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Optimization and comparison of three different methods for the determination of Rn-222 in water
the Science of the Total Environment An I M e n n d m l Jmmal tor SdralMc Rmarcta
ELSEVIER
The Science of the Total Environment 173/174 (1995) 61-67
Optimization and comparison of three different methods for the determination of Rn-222 in water P. Belloni*a, M. Cavaiolib, G. Ingrao3, C. Mancinic, M. Notarob, P. Santaronid, G. Torrib, R. Vassellib "ENEA CRE, CasacciaAMB-BIO, CP2400, 00100 Roma, Italy ANPA, National Environmental Protection Agency, DISPARAMET, v. V. Brancati, 48, 00144 Roma, Italy c Nuclear Engineering Department, University La Sapienza, P. zza S. Pietro in Vmcoli, 10, 00184 Roma, Italy National Institute of Nutrition, v. Ardeatina, 546, 00178 Roma, Italy b
Abstract Three different systems for the determination of radon in water have been examined: liquid scintillation counting (LSC), degassification followed by Lucas cell counting (LCC) and gamma counting (GC). Particular care has been devoted to the sampling methodologies of the water. Comparative results for several environmental samples are given. A critical evaluation is also given on the basis of the final aim of the measurements. Keywords: Lucas cell counting; Radon; Gamma counting; Liquid scintillation counting
1. Introduction The contribution to the mean effective dose due to the exposure to Rn and its progeny is estimated to be about 50% (1.3 mSv) of the total effective dose [1]. This is a mean evaluation, but single values are subject to large variability and differences of 2 or 3 order of magnitude can be found in closed environments (mines, dwellings, etc.) Concerning indoor environments, potential sources of radon are the soil beneath buildings,
" Corresponding author.
the building materials, the water supply, the outdoor air and other minor sources such as natural gases, etc. While soil and building materials are widely studied, minor emphasis has been devoted to radon in water; furthermore, radon in drinkable water represents a source of internal exposure when ingested, even if this is less important than inhaled radon and daughters [2]. The transfer coefficient for radon from water to domestic indoor air, based on normal use, is estimated to be about 10~4. This implies that 1 Bq/1 in water produces about 10 _1 Bq/m 3 in air [3-5]. Very high concentrations of radon in water represents a risk to be considered. Several countries have
0048-9697/95/$09.50 © 1995 Elsevier Science BV. All rights reserved. SSDI 0 0 4 8 - 9 6 9 7 ( 9 5 ) 0 4 7 5 4 - 0
P. Belkmi et al. / The Science of the Total Encironment I73 / I74 (1995) 61-67
62
already taken into consideration the need of recommendations for reference levels of maximum activity of Rn-222 in water for domestic use [6,7]. With these considerations in mind, in view of extensive studies on the evaluation of the importance of radon in water as a potential cause of exposure to the population, and for the control of the environment, it has been decided to study three analytical methods for the determination of the activity of Rn-222 in water: liquid scintillation counting (LX), degassification followed by Lucas cell counting (LCC) and gamma counting (GC). The main objective of the work has been to optimize the methods and compare them not only regarding the precision, accuracy, costs, etc., but also from the point of view of their possible specific applications such as, for example, in large surveys, field measurements, semi-continuous measurements for controlling water supplies, etc. The authors intend to point out that for dosimetry studies, passive integrating techniques, able to measure Rn-222 in water over a long period (months), are the most attractive; nevertheless, methods for instantaneous determination need to be evaluated. The three methods have been calibrated separately and then compared using 21 environmental samples of water with different Rn-222 activity. Particular care has been
devoted to the methodologies of sampling, especially to avoid contact with air. 2. Methods and materials 2.1. Degassijkation and Lucas cell counting (LCC) Radon is extracted from water, transferred into a scintillation flask and counted using a photomultiplier [8,9].The instrumentation used is produced by Pylon (Canada). It consists of a degassing unit (WGlOOl) used to transfer the radon from water into a scintillation flask (the Lucas cell). The internal walls of the flask are coated with a solid scintillator, ZnS(Ag). The counting was performed with a photomultiplier system (Pylon AB5). Sampling. A plastic tube is applied to the water faucet. After a (flow of at least 5 1, a 250 ml graduated cylinder is filled keeping the end of the tube close to the bottom of the cylinder to minimize contact with air. After a water flow of about 1 1, the volume of the water in the cylinder is brought up to 190 ml. The cylinder is immediately tightly closed with the cap used for the degassification. Degassification. During this procedure the radon is transferred from the water to the Lucas cell through the degassing unit WG 1001 (see Fig.
AIRINLET I I METERING
ADDITIONAL
VALVE
VALVE
ON-OFF VALVE Fig. 1. Degassification
unit WG
1001.
P. Belloni et al. / The Science of the Total Environment I73 / 174 (1995) 61-67
1). A 270 ml Lucas cell is evacuated with a vacuum pump and then connected to the cylinder through an on-off valve. A regulated flow of air via a metering valve extracts the radon from the water by bubbling through a diffusion stone. A desiccant column prevents humidity entering into the cell. The extraction time is 5 min in order to guarantee an optimal degassification. At the end of the extraction the Lucas cell is disconnected. The treatment of mineral water with a high CO, content needs particular care. The CO, may cause a fast and violent bubbling when the cylinder is connected to the Lucas cell through the on-off valve 1. This gives rise to a water flow that enters into the WG 1001 system and into the Lucas cell itself. To avoid this problem an additional valve has been inserted between the cylinder and the Lucas cell. This valve is opened very slowly and carefully after the opening of the on-off valve 1. Counting. After a delay time of about 4 h needed to reach equilibrium between radon and its daughters for the optimization of the counting, the Lucas cell is connected to the photomultiplier of the AB5 and a-radiation from Rn-222, PO-218 and PO-214 are measured. A counting time of 10 min was adopted. Calibration. The extraction efficiency of the system has been determined by multiple degassifications on several samples of water prepared using a NIST radium source. The value found for the extraction efficiency is = 93 f 2.5%. The radon counting system (AB5 plus Lucas cell) has been calibrated in a radon chamber at the National Radiological Protection Board (NRPB), Chilton, Didcot, United Kingdom. The efficiency
63
of the counting system was 0.745 f 0.020 cpm/dpm. These parameters, together with the lower limit of detection, are reported in Table 1, where they are compared with the other techniques. 2.2. Liquid scintillation counting (LX) Radon-222 is extracted from water by a scintillation solution and directly counted in a liquid scintillation counter [lO,ll]. Radium-226 is not extracted. The low background Quantulus scintillator system produced by LKB Wallac Pharmacia was employed. The instrument has a pulse shape analyzer (PSA) unit that discriminates the pulses generated by a-radiation from those generated by P-radiation. Sampling. To avoid contact with air a sampling methodology similar to that suggested by EPA [12] has been adopted. A polyethylene tube is connected to the water faucet; after a flow of about 5 1, 10 ml of water are carefully sampled sucking from the tube with a syringe. The sample is transferred directly into a scintillation vial, previously filled with the scintillation liquid, injecting it under the level of the scintillation liquid. Scintillation vial. One problem that might occur when applying the LX technique to radon determination is the tightness of the vial used for the counting because Rn gas can escape from the vials. Eighteen types of commercial and modified vials have been evaluated to compare the tightness with respect to Rn gas. For this purpose, solid state nuclear detectors (SSNTD), used for the determination of radon in air [13], were inserted into 10 vials of each selected type. The
Table 1 Efficiencies, sensitivities, background and LLD of the three investigated methods Efficiency
Lucas Cell counting Liquid scintillation counting Gamma counting
Counting (cpm/dpm)
Rn Extraction (%)
0.745 f 0.020 0.912 * 0.009 0.044 * 0.001
0.925 f 0.025a
LLD formula: 4.66 SD. (background)/sensitivity. “The Rn extraction efficiency is included in the counting efficiency.
Total Sensitivity hm/Bq/l)
BKG (cpm)
@q/l)
LLD
23.57 f 0.898 1.64 0.16 5.33 f 0.02
1.0 f 0.1 0.29 f 0.07 105 * 2
0.02 0.20 1.75
64
P. Belloni et al. / The Science of the Total Environment 173 / 174 (I 995) 61-67
vials were closed and exposed to a high radon concentration. SSNTDs in open vials were used as blanks. After the exposure the mean track density of the SSNTDs was measured. The results of the test are reported in Table 2. The ratio of track density in sealed vials to open vials is reported as the track ratio.From analysis of the data, two vials showed a better tightness: the Zinsser 600 with its cup and the LKB Teflon vial with its copper cup. From a cost analysis it has been decided to adopt the Zinsser 600 vial. The internal surface of the vial is coated with Teflon. When silicon disks are used not all vial types are gas-tight. Scintillation cocktail. The scintillation liquid has been chosen on the basis of the following characteristics: l
l
l
capacity to maximize the difference between the pulses generated by (Y and /? radiation good resolution of the (Y spectra with a good figure of merit capacity to minimize the interference from other radionuclides.
Different scintillation liquids have been tested and the NEF 957 produced by New English Nuclear has been selected because of the best resolution (FWHM P 430 keV). This is a mineral oil with a density less than 1 that extracts the Rn-222 from water selectively. The Ra-226 eventually present is not extracted, avoiding interference from the 4.8 MeV (Y particles. Counting. Counting is performed at least 3 h after the sampling to allow equilibrium to be reached between the Rn-222 and its daughters. To avoid chemiluminescence problems the external surface of the vial is carefully cleaned from dust; the vial remains in the dark for at least 30 min inside the instrument at a temperature of about 8°C before counting. Counting time is 60 min. A typical Rn-222 spectrum is shown in Fig. 2. Calibration. Calibration of the system has been performed using Amersham standard solutions of Ra-226 prepared with the cooperation of the Austrian Federal Institute of Food Control and Research of Wien. Several samples of 10 ml stan-
Table 2 Results of Rn tightness of the vials used for liquid scintillation Vial (material)
Open vials Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Packard (polyethylene) Packard (polyethylene) Packard (glass) Packard (glass) Packard (glass) Packard (glass) Packard (glass) Beckman (polyethylene) Beckman (polyethylene) Beckman (polyethylene) Beckman (polyethylene) LKB (Teflon)
Cover and gasket
Zinsser Zinsser + o-ring Packard Packard + Teflon disk Packard + silicon disk Packard + polyeth. disk Packard Packard + o-ring Packard + aluminium disk Packard + Teflon disk Packard + silicon disk Packard + polyeth. disk Beckman + o-ring Beckman Beckman + o-ring Beckman + Teflon disk Beckman + silicon disk LKB-copper
Mean track density after exposure (t/cm2) 686 6 170 280 529 526 397 128 166 197 193 613 171 445 141 115 90 454 20
“Vials were considered Rn tight when the track density was less than 10% of open vials.
Track ratio (%o)
VialSa
Percent of
1 25 41 77 77 58 19 24 29 28 89 25 65 21 17 13 66 3
100 30 0 20 0 11 0 0 70 60 0 70 20 13 33 56 0 80
P. Behi
65
et al. / The Science of the Total Emkmment 173 / I74 (I 995) 61-67
WATER S-
1
100
200
MO
400
500
600
700
so0
!300
looo
Channels
Fig. 2. Liquid scintillation sample.
spectrum of a 619 Bq/l
radon
dard solution with different Ra-226 activity were introduced into the scintillation vials previously filled with 10 ml of the scintillation liquid. After 30 days, to reach equilibrium between the Ra-226 and the Rn-222, the vials were measured. The mean efficiency obtained was 91.2 f 0.9%. The values for efficiency, background, and the LLD are reported in Table 1. 2.3. Gamma counting (CC)
The instrumentation employed consists of a 3 in. x 3 in. NaI(Tl) scintillator with a phototube produced by Ortec and a multichannel analyzer (4096 channels) produced by Silena. A 7 cm thick lead shielding is used to reduce the background. Sampling. A special 1 1 Marinelli beaker has been designed to sample the water (Fig. 3). The water inlet is provided with a locking female connector. A tube, with a male connector on one side, is connected to the water source. After some liters of water have flowed, the male connector is plugged in the Marinelli beaker. The flow of water in the beaker proceeds from the bottom to the top. After some liters of water have flowed, the tube is disconnected and the screw on top of the beaker is closed. The screw produces a gentle pressure inside the beaker that avoids the development of air bubbles in the beaker itself. Counting. After at least 4 h the beaker is transferred into the counting system. The rradiation of the radon daughters Pb-214 (295 and 352 keV) and Bi-214 (609 keV) are counted for 15 min. A typical spectrum for the rcounting obtained with an 80 Bq/l water sample is shown in Fig. 4.
L J Fig. 3. Modified Marinelli ing.
Ll beaker for water sampling/count-
Calibration. A highly acidic solution (pH r 2) of Ra-226 was equally divided into four parts. The first was used to prepare the modified Marinelli beaker. Two activated charcoal collectors, 10 cm in diameter, were homogeneously sprinkled with two parts of the solution. The amount of Ra-226 in the two charcoal collectors was determined with respect to three calibration collectors with certified Ra-226 activity. The fourth part was diluted in 50 ml of acid solution and inserted in an emanation system [14]. After a month a calibrated cell model Pylon 300A was filled with the radon drawn from the emanation system and counted. The result achieved by the scintillation flask is in good agreement with the results obtained from the y-counting of the charcoal collectors. The efficiency, the background and the LLD are reported in Table 1, where a comparison with other techniques is made.
3. Results and comparison of the methods To compare the three techniques, 21 environmental samples (drinkable water, well water, spa water) with different Rn-222 activity have been measured. The samplings were made simultaneously and each sample was treated following the reported methodologies. The results of the mea-
66
P. Belloni et al. / The Science of the Total Environment I73 / I74 (1995) 61-67
.6
400
I
.
I
l
609 KeV
Fig. 4. Gamma
spectrum
of an 80 Bq/l
radon
4. Conclusion
sample.
surements are reported in Table 3. From these results and from analysis of the data reported in Table 1 several conclusions can be made: l
l
All three investigated methods give good results for the measurement of Rn-222 in water. This is confirmed by a paired T-test analysis (at 0.05 probability level) reported in Table 4. The liquid scintillation technique seems to be the most attractive when large numbers of measurements are required, as in, for example, large scale surveys; the sampling is very easy, and, with the employed instrumentation, up to 150 samples can be measured automatically.
The investigated techniques show very good agreement in the range of the measured activity, especially considering the very different quality of the water, samples. No particular problems seem to occur with the three different sampling methodologies. In the
Table 3 Results of water Sample
No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Grand mean and S.D.
sample
measurements
Radon concentration Gamma counting (Bq/l 20.5 36.0 48.0 88.0 6.0 43.0 92.0 118.0 42.0 12.0 9.0 242.0 95.0 8.0 69.0 49.0 17.0 128.0 7.0 21.0 2.0
f f f f f f f f f f f * f f f f f f f f f
1.0 1.1 1.2 1.4 0.9 1.2 1.4 1.5 1.2 1.0 0.9 2.0 1.4 0.9 1.3 1.2 1.0 1.6 0.9 1.0 0.9
54.9 f 57.8
opinion of the authors the sampling methods used in the LSC and GC techniques are simpler than that used for the LCC. The LCC method has the lowest LLD (20 mBq/l) compared to LX (200 mBq/l) and the GC (1.75 Bq/l).
in the field
f SD.)
Scintillation 19.2 36.5 50.1 96.9 7.4 37.9 108.3 105.6 43.8 11.3 10.4 242.4 108.7 8.9 59.3 47.3 14.3 128.2 4.5 27.0 5.0
f f f f f f f f f * f f f f f f f f f f *
0.8 1.4 1.9 3.7 0.3 1.5 4.2 4.1 1.7 0.5 0.4 9.3 4.2 0.4 2.3 1.8 0.6 4.9 0.2 1.1 0.2
55.9 f 58.5
flask (Bq/l
f SD.)
Liquid 19.8 36.8 53.4 92.8 3.7 46.3 111.0 122.0 44.6 11.3 10.9 250.0 98.0 5.9 73.0 43.0 15.1 125.0 5.6 26.7 1.9
scintillation f f f f f f f f f f f f f f f f f f f f f
0.5 0.6 0.7 0.9 0.2 0.6 1.0 1.0 0.7 0.4 0.4 2.0 1.0 0.2 0.8 0.6 0.7 1.0 0.3 0.7 0.3
57.0 f 60.2
(Bq/l
f S.D.)
P. Belloni et al. / The Science of the Total Environment 173 / 174 (199.5) 61-67
67
Table 4 Results of the paired t-test analysis for the three methods
GC vs. LCC GC vs. LX LCC vs LSC
DF
Mean X-Y
Paired t-value
Probability (two-tail)
20 20 20
- 0.976 -2.11 - 1.133
- 0.681 - 1.86 - 0.836
0.504 0.078 0.413
The Lucas cell counting method is the most attractive in the case of in the field measurements because of the transportability of the instrumentation. The very low LLD makes this technique very useful in the case of water with very low Rn-222 content. The advantage of gamma counting is in the very common instrumentation employed. Almost any radiation measurement laboratory has a NaI or a Ge detector. Good measurements can be made simply adopting a modified, and tight Marinelli beaker. Acknowledgements
The authors wish to thank Dr. F. Shonofer of the Federal Institute of Food Control and Research of Wien and Dr. L. Salonen of the Finnish Centre for Radiation and Nuclear Safety of Helsinki for the precious collaboration in carrying out the calibration for the liquid scintillation technique and for intercomparison measurements. References tll
UNSCEAR, United Nations Scientific Committee on the Effects of Atomic Radiation, 1993 Report to the General Assembly, with Scientific Annexes, 1993. [Z] H. Kahlos and M. Asikainen, Internal radiation doses from radioactivity of drinking water in Finland. Health Phys., 39 (1980) 108.
H.M. Prichard, The transfer of radon from domestic water to indoor air. J. Am. Water Works Assoc., April (1987) 159-161. [41 C.R. Cothem and P.A. Rebers, Radon, Radium and Uranium in Drinking Water. Lewis, 1991. [51 W.W. Nazarof and A. Nero, Radon and its Decay Products in Indoor Air. Wiley, New York, 1988. t61 RPID, The Radiation Protection Institutes in Denmark, Finland, Iceland, Norway and Sweden, Naturally Occurring Radiation in the Nordic Countries - Recommendations, 1986. [71 US EPA, Water Pollution Control: National Primary Drinking Water Regulations; Radionuclides. Fed. Reg., 189 (1986) 34836-62. L31A. Malvicini, Camera a scintillazioneper la misura della emanazione contenuta nell’aria. 11 Nuovo Cimento, 12 (1954) 821-823. 191 H.F. Lucas, Improved low level alpha scintillation counter for radon. Rev. Sci. Instr., 28 (1957) 680. UOI F. Schiinhofer, Measurement of 226Ra in water and 222Rn in water and air by liquid scintillation counting. Radiat. Protect. Dosim., 45(1/4) (1992) 123-125. [Ill L. Salonen, Measurement of low level of 222Rn in water with different commercial liquid scintillation counters and pulse-shape analysis, in J.E. Noakes, F. Schiinhofer and H.A. Pollach (Eds.), Liquid Scintillation Spectrometry. Radiocarbon (1993) 361-372. UZI US EPA, EERF Manual 78-1, Radon in Water Sampling Program 1978. [131 G. Toni, Plastic-bag radon gas monitor and survey results. Proc. First Int. Workshop on Radon Monitoring in Radioprotection, Environmental Radioactivity and Earth Science, ICTP, Trieste, Italy, April 3-14, 1989 pp. 177-188. 1141 H.F. Lucas and F. Markun, Radon in air. Calibration procedure: a primary method. Nucl. Sci. Eng., 99 (1988) 82-87. [31
ELSEVIER
The Science of the Total Environment 173/174 (1995) 61-67
Optimization and comparison of three different methods for the determination of Rn-222 in water P. Belloni*a, M. Cavaiolib, G. Ingrao3, C. Mancinic, M. Notarob, P. Santaronid, G. Torrib, R. Vassellib "ENEA CRE, CasacciaAMB-BIO, CP2400, 00100 Roma, Italy ANPA, National Environmental Protection Agency, DISPARAMET, v. V. Brancati, 48, 00144 Roma, Italy c Nuclear Engineering Department, University La Sapienza, P. zza S. Pietro in Vmcoli, 10, 00184 Roma, Italy National Institute of Nutrition, v. Ardeatina, 546, 00178 Roma, Italy b
Abstract Three different systems for the determination of radon in water have been examined: liquid scintillation counting (LSC), degassification followed by Lucas cell counting (LCC) and gamma counting (GC). Particular care has been devoted to the sampling methodologies of the water. Comparative results for several environmental samples are given. A critical evaluation is also given on the basis of the final aim of the measurements. Keywords: Lucas cell counting; Radon; Gamma counting; Liquid scintillation counting
1. Introduction The contribution to the mean effective dose due to the exposure to Rn and its progeny is estimated to be about 50% (1.3 mSv) of the total effective dose [1]. This is a mean evaluation, but single values are subject to large variability and differences of 2 or 3 order of magnitude can be found in closed environments (mines, dwellings, etc.) Concerning indoor environments, potential sources of radon are the soil beneath buildings,
" Corresponding author.
the building materials, the water supply, the outdoor air and other minor sources such as natural gases, etc. While soil and building materials are widely studied, minor emphasis has been devoted to radon in water; furthermore, radon in drinkable water represents a source of internal exposure when ingested, even if this is less important than inhaled radon and daughters [2]. The transfer coefficient for radon from water to domestic indoor air, based on normal use, is estimated to be about 10~4. This implies that 1 Bq/1 in water produces about 10 _1 Bq/m 3 in air [3-5]. Very high concentrations of radon in water represents a risk to be considered. Several countries have
0048-9697/95/$09.50 © 1995 Elsevier Science BV. All rights reserved. SSDI 0 0 4 8 - 9 6 9 7 ( 9 5 ) 0 4 7 5 4 - 0
P. Belkmi et al. / The Science of the Total Encironment I73 / I74 (1995) 61-67
62
already taken into consideration the need of recommendations for reference levels of maximum activity of Rn-222 in water for domestic use [6,7]. With these considerations in mind, in view of extensive studies on the evaluation of the importance of radon in water as a potential cause of exposure to the population, and for the control of the environment, it has been decided to study three analytical methods for the determination of the activity of Rn-222 in water: liquid scintillation counting (LX), degassification followed by Lucas cell counting (LCC) and gamma counting (GC). The main objective of the work has been to optimize the methods and compare them not only regarding the precision, accuracy, costs, etc., but also from the point of view of their possible specific applications such as, for example, in large surveys, field measurements, semi-continuous measurements for controlling water supplies, etc. The authors intend to point out that for dosimetry studies, passive integrating techniques, able to measure Rn-222 in water over a long period (months), are the most attractive; nevertheless, methods for instantaneous determination need to be evaluated. The three methods have been calibrated separately and then compared using 21 environmental samples of water with different Rn-222 activity. Particular care has been
devoted to the methodologies of sampling, especially to avoid contact with air. 2. Methods and materials 2.1. Degassijkation and Lucas cell counting (LCC) Radon is extracted from water, transferred into a scintillation flask and counted using a photomultiplier [8,9].The instrumentation used is produced by Pylon (Canada). It consists of a degassing unit (WGlOOl) used to transfer the radon from water into a scintillation flask (the Lucas cell). The internal walls of the flask are coated with a solid scintillator, ZnS(Ag). The counting was performed with a photomultiplier system (Pylon AB5). Sampling. A plastic tube is applied to the water faucet. After a (flow of at least 5 1, a 250 ml graduated cylinder is filled keeping the end of the tube close to the bottom of the cylinder to minimize contact with air. After a water flow of about 1 1, the volume of the water in the cylinder is brought up to 190 ml. The cylinder is immediately tightly closed with the cap used for the degassification. Degassification. During this procedure the radon is transferred from the water to the Lucas cell through the degassing unit WG 1001 (see Fig.
AIRINLET I I METERING
ADDITIONAL
VALVE
VALVE
ON-OFF VALVE Fig. 1. Degassification
unit WG
1001.
P. Belloni et al. / The Science of the Total Environment I73 / 174 (1995) 61-67
1). A 270 ml Lucas cell is evacuated with a vacuum pump and then connected to the cylinder through an on-off valve. A regulated flow of air via a metering valve extracts the radon from the water by bubbling through a diffusion stone. A desiccant column prevents humidity entering into the cell. The extraction time is 5 min in order to guarantee an optimal degassification. At the end of the extraction the Lucas cell is disconnected. The treatment of mineral water with a high CO, content needs particular care. The CO, may cause a fast and violent bubbling when the cylinder is connected to the Lucas cell through the on-off valve 1. This gives rise to a water flow that enters into the WG 1001 system and into the Lucas cell itself. To avoid this problem an additional valve has been inserted between the cylinder and the Lucas cell. This valve is opened very slowly and carefully after the opening of the on-off valve 1. Counting. After a delay time of about 4 h needed to reach equilibrium between radon and its daughters for the optimization of the counting, the Lucas cell is connected to the photomultiplier of the AB5 and a-radiation from Rn-222, PO-218 and PO-214 are measured. A counting time of 10 min was adopted. Calibration. The extraction efficiency of the system has been determined by multiple degassifications on several samples of water prepared using a NIST radium source. The value found for the extraction efficiency is = 93 f 2.5%. The radon counting system (AB5 plus Lucas cell) has been calibrated in a radon chamber at the National Radiological Protection Board (NRPB), Chilton, Didcot, United Kingdom. The efficiency
63
of the counting system was 0.745 f 0.020 cpm/dpm. These parameters, together with the lower limit of detection, are reported in Table 1, where they are compared with the other techniques. 2.2. Liquid scintillation counting (LX) Radon-222 is extracted from water by a scintillation solution and directly counted in a liquid scintillation counter [lO,ll]. Radium-226 is not extracted. The low background Quantulus scintillator system produced by LKB Wallac Pharmacia was employed. The instrument has a pulse shape analyzer (PSA) unit that discriminates the pulses generated by a-radiation from those generated by P-radiation. Sampling. To avoid contact with air a sampling methodology similar to that suggested by EPA [12] has been adopted. A polyethylene tube is connected to the water faucet; after a flow of about 5 1, 10 ml of water are carefully sampled sucking from the tube with a syringe. The sample is transferred directly into a scintillation vial, previously filled with the scintillation liquid, injecting it under the level of the scintillation liquid. Scintillation vial. One problem that might occur when applying the LX technique to radon determination is the tightness of the vial used for the counting because Rn gas can escape from the vials. Eighteen types of commercial and modified vials have been evaluated to compare the tightness with respect to Rn gas. For this purpose, solid state nuclear detectors (SSNTD), used for the determination of radon in air [13], were inserted into 10 vials of each selected type. The
Table 1 Efficiencies, sensitivities, background and LLD of the three investigated methods Efficiency
Lucas Cell counting Liquid scintillation counting Gamma counting
Counting (cpm/dpm)
Rn Extraction (%)
0.745 f 0.020 0.912 * 0.009 0.044 * 0.001
0.925 f 0.025a
LLD formula: 4.66 SD. (background)/sensitivity. “The Rn extraction efficiency is included in the counting efficiency.
Total Sensitivity hm/Bq/l)
BKG (cpm)
@q/l)
LLD
23.57 f 0.898 1.64 0.16 5.33 f 0.02
1.0 f 0.1 0.29 f 0.07 105 * 2
0.02 0.20 1.75
64
P. Belloni et al. / The Science of the Total Environment 173 / 174 (I 995) 61-67
vials were closed and exposed to a high radon concentration. SSNTDs in open vials were used as blanks. After the exposure the mean track density of the SSNTDs was measured. The results of the test are reported in Table 2. The ratio of track density in sealed vials to open vials is reported as the track ratio.From analysis of the data, two vials showed a better tightness: the Zinsser 600 with its cup and the LKB Teflon vial with its copper cup. From a cost analysis it has been decided to adopt the Zinsser 600 vial. The internal surface of the vial is coated with Teflon. When silicon disks are used not all vial types are gas-tight. Scintillation cocktail. The scintillation liquid has been chosen on the basis of the following characteristics: l
l
l
capacity to maximize the difference between the pulses generated by (Y and /? radiation good resolution of the (Y spectra with a good figure of merit capacity to minimize the interference from other radionuclides.
Different scintillation liquids have been tested and the NEF 957 produced by New English Nuclear has been selected because of the best resolution (FWHM P 430 keV). This is a mineral oil with a density less than 1 that extracts the Rn-222 from water selectively. The Ra-226 eventually present is not extracted, avoiding interference from the 4.8 MeV (Y particles. Counting. Counting is performed at least 3 h after the sampling to allow equilibrium to be reached between the Rn-222 and its daughters. To avoid chemiluminescence problems the external surface of the vial is carefully cleaned from dust; the vial remains in the dark for at least 30 min inside the instrument at a temperature of about 8°C before counting. Counting time is 60 min. A typical Rn-222 spectrum is shown in Fig. 2. Calibration. Calibration of the system has been performed using Amersham standard solutions of Ra-226 prepared with the cooperation of the Austrian Federal Institute of Food Control and Research of Wien. Several samples of 10 ml stan-
Table 2 Results of Rn tightness of the vials used for liquid scintillation Vial (material)
Open vials Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Zinsser 600 (Teflon coated) Packard (polyethylene) Packard (polyethylene) Packard (glass) Packard (glass) Packard (glass) Packard (glass) Packard (glass) Beckman (polyethylene) Beckman (polyethylene) Beckman (polyethylene) Beckman (polyethylene) LKB (Teflon)
Cover and gasket
Zinsser Zinsser + o-ring Packard Packard + Teflon disk Packard + silicon disk Packard + polyeth. disk Packard Packard + o-ring Packard + aluminium disk Packard + Teflon disk Packard + silicon disk Packard + polyeth. disk Beckman + o-ring Beckman Beckman + o-ring Beckman + Teflon disk Beckman + silicon disk LKB-copper
Mean track density after exposure (t/cm2) 686 6 170 280 529 526 397 128 166 197 193 613 171 445 141 115 90 454 20
“Vials were considered Rn tight when the track density was less than 10% of open vials.
Track ratio (%o)
VialSa
Percent of
1 25 41 77 77 58 19 24 29 28 89 25 65 21 17 13 66 3
100 30 0 20 0 11 0 0 70 60 0 70 20 13 33 56 0 80
P. Behi
65
et al. / The Science of the Total Emkmment 173 / I74 (I 995) 61-67
WATER S-
1
100
200
MO
400
500
600
700
so0
!300
looo
Channels
Fig. 2. Liquid scintillation sample.
spectrum of a 619 Bq/l
radon
dard solution with different Ra-226 activity were introduced into the scintillation vials previously filled with 10 ml of the scintillation liquid. After 30 days, to reach equilibrium between the Ra-226 and the Rn-222, the vials were measured. The mean efficiency obtained was 91.2 f 0.9%. The values for efficiency, background, and the LLD are reported in Table 1. 2.3. Gamma counting (CC)
The instrumentation employed consists of a 3 in. x 3 in. NaI(Tl) scintillator with a phototube produced by Ortec and a multichannel analyzer (4096 channels) produced by Silena. A 7 cm thick lead shielding is used to reduce the background. Sampling. A special 1 1 Marinelli beaker has been designed to sample the water (Fig. 3). The water inlet is provided with a locking female connector. A tube, with a male connector on one side, is connected to the water source. After some liters of water have flowed, the male connector is plugged in the Marinelli beaker. The flow of water in the beaker proceeds from the bottom to the top. After some liters of water have flowed, the tube is disconnected and the screw on top of the beaker is closed. The screw produces a gentle pressure inside the beaker that avoids the development of air bubbles in the beaker itself. Counting. After at least 4 h the beaker is transferred into the counting system. The rradiation of the radon daughters Pb-214 (295 and 352 keV) and Bi-214 (609 keV) are counted for 15 min. A typical spectrum for the rcounting obtained with an 80 Bq/l water sample is shown in Fig. 4.
L J Fig. 3. Modified Marinelli ing.
Ll beaker for water sampling/count-
Calibration. A highly acidic solution (pH r 2) of Ra-226 was equally divided into four parts. The first was used to prepare the modified Marinelli beaker. Two activated charcoal collectors, 10 cm in diameter, were homogeneously sprinkled with two parts of the solution. The amount of Ra-226 in the two charcoal collectors was determined with respect to three calibration collectors with certified Ra-226 activity. The fourth part was diluted in 50 ml of acid solution and inserted in an emanation system [14]. After a month a calibrated cell model Pylon 300A was filled with the radon drawn from the emanation system and counted. The result achieved by the scintillation flask is in good agreement with the results obtained from the y-counting of the charcoal collectors. The efficiency, the background and the LLD are reported in Table 1, where a comparison with other techniques is made.
3. Results and comparison of the methods To compare the three techniques, 21 environmental samples (drinkable water, well water, spa water) with different Rn-222 activity have been measured. The samplings were made simultaneously and each sample was treated following the reported methodologies. The results of the mea-
66
P. Belloni et al. / The Science of the Total Environment I73 / I74 (1995) 61-67
.6
400
I
.
I
l
609 KeV
Fig. 4. Gamma
spectrum
of an 80 Bq/l
radon
4. Conclusion
sample.
surements are reported in Table 3. From these results and from analysis of the data reported in Table 1 several conclusions can be made: l
l
All three investigated methods give good results for the measurement of Rn-222 in water. This is confirmed by a paired T-test analysis (at 0.05 probability level) reported in Table 4. The liquid scintillation technique seems to be the most attractive when large numbers of measurements are required, as in, for example, large scale surveys; the sampling is very easy, and, with the employed instrumentation, up to 150 samples can be measured automatically.
The investigated techniques show very good agreement in the range of the measured activity, especially considering the very different quality of the water, samples. No particular problems seem to occur with the three different sampling methodologies. In the
Table 3 Results of water Sample
No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Grand mean and S.D.
sample
measurements
Radon concentration Gamma counting (Bq/l 20.5 36.0 48.0 88.0 6.0 43.0 92.0 118.0 42.0 12.0 9.0 242.0 95.0 8.0 69.0 49.0 17.0 128.0 7.0 21.0 2.0
f f f f f f f f f f f * f f f f f f f f f
1.0 1.1 1.2 1.4 0.9 1.2 1.4 1.5 1.2 1.0 0.9 2.0 1.4 0.9 1.3 1.2 1.0 1.6 0.9 1.0 0.9
54.9 f 57.8
opinion of the authors the sampling methods used in the LSC and GC techniques are simpler than that used for the LCC. The LCC method has the lowest LLD (20 mBq/l) compared to LX (200 mBq/l) and the GC (1.75 Bq/l).
in the field
f SD.)
Scintillation 19.2 36.5 50.1 96.9 7.4 37.9 108.3 105.6 43.8 11.3 10.4 242.4 108.7 8.9 59.3 47.3 14.3 128.2 4.5 27.0 5.0
f f f f f f f f f * f f f f f f f f f f *
0.8 1.4 1.9 3.7 0.3 1.5 4.2 4.1 1.7 0.5 0.4 9.3 4.2 0.4 2.3 1.8 0.6 4.9 0.2 1.1 0.2
55.9 f 58.5
flask (Bq/l
f SD.)
Liquid 19.8 36.8 53.4 92.8 3.7 46.3 111.0 122.0 44.6 11.3 10.9 250.0 98.0 5.9 73.0 43.0 15.1 125.0 5.6 26.7 1.9
scintillation f f f f f f f f f f f f f f f f f f f f f
0.5 0.6 0.7 0.9 0.2 0.6 1.0 1.0 0.7 0.4 0.4 2.0 1.0 0.2 0.8 0.6 0.7 1.0 0.3 0.7 0.3
57.0 f 60.2
(Bq/l
f S.D.)
P. Belloni et al. / The Science of the Total Environment 173 / 174 (199.5) 61-67
67
Table 4 Results of the paired t-test analysis for the three methods
GC vs. LCC GC vs. LX LCC vs LSC
DF
Mean X-Y
Paired t-value
Probability (two-tail)
20 20 20
- 0.976 -2.11 - 1.133
- 0.681 - 1.86 - 0.836
0.504 0.078 0.413
The Lucas cell counting method is the most attractive in the case of in the field measurements because of the transportability of the instrumentation. The very low LLD makes this technique very useful in the case of water with very low Rn-222 content. The advantage of gamma counting is in the very common instrumentation employed. Almost any radiation measurement laboratory has a NaI or a Ge detector. Good measurements can be made simply adopting a modified, and tight Marinelli beaker. Acknowledgements
The authors wish to thank Dr. F. Shonofer of the Federal Institute of Food Control and Research of Wien and Dr. L. Salonen of the Finnish Centre for Radiation and Nuclear Safety of Helsinki for the precious collaboration in carrying out the calibration for the liquid scintillation technique and for intercomparison measurements. References tll
UNSCEAR, United Nations Scientific Committee on the Effects of Atomic Radiation, 1993 Report to the General Assembly, with Scientific Annexes, 1993. [Z] H. Kahlos and M. Asikainen, Internal radiation doses from radioactivity of drinking water in Finland. Health Phys., 39 (1980) 108.
H.M. Prichard, The transfer of radon from domestic water to indoor air. J. Am. Water Works Assoc., April (1987) 159-161. [41 C.R. Cothem and P.A. Rebers, Radon, Radium and Uranium in Drinking Water. Lewis, 1991. [51 W.W. Nazarof and A. Nero, Radon and its Decay Products in Indoor Air. Wiley, New York, 1988. t61 RPID, The Radiation Protection Institutes in Denmark, Finland, Iceland, Norway and Sweden, Naturally Occurring Radiation in the Nordic Countries - Recommendations, 1986. [71 US EPA, Water Pollution Control: National Primary Drinking Water Regulations; Radionuclides. Fed. Reg., 189 (1986) 34836-62. L31A. Malvicini, Camera a scintillazioneper la misura della emanazione contenuta nell’aria. 11 Nuovo Cimento, 12 (1954) 821-823. 191 H.F. Lucas, Improved low level alpha scintillation counter for radon. Rev. Sci. Instr., 28 (1957) 680. UOI F. Schiinhofer, Measurement of 226Ra in water and 222Rn in water and air by liquid scintillation counting. Radiat. Protect. Dosim., 45(1/4) (1992) 123-125. [Ill L. Salonen, Measurement of low level of 222Rn in water with different commercial liquid scintillation counters and pulse-shape analysis, in J.E. Noakes, F. Schiinhofer and H.A. Pollach (Eds.), Liquid Scintillation Spectrometry. Radiocarbon (1993) 361-372. UZI US EPA, EERF Manual 78-1, Radon in Water Sampling Program 1978. [131 G. Toni, Plastic-bag radon gas monitor and survey results. Proc. First Int. Workshop on Radon Monitoring in Radioprotection, Environmental Radioactivity and Earth Science, ICTP, Trieste, Italy, April 3-14, 1989 pp. 177-188. 1141 H.F. Lucas and F. Markun, Radon in air. Calibration procedure: a primary method. Nucl. Sci. Eng., 99 (1988) 82-87. [31