Radiation Protection Dosimetry (2014), Vol. 160, No. 1–3, pp. 57 –61 Advance Access publication 17 April 2014

doi:10.1093/rpd/ncu091

DIURNAL AND SEASONALVARIABILITY OF OUTDOOR RADON CONCENTRATION IN THE AREA OF THE NRPI PRAGUE K. Jilek*, M. Sleza´kova and J. Thomas National Radiation Protection Institute, Bartosˇkova 28, 140 00 Prague, Czech Republic *Corresponding author: [email protected]

INTRODUCTION The lack of for similar equipment that would facilitate continuous measurement of atmospheric radon (monitoring the surroundings of the old burdens caused by uranium mining, the use of radon as atmospheric tracers or a precursor of increased seismic activity) led to the decision to build a measuring automatic station at the National Radiation Protection Institute (NRPI) in the fall of 2010. The station was equipped with a facility allowing continuous measurement of atmospheric volume activity of radon (av), gamma dose rate (Dg) and relevant meteorological parameters and enables better interpretation of investigated variability in measured radon gas and gamma dose rates or exceeding of the action levels of Dg. The station itself was designed to be low energy consuming and independent of the external power network through their own battery power, supplemented by solar panels. The HW and SW station concept allows, via GPRS, wireless data transfer to a user’s PC, remote control and setting up of the station’s key hardware modules from a user’s PC, and finally driving and reading the network of such stations via a user’s PC. After introduction of the station, its capabilities are presented by means of the annual results of processing of continuous measurements of atmospheric radon gas conducted every 30 minutes in the NRPI area. All calculations and graphs were processed in the R programming environment (1). MATERIALS AND METHODS Description of the monitoring station The station principally comprises the following five blocks placed in an opaque, water-resistant crate.

(1) Central managing and logging unit Aures with a built-in transmission modem. (2) Gas filling system consists of an air sampling manifold, pump, flow meter, valves, a tank with compressed background gas and radon gas sampling head. (3) Measuring sensors and detectors: (a) Meteorological sensors for measuring atmospheric temperature at different heights, relative humidity, absolute pressure, and wind speed and direction; rain gauge; and probe for recording the intensity of solar radiation. (b) Radiometer GMS 3D with GM tubes measuring the equivalent dose rate Dg. (c) High-volume scintillation cell (ZnS) in a volume of 3 l for measuring atmospheric radon gas with evaluation unit JKA-400 equipped with a multichannel analyser. (4) Power supply includes battery power sources, complemented by a trio of solar panels V. (5) A user PC with a modem to receive logged data from the unit Aures and proper SW allowing both the setting up of HW detection modules (code WinCentralTM ) and visualisation and computation of received data (code VisualisTM ). All measured data including those allowing checking the proper operation of all gauges are recorded every minute to the central unit Aures. From the unit they are transmitted on-line wirelessly using the GPRS protocol to a user’s PC at the set interval of a minimum of 1 min for recording and transmission. The setting up of both the managing unit Aures in terms of the frequency and manner of its communication with the connected sensors and the required parameters of the Aures data transfer to the PC is done either manually or remotely from a user’s PC using the

# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]

Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on June 6, 2015

In autumn 2010, an outdoor measuring station for measurement of atmospheric radon, gamma equivalent dose rate in the range of 100 nSv h21 21 Sv h21 and proper meteorological parameters such as thermal air gradient, relative air humidity, wind speed and direction and solar radiation intensity was built in the area of the National Radiation Protection Institute vvi. The station was designed to be independent of an electrical network and enables on-line wireless transfer of all data. After introduction of the station, illustrations of its measurement properties and the results of measured diurnal and seasonal variability of atmospheric radon, based on annual continuous measurement using a high-volume scintillation cell at a height of 2.5 m above the ground, are presented.

K. JILEK ET AL.

measurement of wind speed and and temperature gradient and sensing wind direction, done at a height of 5 m above the ground. Typical measurement accuracy of all the weather variables ranged between about +10 %. (2) Gamma equivalent dose rate Dg The measurement of Dg is performed with a radiometer 3D GMS-based on GM tubes, designed for outdoor use and located at a height of 2.5 m above the ground. The measuring range of dose rates is between 100 nGy h21 and1 Gy h21, with the manufacturer-declared measurement error of up to 15 % within the full range. (3) Atmospheric radon gas Radon gas measurements are performed continuously at a sampling height of 2.5 m above the ground using a high-volume scintillation cell in a volume of 3 l connected to the evaluation unit JKA-400. The station HW and SW equipment allows measurement of radon gas in these two modes that can be easily set up either remotely from the laboratory via a user’s PC or directly in the unit: (a)

Continuous mode: Optionally one can adjust the flow rate through the used scintillation cell up to 10 l min21 and sampling time in the range of 1– 99 999 s. For continuous determination of radon gas a deconvolution algorithm(2) was adopted. (b) Semi-continuous mode: Radon gas concentration is calculated within each step of used the well- known fixed recurring measuring algorithm as follows: (1) Cell’s background measurement-sampling of outdoor air-delay to reach steady state between radon daughter products and radon gas in the cell-measurement of the sample – calculation of radon concentration. (2) Cleaning of the cell with technical gas free of radon – delay for decay of radon daughter products - background measurement- sampling of outdoor air- etc. All the time intervals can be set up optionally in the range of 1–99 999 s. Thus, considering the background of the used scintillation cell and its detection efficiency, MDA below 5 Bq m23 can be measured for a sampling time of active air 2000 s within a single measurement. Overall uncertainty below 15 % (K ¼ 1) for a used continuous mode and measuring interval of 2000 s can be expected for measured radon gas concentration of about 10 Bq m23.

Measuring parameters and sampling points The station is located at the NRPI area at an altitude of 220 m above sea level. The whole area is surrounded by buildings with more than two storeys; it is in the shape of a pentagon and is covered 90 % by an asphalt surface. The whole station is in fact located amidst taller buildings and, unfortunately, its highest point for measuring wind speed and direction does not exceed the elevation level. (1) Meteorological data The station can measure and record outputs of all measurement sensors in fixed arbitrarily selected time intervals of 1, 10, 30 or 60 min. Sampling sites were chosen for each sensor at a height of 2.5 m above the ground, except for

(4) Operation conditions The station has successfully passed the annual outdoor operation in the area of NRPI, where hourly averages of temperature and relative humidity varied in the range of approximately (220)– 58

Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on June 6, 2015

code WinCentral. This code also allows a user’s PC to process data transferred from the Aures by database way and set up and change important parameters of the key HW components used in the station such as measuring time, HV values on the used photomultiplier, width of the ROI, etc. using text commands only. The code WinCentral was designed to allow reading data simultaneously from the multiple units of Aures. That is why it is easy to create a network of monitoring stations based on one central user’s PC. Visualisation, statistical processing, creating user reports and export of stored data from the Aures are done in the user’s PC code Visualis. The gas filling system consists of a compressed tank with cancel technical air, sampling head for atmospheric radon gas, pump, controlled flow meter and a pair of bistable valves. The compressed technical air enables measurement of a real background of used scintillation cells which is necessary to know for radon gas calculations. The key role of that filling system is to assure samplings of atmospheric air with radon into the high-volume scintillation cell and allow determining its own background. The whole filling system is under control of the evaluation unit JKA-400 either manually or using text commands remotely by means of the code WinCentral. The evaluation unit JKA400 enables both setting up of a measuring regime (continuous or semi-continuous) using a scintillation cell and settings and control of the gas filling system. A built-in multichannel analyser in the unit JKA-400 allows connection to and application on any other spectrometric alpha/gamma detector. The unit can be set up and read manually or remotely from the user’s PC. Condensation of air moisture inside the scintillation cell is eliminated. The power supply block consists of four pieces of batteries with a capacity of 100 Ah each for photovoltaic charging. The batteries are deployed inside the cabinet and drop in their voltage during day is reduced by means of solar panels with a smart battery charging regulator.

DIURNAL AND SEASONAL VARIABILITY Table 1. Calculated deconvolution coefficient fk. k

1

2

3

4

5

6

7

6.2 1.7 1.4 0.73 0.33 0.14 0.02 fk 78 48 14 2.8 0.5 0.08 ( fk/f1)2`  1023 26 fk in (imp/(Bq m23)).

Figure 1. Diurnal and monthly averages of measured atmospheric radon.

Continuous monitoring of radon gas For practical reasons, the continuous measuring regime is chosen which enables one to obtain results in high frequency compared with separate measuring. For continuous evaluating of the volume activity of radon an by the scintillation cell the algorithm of Ward and Borak(2) is used adapted to the conditions here. In principle, the number of pulses Nn from the chamber (after subtracting the background pulses) is substituted into a recurrent discrete deconvolution (type D1) given by the following relation with a series of deconvolution coefficients fn acting as the response function: an ¼ ðNn  an1 f2  an2 f3    anþkþ1 fk Þ=f1

Table 2. Monthly averages of atmospheric radon and selected parameters.

ð1Þ

The series of coefficients fn was determined by a calibration experiment according to ref. (2). The determined values of fn are given in Table 1; the given confidence intervals for the volume activities an are determined using the law of propagation of uncertainties of relation (1). For the relevant values of the variance s2an , the following relation is given:

s2an ¼ ð1=f12 Þ2 Nn þ ð f2 =f12 Þ2 Nn1 þ    þ ð fn =f12 Þ2 N1

ð2Þ

Month

av, Bq m23

1 2 3 4 5 6 7 8 9 10 11 12 Annual average

11.6 13.9 14.7 13.7 15.3 14.9 15.2 17.2 17.3 19.2 26.8 13.1 16.1

Dg, nSv T8C RH, Wind, Pressure, h21 % m s21 mbar 169 167 167 171 169 169 168 169 169 169 171 171 169

2.9 2.2 8.6 11.2 17.9 19.9 21.4 22.1 18.0 10.4 4.4 4.7 11.6

73 69 62 59 55 62 65 58 69 76 83 77 67

1.4 1.2 1.0 1.1 1.1 1.1 1.1 1.0 0.8 0.9. 0.8 1.4 1.1

973 985 990 976 985 984 985 986 986 985 979 967 982

temperature, relative humidity, wind speed and barometric pressure. From these data can be seen that the lowest value of av was registered in autumn months, in October 19 Bq m23 and the highest in November,27 Bq m23. For these months frequent inversions were typical in connection with a stable atmosphere with a low vertical mixing of the air and measured positive temperature potential. Therefore, higher radon concentrations are at ground level. The wind speed, which affects the stability of the atmosphere was very low at this time, 0.8–0.9 m s21 on average. In contrast, the relative humidity was the highest, 80 %, due to higher rainfall. The winter months show lowest average values of av, 13 Bq m23, lowest temperatures and highest wind speeds up to 1.4 m s21. The monthly average of the dose rate was not changing significantly with the average of 170 nGy h21. The average daily cycle of the radon concentration and selected meteorological parameters are shown in Figure 2.

Calibration of a used scintillation cell was performed via the NRPI radon facility published elsewhere(3). The combined relative uncertainty of the realised volume activity av in the scintillation cell was better than 5 % (K ¼ 1). RESULTS AND DISCUSSION The time course of daily volume activities of radon from September 2011 up to August 2012 is shown in Figure 1. It can be seen that mean values of av are around 16 Bq m23 with a standard deviation of 7 Bq m23. In the fall of 2011, a significant higher values were measured compared with values during the year and values up to 50 Bq m23 can be seen. In Table 2 are given monthly averages of av, of the dose rate Dg, of meteorological parameters such as the 59

Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on June 6, 2015

(þ45) 8C and 10–95 %, respectively. Typical current consumption of the entire station moved up to 300 mA.

K. JILEK ET AL.

August 2011 was 16 Bq m23, which is comparable with the values of av 5–25 Bq m23 given by WHO. The highest values were registered in autumn due to frequent temperature inversions, and lowest values were measured during winter due to the highest wind speeds and a lower exhalation rate of radon from the ground due to layers of snow. The average daily cycles show maximums around 6–8 h and minimums in the afternoon at 16 h. The daily variations were related with the vertical temperature gradient depending on the intensity of sunshine, atmospheric stability and wind speed.

Among the obtained results a positive correlation between av and relative humidity can be seen and a negative correlation between av and temperature and wind speed. In Figure 2, one also see the sinusoidal course of av with maximums between 6 and 8 h and a minimum in the afternoon, around 16 h. These changes are affected by the periodic changes of the positive vertical temperature gradient at night and in the afternoon with higher radon concentration at ground level and the changes during forenoon followed by an increasing vertical mixing of the atmosphere with a minimum of radon at noon(4 – 8). These variations were more pronounced during spring, when also the daily cycles of the temperature, the relative humidity and of the wind speed had the highest amplitudes. In contrast, in winter the least significant daily cycles were registered. This effect depends on the seasonal variability of the intensity of sunshine(9).

FUNDING This work was sponsored by Technological Agency of the Czech Republic (TACR alfa) under the Contract ID: TA 02020865. REFERENCES 1. R Development Core Team (2012). R: a language and environment for statistical computing, R Foundation for Statistical Computing. ISBN 3-900051-07- 0, Available on URL http://www.R-project.org. 2. Ward, D. C. and Borak, T. B. Determination of time-varying 222Rn concentrations using flow-through scintillation flasks. Health Phys. 61(6), 799– 807 (1991). 3. Jı´lek, K., Thomas, J. and Brabec, M. QA programme for radon and its short-lived progeny measuring instruments

CONCLUSIONS The obtained results of continuous measurement of the volume activity of radon av registered during the trial run of the monitoring station confirm its reliability. The measured average volume activity of radon av in the area of the institute from September 2011 to 60

Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on June 6, 2015

Figure 2. Diurnal averages of atmospheric radon within different seasons and selected meteorological parameters.

DIURNAL AND SEASONAL VARIABILITY in NRPI Prague. Radiat. Prot. Dosim. 130(1), 43–47 (2008). 4. Baciu, A. C. Radon and thoron progeny concentration variability in relation to meteorological conditions at Bucharest (Romania). J. Environ. Radioact. 83, 171– 189 (2005). 5. Duenas, C., Perez, M., Fernandes, M. C. and Carretero, J. Radon concentration in the surface air and vertical atmospheric stability in the lower atmosphere. J. Environ. Radioact. 31, 87– 102 (1996). 6. Porstendo¨rfer, J., Butterweck, G. and Reineking, A. Diurnal variation of concentration of radon and its short-

lived daughters in the atmosphere near the ground. Atmos. Environ. 25, 709–713 (1991). 7. Podstawczyn˜ska, A., Kozak, K., Pawlak, W. and Mazur, J. Seasonal and diurnal variation of outdoor radon (222Rn) concentrations in urban and rural area with reference to meteorological conditions. Nukleonika 55, 543–547 (2010). 8. Bulko, M. Atmospheric radon and its applications. Ph.D. thesis. Univerzita Komenske´ho v Bratislave. Fakulta matematiky, fyziky a informatiky (2010). 9. Sesana, L., Barbieri, L., Facchini, U. and Marcazzan, G. 222 Rn as a tracer of atmospheric motions: a study in Milan. Radiat. Prot. Dosim. 78, 65–71 (1998).

Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on June 6, 2015

61

Diurnal and seasonal variability of outdoor radon concentration in the area of the NRPI Prague.

In autumn 2010, an outdoor measuring station for measurement of atmospheric radon, gamma equivalent dose rate in the range of 100 nSv h(-1)-1 Sv h(-1)...
214KB Sizes 0 Downloads 3 Views