Gamma Radioactivity Of The January 2013 Rainfall In Ostrava / Gama Radioaktivita Deště V Lednu 2013 V Ostravě

Abstract Gamma radioactivity of an 8 mm thick ice layer formed after a modest rain on 21 January 2013 in the VŠB-Technical University of Ostrava campus was analyzed and gamma-ray emissions from primordial 40K have been found. Cosmogenic 7Be produced mainly in galactic cosmic-ray spallation processes on atmospheric nitrogen and oxygen and 226Ra, 235U and 137Cs have been also observed. Abstrakt Při analýze gama radioaktivity 8 mm silné vrstvy ledu, která se vytvořila při dešťových srážkách v areálu VŠB-Technické univerzity Ostrava 21.1.2013, bylo detekováno gama záření z izotopu 40K. Rovněž bylo pozorováno kosmogenní 7Be vznikající především v tříštivých reakcích galaktického kosmického záření s atmosférickým dusíkem a kyslíkem a také 226Ra, 235U and 137Cs.


INTRODUCTION
Naturally occurring radioactive isotopes are practically ubiquitous on Earth and may become even concentrated as a result of human activities (nuclear industry, coal and petroleum industry, water treatment, etc.) [1,2]. According to their origin, they can be divided into four groups: primordial isotopes (present since the formation of Earth), members of radioactive decay chains (radioactive products of primordial isotopes 232 Th, 235 U and 238 U), cosmogenic isotopes (produced in nuclear reactions of cosmic radiation with the gaseous and particulate constituents of the Earth atmosphere), and human produced isotopes (released in nuclear accidents, nuclear-weapon tests and controlled low-activity emissions from nuclear objects as power plants, research and medical facilities, etc.) [3]. Especially primordial 40 K and 235 U, members of the 232 Th chain ( 208 Tl, 212 Pb, 228 Ac) and 238 U chain ( 210 Pb, 214 Pb, 214 Bi, 226 Ra, 234 Th), cosmogenic 7 Be and human produced caesium isotopes 134 Cs and 137 Cs belong to the naturally occurring radioactive isotopes that can be detected in gamma spectrometry [4].
A positive correlation between the air pollution and the specific activity of radioisotopes in air is a wellestablished phenomenon [5]. In the atmosphere, radioisotopes associate with aerosol particles. The activity size distribution of 7 Be and 210 Pb aerosols peaks mostly in submicron region [6,7]. Recently correlations between the 7 Be and PM10 concentrations have been reported from a 14-year monitoring study in Taiwan [8].
The present study was carried out in Ostrava, the most polluted area of the Czech Republic [9,10], where the concentrations of the PM10 air-borne dust (particulate matter containing particles of a diameter < 10 µm) usually exceed 100 µg/m 3 in winter. Gamma spectra of two samples of an ice layer formed after a modest rain in January 2013 are analyzed to identify present radioisotopes and a possible health impact depending on the annual effective dose due to the presence of the identified radioisotopes in air is discussed. Přívoz, Ostrava-Fifejdy and Studénka) during January 21, 2013 [12] can be clearly seen. One can also see two peaks in the rain deposit per hour between 2-6 p.m. and 1-6 a.m. of Central European Time (CET) that resulted in the ice layer formation. The clouds lower boundary increased from 50-100 m towards 300 m during the day [12].

EXPERIMENTAL PROCEDURE
The ice samples collected in the campus of the VŠB-Technical University of Ostrava were kept at room temperature to melt the ice. Approx. 1 ml of nitrogen acid was then added to the water samples to decrease their pH value below 2 [13]. The samples were put into two Marinelli beakers. One of the Marinelli beakers was completely filled with the sample (620 ml), sealed and left for 40 days to reach the secular equilibrium for radium measurements (sample A). The second Marinelli beaker (460 ml) was not sealed (sample B).
Gamma spectra of all samples were accumulated using the Canberra spectroscopic system (semiconductor germanium detector GC3018, multichannel spectrometer DSA1000, spectrometric software Genie 2000 [14], massive 100 mm Pb + 1 mm Cd + 1 mm Cu shielding). Specifically, the gamma spectrum of the sample A was recorded after 40 days and the gamma spectrum of the sample B within a few days after its preparation. The 152 Eu standard source (EG1) provided by Eurostandard, the Czech Republic, was used for energy calibration. Two water solutions in Marinelli beakers (460 ml and 620 ml) were then prepared from the standard 152 Eu solution (325 Bq/ml) provided by the National Radiation Protection Institute, the Czech Republic, for efficiency calibration. All gamma spectra were corrected for laboratory background.

RESULTS AND DISCUSSION
In Fig. 2, an example of the obtained gamma spectrum is shown (sample B and background). The most important identified radioisotopes and their volume activities including standard uncertainties are summarized in Tab. 1. In the sample, B besides the expected primordial 40 K that is the most abundant natural radioisotope (observed gamma emissions from 226 Ra decay products, 214 Pb, 214 Bi, and from 228 Th decay product, 212 Pb are comparable with the laboratory background), the cosmogenic 7 Be was observed (see Fig. 1, the 7 Be peak at 478 keV clearly identified in the sample B gamma spectrum and not found in the background) [4]. 7 Be is produced mainly in stratosphere and upper troposphere in the spallation process on nitrogen and oxygen induced by highenergy galactic cosmic rays (protons and neutrons) and extraordinary intense solar energetic particles (protons) during great coronal mass ejections. The average production rate of 7 Be in the upper troposphere at 11 km was estimated to 0.02 7 Be/cm 2 /s [15]. It is known that 7 Be like man-made pollutants attaches itself to atmospheric dust particles and enters the earth surface during rain events [16]. In the sample A, the measured 7 Be volume activity was lower due to the 7 Be decay, the decay-corrected value (1.72±0.22) Bq/l is in agreement with the value obtained from the sample B within standard uncertainties. The volume activities of 40 K in both samples agree within standard uncertainties, too.

Fig. 2 Gamma spectrum of the sample B (Marinelli beaker 460 ml, 63.2 h detection time) shifted 200 units upwards compared to the laboratory background.
In the sample A, two additional peaks have been observed due to a dry deposit (dust contamination) on the ice layer between its formation on January 21, 2013 and the sample A collection on January 24, 2013. A complex peak at 186 keV can be ascribed to a mutual interference between the 235 U (185.7 keV) and 226 Ra (186.2 keV) peaks. Assuming that the 226 Ra is in radioactive equilibrium with its parent 238 U and that the 235 U/ 238 U isotopic ratio equals the expected natural value, the counts in the 186 keV peak can be mathematically apportioned between 226 Ra and 235 U [4] (for the resulting 226 Ra and 235 U volume activities see Tab. 1). A single peak at 662 keV indicates the presence of the post-Chernobyl and post-Fukushima isotope 137 Cs [17,18].
From the measured volume activities of 7 Be and 40 K in the sample B (no dust contamination), volume activities in air can be estimated (see Tab. 1) assuming homogenous distribution of radioisotopes in air up to 100 m above the earth surface and approximately one half of the radioisotopes captured during the rainfall (supported by the observed decrease of the average PM10 concentration, see Fig. 1). The obtained volume activities in air are below the clearance levels for 7 Be (1,8 kBq/m 3 ) and 40 K (47 Bq/m 3 ) [19]. The level of the volume activity of 40 K in air corresponds to the individual effective inhalation dose of 2 µSv/y max for adults and 2.7 µSv/y for infants if comparable unfavourable meteorological conditions occur in approx. 50 days per year. The effective inhalation dose is computed from the inhalation conversion factors hinh = 2,1x10 -9 Sv/Bq and 2,4x10 -8 Sv/Bq and the total air inhalation per year 8500 m 3 and 1000 m 3 for adults and infants, respectively [19]. The obtained effective inhalation doses represent less than 0.3 % of the annual effective dose limit of 1 mSv [19].

CONCLUSIONS
The measured volume activities of the melted ice samples from the modest rainfall on January 21, 2013 revealed slightly increased volume activities of 7 Be and 40 K. The highest estimated annual effective inhalation dose was obtained for 40 K. The value is less than 0.3 % of the annual effective dose limit for population, i.e. it presents no health risk.
The detection of the cosmogenic radioisotope 7 Be that has been suggested as an independent indicator of air pollution [16] seems to be the most important result of the present measurement. Since rainfalls can clean the atmosphere (washout of the aerosols present below the clouds), an increased 7 Be specific activity (volume activity in rain samples and in air and surface activity due to a deposition on the Earth surface) in winter is expected at high PM10 and PM2,5 air-borne dust concentrations as observed in this study. It is also supported by a recent study of correlations between 7 Be and PM10 concentrations from a 14-year air monitoring in Taiwan [20]. Seasonal variations of the 7 Be specific activity have also been observed [21]. They are ascribed to intrusion of stratospheric 7 Be-rich masses into the troposphere in late spring and summer. On the other hand, potential correlations between 7 Be wet deposition (in individual rain events) and basic rain characteristics as total precipitation, precipitation rate and the elapsed time between two rain events, have been extensively studied by different authors with divergent results [22], e.g. correlations between the 7 Be specific activity and the elapsed time between two subsequent rain events were reported in [21], whereas in [22] no correlations were found. The enhanced 7 Be specific activities due to an anomalous transport of air masses over the continent in the upper troposphere and the lower stratosphere where 7 Be is formed cannot be also excluded in any season [21].
To clarify the significance of the above mentioned factors and to examine their correlations to the 7 Be volume activity in rain, we propose to measure the 7 Be volume activity in individual rain events on the roof of the main building of the Technical University of Ostrava for a period of at least one year. The chosen location in the European PM10 hot spot in Ostrava [9,10] will enable to test the correlations in a wide range of PM10 and PM2,5 values. A collector on the roof of a high building minimizes surface dust contamination. Moreover, an automatic meteorological station of the Institute of Geoinformatics of the Technical University of Ostrava that measures relevant meteorological data every 5 minutes including total precipitation and precipitation rate is located on the same roof. It provides information about precipitation rate evolution during individual rain events that has never been used in analysis of correlations between 7 Be volume activity and basic rain characteristics.