument measurement.
Table 2. F (equilibrium factor) values as calculated from Table 1 data by means of (1 b) and (1 c) curves, F vs concentrations ratio, compared with values given by active instrument measurements in the sample of four houses monitored. Uncertainties are at 68% confidence level. House code
Fa
Fb
FC
1
0.20 f 0.02
0.06 f 0.02
2
0.28 f 0.02
0.41 •t 0.16
0.32 f 0.03
3
0.49 f 0.04
0.51 f 0.19
0.52 k 0.03
4
0.21 f 0.02
0.18 f 0.13
0.34 f 0.03
a)F
=F[
c(214Po) C(222Rn)
1
b) F = F [c(214po) + c(218po)] C(222Rn) C) Data refer to EEC measurements
from two polonium analysis and are closer to the active instrument estimates. However, active instrument detections are short-term ones and their use in representing the mean value of F during the passive dosimeters exposure time must be considered with care. CONCLUSIONS
Spectroscopic analysis on CR-39 detectors to separate *14Po, *‘*PO,and **%I contributions may become a tool for long-term short half-life a-emitters radon daughters exposure assessments. The existence of theoretical relationships connecting the equilibrium factor to these concentrations makes it possible to achieve straight time integrated estimates of the EEC. The EEC is a quantity of primary interest as far as lung dose is concerned. Refinement of the method is necessary to improve the separation of the *14Po and *18Po signals, to fixopen dosimeters exposure criteria to make their response independent of positioning conditions, to design other alternative dosimeter geometries providing better
with active instrument.
spectroscopic results, and to elaborate F-concentration ratio relations.
more flexible
REFERENCES Diirschel, B.; Piesch, E. A new approach to estimating the equilibrium factor between radon and its daughters. Radiat. Prot. Dosim. 48: 145-151; 1993. Faj, Z.; Planinic, J. Dosimetry of radon and its daughters by two SSNT detectors. Radiat. Prot. Dosim. 35: 265-267; 1991. Ilic, R. Damage track detectors for alpha particle registration Track formation and detector processing. In: Tommasino, L.; Furlan, G.; Khan, H.A.; Monnin, M., eds. Proc. int. workshop on radon monitoring in radioprotection, environmental radioactivity and earth sciences. Singapore: World Scientific; 1989: 133-144. Knutson, E.O. Modeling indoor concentrations of radon’s decay products. In: Nazaroff, W.W.; Nero, A.V., eds. Radon and its decay products in indoor air. New York, NY: John Wiley and Sons; 1988: 161. Planinic, J.; Faj, Z. Equilibrium factor and dosimetry of Rn by a nuclear track detector. Health Phys. 59: 349-351; 1990. Somogyi, G. Development of etched nuclear tracks. Nucl. Instrum. Meth. 173: 21-42; 1980. Tommasino, L.; Cherouati, D.E.; Seidel, J.L:; Monnin, M.A. A plastic bag sampler for passive radon monitoring. Nucl. Tracks 12: 681; 1986.
Pergamon
International, Vol. 22, Suppl. 1, pp. S595400, 1996 Copyright 01996 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/96 $lS.OO+.OO
PI1SO160-4120(96)00160-2
a-SPECTROSCOPY ON CR-39 TRACK DETECTORS FOR THE DOSIMETRY OF RADON DAUGHTERS P. Mozzo, F. Trotti, A. Temporin, M. Lanciai, F. Predicatori, F. Righetti, and A. Tacconi CRR PMP ULSS 20 Veneto, Policlinico Borgo Roma 37134 Verona, Italy
EI 9510-296 M (Received 7 October 1995; accepted 12 July 1996)
Long-term radon daughter concentration detection is of great interest in lung dose studies. An automatic image analysis system to carry out a-spectroscopy by tracks analysis on CR-39 (ally1 diglycol carbonate) detectors was developed. Identification and separation of 222Rn, 2’sPo, and 2’4Po contributions to the a-tracks in CR-39 exposed inside an open dosimeter were possible using the system. The calibration of the dosimeters with regard to 218Poand 2’4Po was performed through an exposure in the National Radiological Protection Board (U.K.) radon chamber. In four houses, long-term 218Poand 2’4Po concentrations were measured by means of open dosimeters; u2Rn levels were simultaneously assessed with permeation dosimeters. Estimates of the equilibrium factor, F, were made using theoretical equations giving F values as a Iimction of radon daughters to radon gas concentrations ratios. Based on the most sensitive of those relations, F values varying between 0.2 and 0.5 were obtained, in good agreement with data recorded by an active Equilibrium Equivalent Concentration (EEC) detection instrument used for comparison. Copyright ~1996 EISOGW Science od
INTRODUCTION gradient), while the F trend versus normalized 214Po activity alone is less critically dependent, giving the opportunity for more accurate F estimates.
The most common methods for long-term indoor radon exposure estimates are based on passive dosimeters which selectively assess radon gas. As far as dose is concerned, cl-energy released in the respiratory tract by inhaled radon short half-life daughters is of major interest. In recent years, several authors have established new methodologies for time integrated Equilibrium Equivalent Concentration (EEC) analysis. In one of those studies (Planinic and Faj 1990; Faj and Planinic 1991), from the ratio between tracks produced by 222Rn, 218Po, and 214Po cl-particles on a bare solid-state detector, and the tracks of cl-particles generated within a radon selective dosimeter, the equilibrium factor, F, was obtained. From this F value and using the radon gas concentration, the EEC was derived. It has been pointed out (Dorschel and Piesch 1993) that the F functional dependence from 222Rn, 2’8Po, and 214Po activity sum (for unit radon activity) is critical (elevated
Basis Theoretically, the equilibrium factor, F, can be expressed as a function of the ratio between a single radon decay product concentration and the radon concentration or the daughters concentrations sum for unit radon concentration (Planinic and Faj 1990; Faj and Planinic 1991; Dorschel and Piesch 1993). Those relations can be achieved by setting correspondences between ventilation rate and individual concentration ratios (plate out constant and attached daughters atom fractions held fixed), through solution of indoor air radon daughter behaviour equations (Knutson 1988). The following relationships, in particular, are obtained: s595
P. Mozzoet al.
c(*‘aPo)
F =F[
C (222Rn)
F =F[
c(*i4Po)
1
(lb)
C(***Rn)
F =F[
c(*14Po)
+ c(218Po)
C(***Rn)
1
UC)
Curves described by Eqs. la, 1b, and lc are shown in Fig. 1. Due to its steep slope, the curve following Eq. la is not suitable for practical use. The curve described by Eq. 1b shows a slower F increase as the concentration ratio grows compared to what occurs following Eq. 1c, and appears to be the best choice for practical purposes. Preliminary
system setup
A system aimed at separating 222Rn, 2’8Po, and 214Po specific contributions was developed. It is based on the analysis of the geometric parameters of the tracks induced by a-particles coming from the nuclides and detected by CR-39 (ally1 diglycol carbonate) plastic detectors. The CR-39 image analysis system consists of an optical microscope supplied with a telecamera connected to an image digitizer PC board. Self-developed software enables collection and processing of the images as well as control of the motion of the microscope table where the detector lies. For each detector (which has previously undergone standard chemical etching (Ilic 1989)), 289 image frames are scanned, resulting in total areas of 0.85 cm2. In Fig. 2, an image of a tracks from 222Rn, 218Po, and 214Po on an indoor exposed CR-39 is presented. The analysis software bases tracks identification upon the local gradient value existing between the track and background grey level. It not only counts tracks but also measures their area and axes lengths (whose ratio defines the eccentricity “e”). Track eccentricity depends on the a-radiation angle of incidence on the detector. For equal eccentricities, track area is in biunique correspondence with particle energy. More precisely, area and energy are in inverse relation, the former being a function of the radiation specific energy loss (Somogyi 1980).
The opportunity to perform a spectroscopy on CR-39 thus depends on selecting eccentricity windows narrow enough to directly assign an energy value to an area, being sure not to make statistics excessively poor. The window chosen as the most convenient compromise solution, is between e values of 1.O and 1.2 (roughly corresponding to respective incidences of 90’ and 60’ with respect to the detector surface). In this paper, the preliminary results of the application of the above described spectroscopic criteria to the analysis of CR-39 tracks from air 222Rn, 218Po, and 2’4Po a-particles aiming at individual concentrations of these radionuclides are discussed.The equilibrium factor is then derived according to the theoretical model described. MATERIALS
AND METHODS
Four dosimeters for the detection of radon and seven dosimeters for the detection of 218Po and 214PO were exposed in four houses for time periods ranging between 1 and 6 months. The radon dosimeter was made of a plastic shell with two cellulose nitrate (LR 115) opposing detectors inside wrapped in a polyethylene envelope permeable to radon gas but not to its daughters (Tommasino et al. 1986). The same kind of dosimeter, containing CR-39 detectors (Pershore PM500), was chosen for assessing the two polonium levels without the polyethylene envelope barrier (open dosimeter). These measurements were used for F estimates. To check the dosimeters data for each of the houses, mean daily EEC levels were detected three different times with an active instrument (Silena 3s) that analyses air particles in a spectrometry. Passive dosimeter 2’8Po and 214Po detecting efficiency was computed from the calibration data of a group of dosimeters exposed for 10 d inside the National Radiological Protection Board (U.K.) radon chamber to known concentrations of the two isotopes. The efficiencies were, respectively, for 2’4Po and 218Po, (2.95 f 0.27) 10m3and (1.34* 0.21) 10-4tracks cmm2per Bq h rne3. The area (A)-energy (E) calibration curve, carried out for the energy range of interest with regard to radon and its a-emitting daughters is shown in Fig. 3. The curve is described by the following equation:
A = i*(lnE
+b) +c
(2)
u-spectroscopy
s597
on CR-39 track detectors for radon daughters
2
0.7
? 2
0.6
I 2 0.5 S i 5 0.4 wa z
0.3 0.2 0.1 0 CONCENTRATION RATIO
Fig. 1. Equilibrium factor (F) as a tinction of concentration ratios: C( ~‘sPo)/c(%n),
C(2’4Po)/C(2%l),
[C(2’4Po) + c(2’sPo)]/c(wll).
Fig. 2. Image of a CR-39 nuclear track detector, exposed to indoor atmosphere inside an open dosimeter, as captured by the analysis system.
P. Mezzo et al.
S598
1200 1150 NE, 1100 2 9
1050
:: gj 1000 l950 900 850 800 5
6
5.5
6.5
7.5
7
8
ENERGY (MeV)
Fig. 3. Energy calibration curve of the analysis system. Full circles correspond to the energies of interest for the dosimeter. 60 r
2’8Po(po2)
4, ??
0
400
600
800
1000
1200
1400
1600
1800
2000
TRACK AREA (q?)
Fig. 4. Track a rea distributions from u-particles detected by a CR-39 film exposed inside an open dosimeter: pol refers to the plate-out peak of both 214Poand 218Podeposited on the film; po2 is the analogue peak for a-particles emitted by polonium deposited on the opposite film; and, FE is the Full-Energy peak of ***Rn.
a-spectroscopy
s599
on CR-39 track detectors for radon daughters
300
250 F f
200
8 150
_ __._...._......._.. 400
600
800
1000
1200
1400
1600
1800
2000
TRACK AREA (pm*)
Fig. 5. Monte Carlo simulation of the area frequency distribution coming from the detection of a-particles by a CR-39 film exposed inside an open dosimeter.
where, a, b, and c are constants. Full circles in Fig. 3 indicate a-energies of interest for the dosimeter. To check experimental area frequency distributions of the tracks of ***Rn, *l*Po, and *14Po on CR-39 in the open dosimeter, simulations were carried out with Monte Carlo software. RESULTS AND DISCUSSION
In Fig. 4, the area distribution of a CR-39 detector belonging to an open dosimeter exposed in a house is presented. The 880 and 1030 urn* peaks (pol) refer to a-particles, respectively, from *14Po and *l*Po plated out on film. The 1180 pm* peak is formed by alphas from ***Rn close to the detector, while the 940 and 1270 urn* maxima (~02) are respectively related to *14Po and *‘*PO plated out on the CR-39 surface opposite, inside the dosimeter, the detector under consideration. High area values (low energies) cut-off comes from detector thickness reduction in chemical etching. Monte Carlo simulation of the area frequency distribution is given in Fig. 5. To compute the emitters distribution, constant radon concentration during exposure is assumed and the daughter fractions (free, attached, and plated out) were calculated by solving
Jacobi-Porstendorfer equations (Knutson 1988) with dosimeter geometrical parameters (specifically, surface and volume). For the emitted a distribution, dosimeter local geometry together with a limiting angle of incidence on the detector of 60” was used. Based on area spectrum (Fig. 4), both *14Po peaks (po 1, ~02) were selected for quantitative assessments, whilst ‘pol’ peaks only were used for *‘*PO determinations. In Table 1, *14Po, *‘*PO (open dosimeters), and ***Rn (permeation dosimeters) measurement data for four monitored houses are summarized. Single dosimeter concentrations are the average over two detector estimates. In Table 1, EEC levels as recorded by the active device, and averaged over different detection periods during the passive dosimeters exposure time, are also shown. From *14Po, *‘*PO, and ***Rn concentrations detected by means of passive dosimeters, as given in Table 1, estimates of F for the monitored houses were made using Eqs. lb and lc. This data is presented in Table 2. F estimates provided by the passive dosimeters are also compared in Table 2 with values obtained from EEC active instrument assessments. F values computed referring to passive dosimeters as far as *14Po only is concerned are in general agreement with those resulting
S600
P. Mozzo et al.
Table I. ‘14Po and “*PO indoor concentrations measured with passive open dosimeters compared with 222Rn concentration (measured with passive permeation dosimeters) and EEC values given by multiple measurements with an active instrument. Uncertainties are at 68% confidence level. House code
Exposure
Room
214pOa
time
concentration (Bq me3)
(d) Basement
1
Basement
2
218poa
concentration (Bq mT3)
222Rn concentration (Bq m”) 1568=t 19
30
143 5 16
579 f 146
30
123+ 10
602+
178
8.2 f 0.9
56*
11
6.5 f 0.8
27k
11
48.6 jz 1.4
15.8 * 2.1
108 zt 3.0
55.4 f 3.4
86.9 f 2.8
29.2 f 2.8
95 * 21
3
Basement
113
4
Understairs
169
7.8 f 0.9
45*
169
8.1 f 0.9
56 + 12
a)
(Bq m-‘)
102
178
31.6*3.8
EECb
10
Passive dosimeters measurement. b) Active instrument measurement.
Table 2. F (equilibrium factor) values as calculated from Table 1 data by means of (1 b) and (1 c) curves, F vs concentrations ratio, compared with values given by active instrument measurements in the sample of four houses monitored. Uncertainties are at 68% confidence level. House code
Fa
Fb
FC
1
0.20 f 0.02
0.06 f 0.02
2
0.28 f 0.02
0.41 •t 0.16
0.32 f 0.03
3
0.49 f 0.04
0.51 f 0.19
0.52 k 0.03
4
0.21 f 0.02
0.18 f 0.13
0.34 f 0.03
a)F
=F[
c(214Po) C(222Rn)
1
b) F = F [c(214po) + c(218po)] C(222Rn) C) Data refer to EEC measurements
from two polonium analysis and are closer to the active instrument estimates. However, active instrument detections are short-term ones and their use in representing the mean value of F during the passive dosimeters exposure time must be considered with care. CONCLUSIONS
Spectroscopic analysis on CR-39 detectors to separate *14Po, *‘*PO,and **%I contributions may become a tool for long-term short half-life a-emitters radon daughters exposure assessments. The existence of theoretical relationships connecting the equilibrium factor to these concentrations makes it possible to achieve straight time integrated estimates of the EEC. The EEC is a quantity of primary interest as far as lung dose is concerned. Refinement of the method is necessary to improve the separation of the *14Po and *18Po signals, to fixopen dosimeters exposure criteria to make their response independent of positioning conditions, to design other alternative dosimeter geometries providing better
with active instrument.
spectroscopic results, and to elaborate F-concentration ratio relations.
more flexible
REFERENCES Diirschel, B.; Piesch, E. A new approach to estimating the equilibrium factor between radon and its daughters. Radiat. Prot. Dosim. 48: 145-151; 1993. Faj, Z.; Planinic, J. Dosimetry of radon and its daughters by two SSNT detectors. Radiat. Prot. Dosim. 35: 265-267; 1991. Ilic, R. Damage track detectors for alpha particle registration Track formation and detector processing. In: Tommasino, L.; Furlan, G.; Khan, H.A.; Monnin, M., eds. Proc. int. workshop on radon monitoring in radioprotection, environmental radioactivity and earth sciences. Singapore: World Scientific; 1989: 133-144. Knutson, E.O. Modeling indoor concentrations of radon’s decay products. In: Nazaroff, W.W.; Nero, A.V., eds. Radon and its decay products in indoor air. New York, NY: John Wiley and Sons; 1988: 161. Planinic, J.; Faj, Z. Equilibrium factor and dosimetry of Rn by a nuclear track detector. Health Phys. 59: 349-351; 1990. Somogyi, G. Development of etched nuclear tracks. Nucl. Instrum. Meth. 173: 21-42; 1980. Tommasino, L.; Cherouati, D.E.; Seidel, J.L:; Monnin, M.A. A plastic bag sampler for passive radon monitoring. Nucl. Tracks 12: 681; 1986.