Complementary system for long term measurements of radon exhalation rate fromsoil J. Mazur and K. Kozak Citation: Review of Scientific Instruments 85, 022104 (2014); doi: 10.1063/1.4865156 View online: http://dx.doi.org/10.1063/1.4865156 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Development and application of a continuous measurement system for radon exhalation rate Rev. Sci. Instrum. 82, 015101 (2011); 10.1063/1.3527065 Radon and Thoron exhalation rate map in Japan AIP Conf. Proc. 1034, 177 (2008); 10.1063/1.2991202 Estimation of global radon exhalation rate distribution AIP Conf. Proc. 1034, 169 (2008); 10.1063/1.2991199 Characteristic of Soils and Behavior of Hexavalent Chromium in SoilWater Interaction AIP Conf. Proc. 987, 62 (2008); 10.1063/1.2896980 Longterm Persistence of Cyclodiene Pesticide in Soil AIP Conf. Proc. 898, 32 (2007); 10.1063/1.2721245
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 022104 (2014)
Complementary system for long term measurements of radon exhalation rate from soil J. Mazur and K. Kozaka) Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Kraków, Poland
(Received 31 July 2013; accepted 10 October 2013; published online 18 February 2014) A special set-up for continuous measurements of radon exhalation rate from soil is presented. It was constructed at Laboratory of Radiometric Expertise, Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Krakow, Poland. Radon exhalation rate was determined using the AlphaGUARD PQ2000 PRO (Genitron) radon monitor together with a special accumulation container which was put on the soil surface during the measurement. A special automatic device was built and used to raise and lower back onto the ground the accumulation container. The time of raising and putting down the container was controlled by an electronic timer. This set-up made it possible to perform 4–6 automatic measurements a day. Besides, some additional soil and meteorological parameters were continuously monitored. In this way, the diurnal and seasonal variability of radon exhalation rate from soil can be studied as well as its dependence on soil properties and meteorological conditions. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865156] I. INTRODUCTION
Radon (222 Rn) gas and its short-lived decay products as natural radioactive isotopes widely occurring in nature are a major factor responsible for the dose of ionising radiation that people receive. Therefore, radon has been in scope of interest for many years – both in terms of science (its transport in the ground, exhalation from soil, migration into buildings, etc.) and in terms of dosimetry (its impact on the human health). The radiation dose to human population due to inhalation of radon and its progeny contribute more than 50% of the dose from all sources of radiation both naturally occurring and man made.1 (UNSCEAR, 2000). Radon exhalation is the process of its escaping from the ground or other materials (e.g., building materials) into the atmosphere. It is defined as the radon activity released from an area unit during a time unit. This process is interesting and complex, depending on many factors, mainly those which influence radon emanation coefficient and radon transport, i.e., radium (226 Ra) concentration in soil or rock grains, soil properties (porosity, permeability, humidity, temperature), and meteorological conditions (air pressure, air temperature, wind speed, precipitation).2 These relationships are not yet completely explained, and have been still studied. The soil properties influence directly the dynamics of radon exhalation. Meteorological parameters are factors which indirectly affect the process of radon exhalation from soil. The investigation of radon exhalation rate from soil using different techniques was presented by several researchers. Many papers describe the “laboratory techniques” for determination of mass radon exhalation rate from soil samples and/or from building materials samples.3–7 The most common methods for radon registration are solid state nuclear track detectors (SSNTD technique), activated charcoal canisters, Lucas cells (scintillation counters), and the active technique with a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(2)/022104/7/$30.00
the ionisation chamber. The in situ measurements of radon exhalation from soil are more interesting in terms of its dependencies on soil properties and meteorological parameters. Some authors described this kind of measurements but the presented results regarded only a single or short term measurements of radon exhalation rate from soil.8–12 The study of relationship between radon exhalation rate from soil and meteorological conditions has been previously carried out by Ferry et al.13 and Hosoda et al.14 The authors used an automatic devices for radon exhalation measurements but the duration of measurements did not exceed one month time. At the Institute of Nuclear Physics PAN (Krakow, Poland) a special device, called AutoEXH, was constructed for long term, continuous survey of radon exhalation rate from soil. In order to limit the influencing factors to meteorological conditions the device was set up at one specific place (at the Institute’s area) where the soil properties (radium content, density, porosity) were determined and stayed the same for the whole period of measurements. The facility made it possible to perform 4 measurements per day, i.e., the new measurement started every 6 h. The measurement campaign lasted one year time. The aim of this research was to broaden the knowledge on radon exhalation process and to study of radon exhalation dynamics during long period of time in connection with meteorological conditions. In the paper, the construction of AutoEXH is presented as well as some results obtained using the facility.
II. MATERIALS AND METHODS
At the Institute of Nuclear Physics PAN (Krakow, Poland) a special site (RSF – Radon Study Field) was created for long-term measurements of radon exhalation rate. The RSF was situated in the open air at the distance of more than 50 m from other buildings. At the RSF there was also a set of sensors for monitoring of meteorological conditions
85, 022104-1
© 2014 AIP Publishing LLC
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J. Mazur and K. Kozak
Rev. Sci. Instrum. 85, 022104 (2014)
TABLE I. The characteristics of soil at the Radon Study Field. Density 2.631 ± 0.002 (g cm−3 ) Permeability 5.22 × 10−12 ÷ 8.88× 10−12 (m2 ) Temperature 2,4 ◦ C ÷ 19 ◦ C Relative humidity 10% ÷ 16% Concentration of natural radioactive isotopes in soil 40 K 340 ± 20 (Bq kg−1 ) 226 Ra 22 ± 2 (Bq kg−1 ) 232 Th 31 ± 3 (Bq kg−1 ) 222 Rn 20 ÷ 56 (kBq m−3 )
Alpha Pump AlphaGUARD PQ2000 PRO
Filter
ExhBox
(Weather Monitor, DAVIS). The characteristics of the RSF are presented in Table I. Soil density was determined with multipycnometer (Quantachrome Corporation). Soil permeability and Rn-222 concentration in soil gas at the depth of 1 m were determined using the methods described in Zunic et al.15 At the same depth soil temperature and relative humidity were measured. A thermocouple was used for soil temperature and Theta Probe ML2x sensor was applied for soil humidity.16 Radon in soil gas was measured systematically from September 2003 to April 2006 once or twice a month. The concentrations of gamma radio-nuclides in soil (40 K, 226 Ra, 232 Th) have been determined by low background gamma spectrometry with three-window method. The geological identification of subsurface bedrock structure of the RSF area was made using a shallow acoustic seismic logging method.17 The interpretation of the obtained seismograms showed that the area of RSF is uniform and no tectonic faults occur there. The layer of soil has a thickness of 5–8 m, beneath there is a layer of loess with a thickness of about 10 m. At the depth of 15–70 m, there are Miocene clays that are in turn located over a layer of Jurassic limestone. The idea of radon exhalation measurement was based on a known “accumulation method” (Fig. 1). A container ExhBox was placed on the ground and the increase of radon concentration inside the container was registered by the ionisation chamber AlphaGUARD. Radon concentration was registered in 10-min intervals, the AlphaGUARD was working in a flow mode with the flow rate of 0,3 dm3 min−1 forced by the AlphaPUMP. The small flow rate was chosen in order to keep the natural conditions and not to force the additional radon release from soil surface. The radon exhalation rate E was then calculated according to the formula (1), E=
V · B, S
(1)
where V – volume of the system (m3 ); S – area covered by the container (m2 ); B – slope of the line CRn = f(t) fitted to experimental points; CRn – radon concentration (Bq m−3 ); t – time (min). In this way, single measurements have been made, first 2–3 times a week, then the frequency has been increased to 1 measurement a day. It appeared, however, that they were insufficient to search for correlations between radon exhalation and meteorological conditions.
SOIL FIG. 1. Idea of radon exhalation measurement.
The next step was the construction of the automatic facility AutoEXH that replaced the container ExhBox and enabled to carry out the measurements several times a day. The schematic view of AutoEXH is presented in Fig. 2. The facility consisted of the following main components: (a) fixing base plate (dimensions 590 mm × 280 mm) with ground spikes (70 mm), (b) power supply system, (c) automatic electronic control unit, (d) mechanism for raising and lowering the ExhBox chamber, (e) ExhBox chamber (diameter of 215 mm, height of 145 mm) with valves. It is essential to know exactly the surface area under the ExhBox to calculate precisely the area covered by the ExhBox container during each measurement. The fixing base plate had a hole (diameter of 216 mm) ended with a flange that was pressed into the ground to a depth of 60 mm. It was also crucial to ensure the tightness of the system. There was also a pressure valve in the fixing base plate which was used for the determination of pressure difference. The measurements showed that the difference between atmospheric air pressure and air pressure under the ExhBox did not exceed 2 Pa. The height of the whole device was 720 mm. The work cycle of AutoEXH consisted of 3 stages:
r ExhBox was lowered and put on the ground. r Radon exhalation rate measurement, ExhBox stayed down.
r ExhBox was lifted up to 380 mm above the ground, the system was ventilated. The pump was working during all the cycle which made it possible to ventilate both ionisation chamber of AlphaGUARD and ExhBox when it was up. After the ventilation period the ExhBox was lowered again and the next measurement cycle started. After carrying out test measurements the following cycle was established: 1,5-h measurement and 4,5-h ventilation. It meant only four radon exhalation measurements a day but this ensured good
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J. Mazur and K. Kozak
Rev. Sci. Instrum. 85, 022104 (2014)
FIG. 2. Schematic and real view of AutoEXH.
ventilation of the set-up and retaining environmental conditions as natural as possible. The example of the results obtained using AutoEXH is shown in Fig. 3. The values of radon exhalation rate were calculated for every increasing course of the curve. III. RESULTS AND DISCUSSION
The total number of 1346 radon exhalation values were gathered using AutoEXH during one year time. Measurements were made in changing meteorological conditions and changing soil temperature: air temperature: from –24 ◦ C to 32 ◦ C air pressure: from 950 hPa to 1023 hPa
soil temperature: from 2,4 ◦ C to 19 ◦ C. The measured values of radon exhalation rate ranged from 0 to 386 mBq m−2 s−1 . The values 0 mBq m−2 s−1 mean that it was impossible to register increase of Rn concentration inside ExhBox, this occurred in winter time when surface soil layer was frozen. The obtained results served for study of annual and diurnal changeability of radon exhalation rate, dependence of exhalation rate on radon concentration in soil, and dependence of exhalation rate on chosen meteorological parameters. The relationship between E and radon in soil gas is shown in Fig. 4. The results are grouped in two distinct populations. Within one of them the simple proportionality can be observed – the higher the concentration of radon in the soil
FIG. 3. Example of radon concentration registered by AlphaGUARD inside the ExhBox using AutoEXH.
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J. Mazur and K. Kozak
Rev. Sci. Instrum. 85, 022104 (2014)
160 140 120 100 80 60 40 20 0 20
25
30
35
40
45
222
50
55
3
Rn concentration in soil [kBq/m ]
FIG. 4. Dependence of radon exhalation rate on radon in soil concentration at 1 m depth.
the higher values of E. The second population (more numerous) does not show this type of dependence. This suggests that within this population radon exhalation rate is much more influenced by meteorological conditions than by radon in soil concentration at 1 m depth. Moreover, the values of E are lower than in first population (from 0 to ca. 20 mBq m−2 s−1 ). These values occur during winter seasons or during more intense rainfall, i.e., situations when the influence of meteorological conditions on E is significant.
A. Diurnal variability of radon exhalation rate from soil
The application of AutoEXH device made it possible to observe not only seasonal but also the diurnal changeability of radon exhalation rate from soil. It was found that there are periods of time when radon exhalation rate E shows regular changes according to changes of air temperature (Fig. 5(a)). 70
The meteorological conditions, i.e., air temperature, wind speed, precipitation, influence directly the soil humidity, and temperature, which in term influence the rate of radon exhalation. With the AutoEXH device it was possible to
25
25 18
radon exhalation rate E air temperature
-1
20 40
30 15 20
10
0 0
1 July 2006 00 00
30
60
90
HOURS
120
10 150
3 July 2006 30 23
o
50
Air temperature [ C]
-2
-1 -2
B. Relationships between radon exhalation rate and meteorological conditions
Rn exhalation rate E air temperature
60
Rn exhalaton rae E [mBq m s ]
However, often no such kind of regularity was observed (Fig. 5(b)). In the periods of time when the regular changes of air temperature occur, the corresponding periodic variability of soil temperature is also observed (with some delay). The decrease of air temperature with simultaneously increasing of soil temperature is responsible for regular changes of radon exhalation rate at those periods of time. This indicates also the significant contribution of convection in radon transport in soil. For further analysis of diurnal changeability of radon exhalation the results have been divided into groups corresponding with the particular hours of the measurement: 5:00, 11:00, 17:00, and 23:00 during three seasons of the year: winter, spring, and summer (Fig. 6). The autumn season is not included in the figure since the AutoEXH was under tests and the measurements were made in different hours of the day. The smallest diurnal variability can be observed in the winter season regardless of the hour of the measurement, whereas during spring and summer seasons the variability is more significant. Radon exhalation rate shows the lowest seasonal discrepancy at 5:00 and at 23:00. The highest values of E were found at 11:00 and at 17:00 during summer and spring seasons. This may be due to the bigger impact of meteorological conditions on radon exhalation rate than in winter season. The statistical analysis of mean values of E, measured at particular hours of the day during winter and summer seasons, showed that they do not differ in a statistically significant way. During spring season the statistically significant differences in E mean values were observed for 5:00 and 17:00 h.
Rn exhalation rate E [mBq m s ]
222
2
Rn exhalation rate [mBq/m s]
180
20
16 14
15 12 10
10
Air temperature [oC]
022104-4
8 5 6 0
4 0
50
1 Oct. 2005 00 00
100
150
HOURS
200
250
6 Oct. 2005 00 5
(b)
(a) FIG. 5. (a) and (b) Diurnal variability of radon exhalation rate.
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Rev. Sci. Instrum. 85, 022104 (2014)
FIG. 6. Radon exhalation rate from soil at the same hours of the day during three seasons of the year.
TABLE II. Correlation coefficients WK for E and chosen meteorological parameters in different seasons of the year.
E Autumn 2005 E Winter 2006 E Spring 2006 E Summer 2006
T
V_wind
P
Rain
T_soil
Rain_24
− 0.33 − 0.14 0.52 0.08
0.00 0.17 0.25 − 0.02
− 0.16 0.26 0.32 0.06
0.03 − 0.05 0.05 − 0.03
− 0.40 0.31 0.69 − 0.01
0.01 − 0.03 − 0.03 − 0.21
24
-1
The values of correlation coefficients WK (at 95% of confidence interval) for radon exhalation rate E and the above described meteorological parameters are presented in Table II. The highest values of WK were found for E and T (0.52) as well as for E and T_soil (0.69) in spring season. In summer, there is no significant correlation between E and air/soil temperature. The weakest correlations during all seasons were observed for E and wind speed and for E and precipitation. In summer 2006 the weak, negative correlation was found between E and Rain_24 (−0.21). The more detailed analysis of relationship between E and T has been made. The values of air temperature were grouped
22
-2
T, V_wind, P, Rain – the values of: air temperature, wind speed, air pressure, and precipitation registered 0.5 h before the end of the E measurement. T_soil – soil temperature at 1 m depth. Rain_24 – sum of precipitation during 24 h before the E measurement was ended.
into 8 intervals covering the range from −10 ◦ C to 32 ◦ C, i.e., temperatures in all seasons of the year. The width of each interval was 5 ◦ C. For each interval the mean value of air temperature Ti and the mean value of E were calculated. The relationship E = f(Ti ) is shown in Fig. 7. It can be seen that within the range from −5 ◦ C to 25 ◦ C the linear increase of E with increasing Ti was observed. The linear fit within that range is described by the equation E = 10,63 (±0,72) + 0,44 (±0,06) · Ti (with R = 0,97). There direct correlation between E and P was not observed. This result is similar to that obtained by Kojima and Nagano,18 although their study was much shorter, i.e., about three months. Thus, further analysis consisted in dividing the whole range of registered air pressure values into 5 hPa
Rn exhalation rate E [mBq m s ]
search for correlations between E and the single meteorological parameter. The registration of meteorological condition was made every 30 min, whereas E was measured every 6 h. For analysis, the following values of meteorological parameters were attributed to each value of E:
20 18 16 14 12 10 8 -10
-5
0
5
10
15
20
25
30
o
Air teperatureTi [ C]
FIG. 7. Relationship between mean values of E and mean values of Ti .
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J. Mazur and K. Kozak
Rev. Sci. Instrum. 85, 022104 (2014)
28 o
26
T 15 C
-1
22
-2
Rn exhalation rate E [mBq m s ]
24 20 18 16 14 12 10 8 6 4 2 0 < 985
985-990
990-995
995-1000
> 1000
Air pressure P [hPa] FIG. 8. Mean values of E for three ranges of air temperature in five ranges of air pressure. FIG. 9. Variability of radon exhalation rate as a function of soil temperature and precipitation.
intervals. Then, for each P interval three mean values of E were calculated which corresponded with three ranges of air temperature T: 15 ◦ C. The obtained results are shown in Fig. 8. The influence of air pressure on radon exhalation rate is different for different air temperature ranges. It can be seen that E is not influenced by air pressure when the air temperature ranges from 0 ◦ C to 15 ◦ C. The greatest influence of P is observed when the air temperature is higher than 15 ◦ C. It is worth mentioning that some authors reported the direct, significant correlation between E and P,10 however this was not confirmed in our study. The reason for this may be much longer time of our measurements. Basing on the presented results and on other authors’ research, it can be seen that within short periods of time (e.g., a few days/weeks) the significant correlations between E and
a single meteorological parameter can be found. After our long-term measurements (one year, 4 measurements a day) it appeared that the process of radon exhalation from soil is very complex and it depends on several factors, and even on the combination of these factors. Thus, an attempt was undertaken to examine the dynamics of this process using multiparametric analysis. This analysis made it possible to show the complex, nonlinear dependencies between variables. As an example, Fig. 9 presents the radon exhalation rate E as a function of two parameters: soil temperature T_soil and sum of precipitation during 24 h before the E measurement was ended Rain_24 basing on the results from the whole year of measurements. The same analysis has been made for each season.
FIG. 10. Variability of radon exhalation rate as a function of air pressure and precipitation with soil temperature as a categorization factor.
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022104-7
J. Mazur and K. Kozak
The highest value of E can be observed for the high soil temperature (E gradient is in agreement with T_soil gradient) with no significant dependency on the precipitation. The similar tendency was found for spring season, whereas the “autumn results” were completely different. In this season, the highest values of E were observed for low soil temperature and low precipitation. The development of the multi-parametric analysis consisted in application of the categorization, i.e., introducing the third variable which divided the data set into groups (categories) corresponding with the ranges of this third variable. Fig. 10 presents the radon exhalation rate E as a function of two parameters: air pressure P and sum of precipitation during 24 h before the E measurement was ended Rain_24 basing on the results from the whole year of measurements. The categorization factor is soil temperature T_soil with two ranges: below 10 ◦ C and over 10 ◦ C. The significantly increasing values of E can be observed for increasing precipitation and decreasing air pressure when soil temperature is lower than 10 ◦ C. This tendency was not found when the simple relationships E = f(P) and E = F(Rain_24) were studied. For soil temperature higher than 10 ◦ C, the E dependence on P and Rain_24 is completely different (Fig. 10).
IV. CONCLUSIONS
The construction of the special set-up AutoEXH for continuous measurements of radon exhalation rate from soil was presented. The device was controlled automatically, so the measurements were made without the necessity of man presence. The facility made it possible to perform several measurements a day. Thus, the study of dependencies of radon exhalation rate on meteorological conditions was undertaken. The obtained results confirmed the complexity of radon exhalation process and its dependence on many factors, including combination of meteorological parameters. The dependencies of E on soil humidity and temperature are more complicated than described by the theory of diffusive radon flux from soil. The lack of simple relationships between E and a single meteorological parameter was observed in long-term measurements. The linear dependence of E on air temperature was found in the range from −5 ◦ C to 25 ◦ C. This shows that the contribution of convection in radon transport is significant. The presented system is promising for further investigation and for better understanding of radon exhalation process in connection with meteorological conditions. The next step can be, e.g., study of radon exhalation rate from soil with relation to soil heat flux.
Rev. Sci. Instrum. 85, 022104 (2014)
The multi-parametric analysis is a good way to study relationship between E and meteorological conditions as shown in the examples. ACKNOWLEDGMENTS
The authors thank the colleagues, Mr. Tadeusz Zdziarski and Andrzej Igielski, M.Sc. Eng., for their help in engineering the AutoEXH device. 1 UNSCEAR,
United Nation Scientific Committee on the Effects of Atomic Radiation: Sources, Effects and Risks of Ionizing Radiation, Report to the General Assembly (United Nations, New York, 2000). 2 S. D. Schery, D. H. Gaeddert, and M. H. Wilkening, “Factors affecting exhalation of radon from a gravelly sandy loam,” J. Geophys. Res. 89, 7299, doi:10.1029/JD089iD05p07299 (1984). 3 A. Sroor, S. M. El-Bahi, F. Ahmed, and A. S. Abdel-Haleem, “Natural radioactivity and radon exhalation rate of soil in southern Egypt,” Appl. Radiat. Isot. 55, 873–879 (2001). 4 A. F. Saad, R. M. Abdallah, and N. A. Hussein, “Radon exhalation from Libyan soil samples measured with the SSNTD technique,” Appl. Radiat. Isot. 72, 163–168 (2013). 5 H. Singh, J. Singh, S. Singh, and B. S. Bajwa, “Radon exhalation rate and uranium estimation study of some soil and rock samples from Tusham ring complex, India using SSNTD technique,” Radiat. Meas. 43, S459–S462 (2008). 6 M. Faheem and M. Matiullah, “Radon exhalation and its dependence on moisture content from samples of soil and building materials,” Radiat. Meas. 43, 1458–1462 (2008). 7 S. Rehman Matiullah, S. Rehman, and S. Rahman, “Studying 222 Rn exhalation rate from soil and sand samples using CR-39 detector,” Radiat. Meas. 41, 708–713 (2006). 8 R. Shweikani and M. Hushari, “The correlations between radon in soil gas and its exhalation and concentration in air in the southern part of Syria,” Radiat. Meas. 40, 699–703 (2005). 9 C. Dueñas, M. C. Fernández, J. Carretero, E. Liger, and M. Pérez, “Release of 222 Rn from some soils,” Ann. Geophys. 15, 124 (1997). 10 D. E. Liger, S. Canete, M. Perez, and J. P. Bolivar, “Exhalation of 222 Rn from phosphogypsum piles located at the southwest of Spain,” J. Environ. Radioact. 95, 63 (2007). 11 S. Oberstedt and H. Vanmarcke, “A radon exhalation monitor,” Radiat. Prot. Dosim. 63, 69 (1996). 12 T. Iimoto, Y. Akasaka, Y. Koike, and T. Kosako, “Development of a technique for the measurement of the radon exhalation rate using an activated charcoal collector,” J. Environ. Radioact. 99, 587–595 (2008). 13 C. Ferry, A. Beneito, P. Richon, and M.-C. Robe, “An automatic device for measuring the effect of meteorological factors on Rn-222 flux from soils in the long term,” Radiat. Prot. Dosim. 93, 271 (2001). 14 M. Hosoda, T. Ishikawa, A. Sorimachi, and S. Tokonami, “Development and application of a continuous measurement system for radon exhalation rate,” Rev. Sci. Instrum. 82, 015101 (2011). 15 Z. S. Zunic, I. Kobal, J. Vaupotic, K. Kozak, J. Mazur, A. Birovljev, M. Janik, I. Celokovic, P. Ujic, A. Demajo, G. Krstic, B. Jakupi, M. Quarto, and F. Bochicchio, “High natural radiation exposure in radon spa areas: A detailed field investigation in Niska Banja (Balkan region),” J. Environ. Radioact. 89, 249 (2006). 16 ThetaProbe Soil Moisture Sensor type ML2x, User Manual, ML2x-IU1.21, Delta-T Devices Ltd., 1999. 17 N. L. Anderson, R. C. Hinds, J. A. Baker, and G. B. Rupert, “Mapping of complex bedrock structure using the high-resolution reflection seismic technique,” Comput. Geosci. 23(10), 1101 (1997). 18 H. Kojima and K. Nagano, “Simulation of radon exhalation from the ground,” private communication.
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 022104 (2014)
Complementary system for long term measurements of radon exhalation rate from soil J. Mazur and K. Kozaka) Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Kraków, Poland
(Received 31 July 2013; accepted 10 October 2013; published online 18 February 2014) A special set-up for continuous measurements of radon exhalation rate from soil is presented. It was constructed at Laboratory of Radiometric Expertise, Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Krakow, Poland. Radon exhalation rate was determined using the AlphaGUARD PQ2000 PRO (Genitron) radon monitor together with a special accumulation container which was put on the soil surface during the measurement. A special automatic device was built and used to raise and lower back onto the ground the accumulation container. The time of raising and putting down the container was controlled by an electronic timer. This set-up made it possible to perform 4–6 automatic measurements a day. Besides, some additional soil and meteorological parameters were continuously monitored. In this way, the diurnal and seasonal variability of radon exhalation rate from soil can be studied as well as its dependence on soil properties and meteorological conditions. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865156] I. INTRODUCTION
Radon (222 Rn) gas and its short-lived decay products as natural radioactive isotopes widely occurring in nature are a major factor responsible for the dose of ionising radiation that people receive. Therefore, radon has been in scope of interest for many years – both in terms of science (its transport in the ground, exhalation from soil, migration into buildings, etc.) and in terms of dosimetry (its impact on the human health). The radiation dose to human population due to inhalation of radon and its progeny contribute more than 50% of the dose from all sources of radiation both naturally occurring and man made.1 (UNSCEAR, 2000). Radon exhalation is the process of its escaping from the ground or other materials (e.g., building materials) into the atmosphere. It is defined as the radon activity released from an area unit during a time unit. This process is interesting and complex, depending on many factors, mainly those which influence radon emanation coefficient and radon transport, i.e., radium (226 Ra) concentration in soil or rock grains, soil properties (porosity, permeability, humidity, temperature), and meteorological conditions (air pressure, air temperature, wind speed, precipitation).2 These relationships are not yet completely explained, and have been still studied. The soil properties influence directly the dynamics of radon exhalation. Meteorological parameters are factors which indirectly affect the process of radon exhalation from soil. The investigation of radon exhalation rate from soil using different techniques was presented by several researchers. Many papers describe the “laboratory techniques” for determination of mass radon exhalation rate from soil samples and/or from building materials samples.3–7 The most common methods for radon registration are solid state nuclear track detectors (SSNTD technique), activated charcoal canisters, Lucas cells (scintillation counters), and the active technique with a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(2)/022104/7/$30.00
the ionisation chamber. The in situ measurements of radon exhalation from soil are more interesting in terms of its dependencies on soil properties and meteorological parameters. Some authors described this kind of measurements but the presented results regarded only a single or short term measurements of radon exhalation rate from soil.8–12 The study of relationship between radon exhalation rate from soil and meteorological conditions has been previously carried out by Ferry et al.13 and Hosoda et al.14 The authors used an automatic devices for radon exhalation measurements but the duration of measurements did not exceed one month time. At the Institute of Nuclear Physics PAN (Krakow, Poland) a special device, called AutoEXH, was constructed for long term, continuous survey of radon exhalation rate from soil. In order to limit the influencing factors to meteorological conditions the device was set up at one specific place (at the Institute’s area) where the soil properties (radium content, density, porosity) were determined and stayed the same for the whole period of measurements. The facility made it possible to perform 4 measurements per day, i.e., the new measurement started every 6 h. The measurement campaign lasted one year time. The aim of this research was to broaden the knowledge on radon exhalation process and to study of radon exhalation dynamics during long period of time in connection with meteorological conditions. In the paper, the construction of AutoEXH is presented as well as some results obtained using the facility.
II. MATERIALS AND METHODS
At the Institute of Nuclear Physics PAN (Krakow, Poland) a special site (RSF – Radon Study Field) was created for long-term measurements of radon exhalation rate. The RSF was situated in the open air at the distance of more than 50 m from other buildings. At the RSF there was also a set of sensors for monitoring of meteorological conditions
85, 022104-1
© 2014 AIP Publishing LLC
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TABLE I. The characteristics of soil at the Radon Study Field. Density 2.631 ± 0.002 (g cm−3 ) Permeability 5.22 × 10−12 ÷ 8.88× 10−12 (m2 ) Temperature 2,4 ◦ C ÷ 19 ◦ C Relative humidity 10% ÷ 16% Concentration of natural radioactive isotopes in soil 40 K 340 ± 20 (Bq kg−1 ) 226 Ra 22 ± 2 (Bq kg−1 ) 232 Th 31 ± 3 (Bq kg−1 ) 222 Rn 20 ÷ 56 (kBq m−3 )
Alpha Pump AlphaGUARD PQ2000 PRO
Filter
ExhBox
(Weather Monitor, DAVIS). The characteristics of the RSF are presented in Table I. Soil density was determined with multipycnometer (Quantachrome Corporation). Soil permeability and Rn-222 concentration in soil gas at the depth of 1 m were determined using the methods described in Zunic et al.15 At the same depth soil temperature and relative humidity were measured. A thermocouple was used for soil temperature and Theta Probe ML2x sensor was applied for soil humidity.16 Radon in soil gas was measured systematically from September 2003 to April 2006 once or twice a month. The concentrations of gamma radio-nuclides in soil (40 K, 226 Ra, 232 Th) have been determined by low background gamma spectrometry with three-window method. The geological identification of subsurface bedrock structure of the RSF area was made using a shallow acoustic seismic logging method.17 The interpretation of the obtained seismograms showed that the area of RSF is uniform and no tectonic faults occur there. The layer of soil has a thickness of 5–8 m, beneath there is a layer of loess with a thickness of about 10 m. At the depth of 15–70 m, there are Miocene clays that are in turn located over a layer of Jurassic limestone. The idea of radon exhalation measurement was based on a known “accumulation method” (Fig. 1). A container ExhBox was placed on the ground and the increase of radon concentration inside the container was registered by the ionisation chamber AlphaGUARD. Radon concentration was registered in 10-min intervals, the AlphaGUARD was working in a flow mode with the flow rate of 0,3 dm3 min−1 forced by the AlphaPUMP. The small flow rate was chosen in order to keep the natural conditions and not to force the additional radon release from soil surface. The radon exhalation rate E was then calculated according to the formula (1), E=
V · B, S
(1)
where V – volume of the system (m3 ); S – area covered by the container (m2 ); B – slope of the line CRn = f(t) fitted to experimental points; CRn – radon concentration (Bq m−3 ); t – time (min). In this way, single measurements have been made, first 2–3 times a week, then the frequency has been increased to 1 measurement a day. It appeared, however, that they were insufficient to search for correlations between radon exhalation and meteorological conditions.
SOIL FIG. 1. Idea of radon exhalation measurement.
The next step was the construction of the automatic facility AutoEXH that replaced the container ExhBox and enabled to carry out the measurements several times a day. The schematic view of AutoEXH is presented in Fig. 2. The facility consisted of the following main components: (a) fixing base plate (dimensions 590 mm × 280 mm) with ground spikes (70 mm), (b) power supply system, (c) automatic electronic control unit, (d) mechanism for raising and lowering the ExhBox chamber, (e) ExhBox chamber (diameter of 215 mm, height of 145 mm) with valves. It is essential to know exactly the surface area under the ExhBox to calculate precisely the area covered by the ExhBox container during each measurement. The fixing base plate had a hole (diameter of 216 mm) ended with a flange that was pressed into the ground to a depth of 60 mm. It was also crucial to ensure the tightness of the system. There was also a pressure valve in the fixing base plate which was used for the determination of pressure difference. The measurements showed that the difference between atmospheric air pressure and air pressure under the ExhBox did not exceed 2 Pa. The height of the whole device was 720 mm. The work cycle of AutoEXH consisted of 3 stages:
r ExhBox was lowered and put on the ground. r Radon exhalation rate measurement, ExhBox stayed down.
r ExhBox was lifted up to 380 mm above the ground, the system was ventilated. The pump was working during all the cycle which made it possible to ventilate both ionisation chamber of AlphaGUARD and ExhBox when it was up. After the ventilation period the ExhBox was lowered again and the next measurement cycle started. After carrying out test measurements the following cycle was established: 1,5-h measurement and 4,5-h ventilation. It meant only four radon exhalation measurements a day but this ensured good
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Rev. Sci. Instrum. 85, 022104 (2014)
FIG. 2. Schematic and real view of AutoEXH.
ventilation of the set-up and retaining environmental conditions as natural as possible. The example of the results obtained using AutoEXH is shown in Fig. 3. The values of radon exhalation rate were calculated for every increasing course of the curve. III. RESULTS AND DISCUSSION
The total number of 1346 radon exhalation values were gathered using AutoEXH during one year time. Measurements were made in changing meteorological conditions and changing soil temperature: air temperature: from –24 ◦ C to 32 ◦ C air pressure: from 950 hPa to 1023 hPa
soil temperature: from 2,4 ◦ C to 19 ◦ C. The measured values of radon exhalation rate ranged from 0 to 386 mBq m−2 s−1 . The values 0 mBq m−2 s−1 mean that it was impossible to register increase of Rn concentration inside ExhBox, this occurred in winter time when surface soil layer was frozen. The obtained results served for study of annual and diurnal changeability of radon exhalation rate, dependence of exhalation rate on radon concentration in soil, and dependence of exhalation rate on chosen meteorological parameters. The relationship between E and radon in soil gas is shown in Fig. 4. The results are grouped in two distinct populations. Within one of them the simple proportionality can be observed – the higher the concentration of radon in the soil
FIG. 3. Example of radon concentration registered by AlphaGUARD inside the ExhBox using AutoEXH.
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J. Mazur and K. Kozak
Rev. Sci. Instrum. 85, 022104 (2014)
160 140 120 100 80 60 40 20 0 20
25
30
35
40
45
222
50
55
3
Rn concentration in soil [kBq/m ]
FIG. 4. Dependence of radon exhalation rate on radon in soil concentration at 1 m depth.
the higher values of E. The second population (more numerous) does not show this type of dependence. This suggests that within this population radon exhalation rate is much more influenced by meteorological conditions than by radon in soil concentration at 1 m depth. Moreover, the values of E are lower than in first population (from 0 to ca. 20 mBq m−2 s−1 ). These values occur during winter seasons or during more intense rainfall, i.e., situations when the influence of meteorological conditions on E is significant.
A. Diurnal variability of radon exhalation rate from soil
The application of AutoEXH device made it possible to observe not only seasonal but also the diurnal changeability of radon exhalation rate from soil. It was found that there are periods of time when radon exhalation rate E shows regular changes according to changes of air temperature (Fig. 5(a)). 70
The meteorological conditions, i.e., air temperature, wind speed, precipitation, influence directly the soil humidity, and temperature, which in term influence the rate of radon exhalation. With the AutoEXH device it was possible to
25
25 18
radon exhalation rate E air temperature
-1
20 40
30 15 20
10
0 0
1 July 2006 00 00
30
60
90
HOURS
120
10 150
3 July 2006 30 23
o
50
Air temperature [ C]
-2
-1 -2
B. Relationships between radon exhalation rate and meteorological conditions
Rn exhalation rate E air temperature
60
Rn exhalaton rae E [mBq m s ]
However, often no such kind of regularity was observed (Fig. 5(b)). In the periods of time when the regular changes of air temperature occur, the corresponding periodic variability of soil temperature is also observed (with some delay). The decrease of air temperature with simultaneously increasing of soil temperature is responsible for regular changes of radon exhalation rate at those periods of time. This indicates also the significant contribution of convection in radon transport in soil. For further analysis of diurnal changeability of radon exhalation the results have been divided into groups corresponding with the particular hours of the measurement: 5:00, 11:00, 17:00, and 23:00 during three seasons of the year: winter, spring, and summer (Fig. 6). The autumn season is not included in the figure since the AutoEXH was under tests and the measurements were made in different hours of the day. The smallest diurnal variability can be observed in the winter season regardless of the hour of the measurement, whereas during spring and summer seasons the variability is more significant. Radon exhalation rate shows the lowest seasonal discrepancy at 5:00 and at 23:00. The highest values of E were found at 11:00 and at 17:00 during summer and spring seasons. This may be due to the bigger impact of meteorological conditions on radon exhalation rate than in winter season. The statistical analysis of mean values of E, measured at particular hours of the day during winter and summer seasons, showed that they do not differ in a statistically significant way. During spring season the statistically significant differences in E mean values were observed for 5:00 and 17:00 h.
Rn exhalation rate E [mBq m s ]
222
2
Rn exhalation rate [mBq/m s]
180
20
16 14
15 12 10
10
Air temperature [oC]
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8 5 6 0
4 0
50
1 Oct. 2005 00 00
100
150
HOURS
200
250
6 Oct. 2005 00 5
(b)
(a) FIG. 5. (a) and (b) Diurnal variability of radon exhalation rate.
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Rev. Sci. Instrum. 85, 022104 (2014)
FIG. 6. Radon exhalation rate from soil at the same hours of the day during three seasons of the year.
TABLE II. Correlation coefficients WK for E and chosen meteorological parameters in different seasons of the year.
E Autumn 2005 E Winter 2006 E Spring 2006 E Summer 2006
T
V_wind
P
Rain
T_soil
Rain_24
− 0.33 − 0.14 0.52 0.08
0.00 0.17 0.25 − 0.02
− 0.16 0.26 0.32 0.06
0.03 − 0.05 0.05 − 0.03
− 0.40 0.31 0.69 − 0.01
0.01 − 0.03 − 0.03 − 0.21
24
-1
The values of correlation coefficients WK (at 95% of confidence interval) for radon exhalation rate E and the above described meteorological parameters are presented in Table II. The highest values of WK were found for E and T (0.52) as well as for E and T_soil (0.69) in spring season. In summer, there is no significant correlation between E and air/soil temperature. The weakest correlations during all seasons were observed for E and wind speed and for E and precipitation. In summer 2006 the weak, negative correlation was found between E and Rain_24 (−0.21). The more detailed analysis of relationship between E and T has been made. The values of air temperature were grouped
22
-2
T, V_wind, P, Rain – the values of: air temperature, wind speed, air pressure, and precipitation registered 0.5 h before the end of the E measurement. T_soil – soil temperature at 1 m depth. Rain_24 – sum of precipitation during 24 h before the E measurement was ended.
into 8 intervals covering the range from −10 ◦ C to 32 ◦ C, i.e., temperatures in all seasons of the year. The width of each interval was 5 ◦ C. For each interval the mean value of air temperature Ti and the mean value of E were calculated. The relationship E = f(Ti ) is shown in Fig. 7. It can be seen that within the range from −5 ◦ C to 25 ◦ C the linear increase of E with increasing Ti was observed. The linear fit within that range is described by the equation E = 10,63 (±0,72) + 0,44 (±0,06) · Ti (with R = 0,97). There direct correlation between E and P was not observed. This result is similar to that obtained by Kojima and Nagano,18 although their study was much shorter, i.e., about three months. Thus, further analysis consisted in dividing the whole range of registered air pressure values into 5 hPa
Rn exhalation rate E [mBq m s ]
search for correlations between E and the single meteorological parameter. The registration of meteorological condition was made every 30 min, whereas E was measured every 6 h. For analysis, the following values of meteorological parameters were attributed to each value of E:
20 18 16 14 12 10 8 -10
-5
0
5
10
15
20
25
30
o
Air teperatureTi [ C]
FIG. 7. Relationship between mean values of E and mean values of Ti .
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Rev. Sci. Instrum. 85, 022104 (2014)
28 o
26
T 15 C
-1
22
-2
Rn exhalation rate E [mBq m s ]
24 20 18 16 14 12 10 8 6 4 2 0 < 985
985-990
990-995
995-1000
> 1000
Air pressure P [hPa] FIG. 8. Mean values of E for three ranges of air temperature in five ranges of air pressure. FIG. 9. Variability of radon exhalation rate as a function of soil temperature and precipitation.
intervals. Then, for each P interval three mean values of E were calculated which corresponded with three ranges of air temperature T: 15 ◦ C. The obtained results are shown in Fig. 8. The influence of air pressure on radon exhalation rate is different for different air temperature ranges. It can be seen that E is not influenced by air pressure when the air temperature ranges from 0 ◦ C to 15 ◦ C. The greatest influence of P is observed when the air temperature is higher than 15 ◦ C. It is worth mentioning that some authors reported the direct, significant correlation between E and P,10 however this was not confirmed in our study. The reason for this may be much longer time of our measurements. Basing on the presented results and on other authors’ research, it can be seen that within short periods of time (e.g., a few days/weeks) the significant correlations between E and
a single meteorological parameter can be found. After our long-term measurements (one year, 4 measurements a day) it appeared that the process of radon exhalation from soil is very complex and it depends on several factors, and even on the combination of these factors. Thus, an attempt was undertaken to examine the dynamics of this process using multiparametric analysis. This analysis made it possible to show the complex, nonlinear dependencies between variables. As an example, Fig. 9 presents the radon exhalation rate E as a function of two parameters: soil temperature T_soil and sum of precipitation during 24 h before the E measurement was ended Rain_24 basing on the results from the whole year of measurements. The same analysis has been made for each season.
FIG. 10. Variability of radon exhalation rate as a function of air pressure and precipitation with soil temperature as a categorization factor.
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J. Mazur and K. Kozak
The highest value of E can be observed for the high soil temperature (E gradient is in agreement with T_soil gradient) with no significant dependency on the precipitation. The similar tendency was found for spring season, whereas the “autumn results” were completely different. In this season, the highest values of E were observed for low soil temperature and low precipitation. The development of the multi-parametric analysis consisted in application of the categorization, i.e., introducing the third variable which divided the data set into groups (categories) corresponding with the ranges of this third variable. Fig. 10 presents the radon exhalation rate E as a function of two parameters: air pressure P and sum of precipitation during 24 h before the E measurement was ended Rain_24 basing on the results from the whole year of measurements. The categorization factor is soil temperature T_soil with two ranges: below 10 ◦ C and over 10 ◦ C. The significantly increasing values of E can be observed for increasing precipitation and decreasing air pressure when soil temperature is lower than 10 ◦ C. This tendency was not found when the simple relationships E = f(P) and E = F(Rain_24) were studied. For soil temperature higher than 10 ◦ C, the E dependence on P and Rain_24 is completely different (Fig. 10).
IV. CONCLUSIONS
The construction of the special set-up AutoEXH for continuous measurements of radon exhalation rate from soil was presented. The device was controlled automatically, so the measurements were made without the necessity of man presence. The facility made it possible to perform several measurements a day. Thus, the study of dependencies of radon exhalation rate on meteorological conditions was undertaken. The obtained results confirmed the complexity of radon exhalation process and its dependence on many factors, including combination of meteorological parameters. The dependencies of E on soil humidity and temperature are more complicated than described by the theory of diffusive radon flux from soil. The lack of simple relationships between E and a single meteorological parameter was observed in long-term measurements. The linear dependence of E on air temperature was found in the range from −5 ◦ C to 25 ◦ C. This shows that the contribution of convection in radon transport is significant. The presented system is promising for further investigation and for better understanding of radon exhalation process in connection with meteorological conditions. The next step can be, e.g., study of radon exhalation rate from soil with relation to soil heat flux.
Rev. Sci. Instrum. 85, 022104 (2014)
The multi-parametric analysis is a good way to study relationship between E and meteorological conditions as shown in the examples. ACKNOWLEDGMENTS
The authors thank the colleagues, Mr. Tadeusz Zdziarski and Andrzej Igielski, M.Sc. Eng., for their help in engineering the AutoEXH device. 1 UNSCEAR,
United Nation Scientific Committee on the Effects of Atomic Radiation: Sources, Effects and Risks of Ionizing Radiation, Report to the General Assembly (United Nations, New York, 2000). 2 S. D. Schery, D. H. Gaeddert, and M. H. Wilkening, “Factors affecting exhalation of radon from a gravelly sandy loam,” J. Geophys. Res. 89, 7299, doi:10.1029/JD089iD05p07299 (1984). 3 A. Sroor, S. M. El-Bahi, F. Ahmed, and A. S. Abdel-Haleem, “Natural radioactivity and radon exhalation rate of soil in southern Egypt,” Appl. Radiat. Isot. 55, 873–879 (2001). 4 A. F. Saad, R. M. Abdallah, and N. A. Hussein, “Radon exhalation from Libyan soil samples measured with the SSNTD technique,” Appl. Radiat. Isot. 72, 163–168 (2013). 5 H. Singh, J. Singh, S. Singh, and B. S. Bajwa, “Radon exhalation rate and uranium estimation study of some soil and rock samples from Tusham ring complex, India using SSNTD technique,” Radiat. Meas. 43, S459–S462 (2008). 6 M. Faheem and M. Matiullah, “Radon exhalation and its dependence on moisture content from samples of soil and building materials,” Radiat. Meas. 43, 1458–1462 (2008). 7 S. Rehman Matiullah, S. Rehman, and S. Rahman, “Studying 222 Rn exhalation rate from soil and sand samples using CR-39 detector,” Radiat. Meas. 41, 708–713 (2006). 8 R. Shweikani and M. Hushari, “The correlations between radon in soil gas and its exhalation and concentration in air in the southern part of Syria,” Radiat. Meas. 40, 699–703 (2005). 9 C. Dueñas, M. C. Fernández, J. Carretero, E. Liger, and M. Pérez, “Release of 222 Rn from some soils,” Ann. Geophys. 15, 124 (1997). 10 D. E. Liger, S. Canete, M. Perez, and J. P. Bolivar, “Exhalation of 222 Rn from phosphogypsum piles located at the southwest of Spain,” J. Environ. Radioact. 95, 63 (2007). 11 S. Oberstedt and H. Vanmarcke, “A radon exhalation monitor,” Radiat. Prot. Dosim. 63, 69 (1996). 12 T. Iimoto, Y. Akasaka, Y. Koike, and T. Kosako, “Development of a technique for the measurement of the radon exhalation rate using an activated charcoal collector,” J. Environ. Radioact. 99, 587–595 (2008). 13 C. Ferry, A. Beneito, P. Richon, and M.-C. Robe, “An automatic device for measuring the effect of meteorological factors on Rn-222 flux from soils in the long term,” Radiat. Prot. Dosim. 93, 271 (2001). 14 M. Hosoda, T. Ishikawa, A. Sorimachi, and S. Tokonami, “Development and application of a continuous measurement system for radon exhalation rate,” Rev. Sci. Instrum. 82, 015101 (2011). 15 Z. S. Zunic, I. Kobal, J. Vaupotic, K. Kozak, J. Mazur, A. Birovljev, M. Janik, I. Celokovic, P. Ujic, A. Demajo, G. Krstic, B. Jakupi, M. Quarto, and F. Bochicchio, “High natural radiation exposure in radon spa areas: A detailed field investigation in Niska Banja (Balkan region),” J. Environ. Radioact. 89, 249 (2006). 16 ThetaProbe Soil Moisture Sensor type ML2x, User Manual, ML2x-IU1.21, Delta-T Devices Ltd., 1999. 17 N. L. Anderson, R. C. Hinds, J. A. Baker, and G. B. Rupert, “Mapping of complex bedrock structure using the high-resolution reflection seismic technique,” Comput. Geosci. 23(10), 1101 (1997). 18 H. Kojima and K. Nagano, “Simulation of radon exhalation from the ground,” private communication.
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