Radiation Protection and Environment

ORIGINAL ARTICLE
Year
: 2018  |  Volume : 41  |  Issue : 3  |  Page : 148--151

Determination of atmospheric concentration of beryllium-7 at ground level


Pablo A Andres 
 Radiological Protection Department, Bariloche Atomic Centre, National Atomic Energy Commission, Bariloche, Argentina

Correspondence Address:
Pablo A Andres
Av. E. Bustillo Km. 9,5; R8402AGP San Carlos de Bariloche
Argentina

Abstract

Atmospheric concentration of beryllium-7 (7Be) at ground level was measured from September 2012 to October 2013 in San Carlos de Bariloche, Argentina, using a HPV-5300AFC brushless, automatic flow control outdoor high volume air sampler for continuous use, and 8”×10” 0.8 microns fiberglass filters. The activity of 7Be was determined on a high-purity germanium (HPGe) detector (relative efficiency 12.3%) by standard gamma spectrometry. The average value found was 4.1 ± 0.1 mBq/m3. Seasonal changes in the atmospheric concentration of 7Be were compared with other environmental variables (temperature, humidity, precipitations, and air kerma rate), and correlation, both positive and negative, was observed as expected. Measurements were carried out following the Radiological Environmental Monitoring Programme at Bariloche Atomic Centre. The aim of the study was to determine atmospheric concentration of 7Be at ground level and to correlate its seasonal changes with environmental variables. Air samplers and environmental dosemeter locations were far away from buildings and any other thing that could influence or alter the measurements. Pearson's correlation coefficient was calculated to evaluate the relation between atmospheric 7Be concentration and environmental variables of interest. These changes in 7Be concentration can be attributed to the variations and dynamic of air exchange between the stratosphere and the troposphere and to the variations of the tropopause height. To fully understand and correctly interpret the meaning of these seasonal variations in the concentrations of atmospheric 7Be, active atmospheric processes must be taken into account. According to the measurements carried out, contribution of 7Be concentration to the natural background radiation dose is negligible.



How to cite this article:
Andres PA. Determination of atmospheric concentration of beryllium-7 at ground level.Radiat Prot Environ 2018;41:148-151


How to cite this URL:
Andres PA. Determination of atmospheric concentration of beryllium-7 at ground level. Radiat Prot Environ [serial online] 2018 [cited 2021 May 9 ];41:148-151
Available from: https://www.rpe.org.in/text.asp?2018/41/3/148/245795


Full Text



 Introduction



Beryllium-7 (7Be) is a naturally occurring radionuclide (Eγ= 477.6 keV; T½=53.2 days) produced as a result of spallation reactions with light elements such as nitrogen, oxygen, and carbon in high atmosphere, throughout a 7Be + e−→7 Li + v electronic capture process.[1],[2] This radionuclide enters the environment by wet deposition mainly (rain and snow), while dried deposition mechanism (gravity action) is below 10%.[3]

Atmospheric concentration of 7Be is not uniform; it depends on the geographical location, solar activity, exchange of air masses in the atmosphere, and the removal efficiency from the troposphere. After 7Be is formed, it rapidly associates with submicron-sized aerosol particles.[4] The world concentration of 7Be is altitude and latitude dependent and is equal to 12.5 mBq/m3, on average.[5] Several authors have reported changes in 7Be content in rain related to the magnitude and intensity of precipitations, length of the event, and time elapsed between events.[6],[7],[8],[9]

When 7Be falls down on the earth surface, it does not scatter in a homogeneous way, but it tends to affix itself in superficial and aquatic plants and on superficial soils to a lesser degree. The deposition velocity of aerosols can be affected by precipitation and seasonal factors. However, the deposition velocity reflects the effectiveness of aerosol scavenging by precipitation; thus, both the amount of precipitation and duration of rainfall must be considered. Rainfall reduces the air concentration of 7Be[10],[11] and increases the deposition velocity, which provides important information on the behavior of atmospheric particulates in the environment.[12],[13]

The production rate of 7Be in the atmosphere increases with a decrease in latitude, while the maximum deposition flux occurs in the middle latitudes due to vertical/horizontal air mass transport and the frequency of rainfall events.[14] This radionuclide is often used in climate change studies in several countries nowadays.[15]

This paper focuses on the determination of concentration of 7Be during a 12-month period in San Carlos de Bariloche, Argentina (41° 09' S; 71° 18' W; 893 m above sea level). Its relationship with other environmental variables was analyzed.

 Materials and Methods



The city of San Carlos de Bariloche is located in Northern Patagonia, Argentina, close to the Andes mountain chain (41° 09' S; 71° 18' W; 893 m above sea level). Combination of altitude, latitude, and winds dominance from the West-Northeast quadrant and South generates a mild-cold climate with dry season, which shows a noticeable variation of rains from West to East. The precipitations' gradient fluctuates from 4000 mm/year in mountain points (Valdivian temperate forests) to only 600 mm/year in the Limay river area. The most urban population is located in the valleys where annual precipitations are in the 800–1000 mm/year range. About 70% of precipitations occur during autumn and winter months.

The area – according to Thornthwaite classification – is a climate region defined as follows:[16]

AC' 2ra': This stands for a microthermal high wet climate, with little or no water deficit and low thermal concentration in summerB3C' 2sa'+: This stands for a microthermal wet climate, with moderate water deficit and low thermal concentration in summer.

Each aerosol sample was collected during 20 h on 8” × 10” 0.8 μ fiberglass filters by constant flow rate samplers (HPV-5300AFC brushless and automatic flow control outdoor high volume air sampler for continuous use). The activity of 7Be was determined on a high-purity germanium detector (relative efficiency 12.3%) by standard gamma spectrometry. Detector calibration was performed using gamma standard calibrated sources (241Am, 57Co, 22Na, 137Cs,54 Mn, 60Co, 133Ba, and 152Eu) following conventional acknowledged methods.[17],[18] The photopeak efficiency was 1.8% at 477 keV. Pulse height spectrum analysis was made with a computer-based multichannel analysis (4096 energy channels), and each filter was measured for at least 48 h. Samples were collected once a month from September 2012 to October 2013. The combined measurement uncertainty of the results was calculated at the 95% level of confidence (k = 2).

 Results



Concentration of 7Be at ground level is shown in [Table 1]. The average concentration of 7Be can be determined as 4.1 ± 0.1 mBq/m3.{Table 1}

[Table 2] shows weather conditions during the sampling period. Data were obtained from the local aerodrome meteorological station. Values are indicated as an average of daily measurements followed by one standard deviation. Air kerma rate values were measured at the sampling point with thermoluminescent dosemeters (LiF 700H), and the combined measurement uncertainty of the results was calculated at the 95% level of confidence (k = 2).[19]{Table 2}

From the analysis of the concentration changes of 7Be and its relationship with changes of other environmental variables over the year, a positive correlation can be observed between concentration of 7Be and temperature changes (Pearson's correlation coefficient, r = 0.70, average temperature; r = 0.61, average maximum temperature; and r = 0.72, average minimum temperature). These variations can be seen in [Figure 1]. According to the literature, an important factor in producing higher concentrations of 7Be during warmer months is the increase of vertical transport within the troposphere occurring during those months.[20] This vertical transport carries down to the surface layer of 7Be that has been produced within the upper troposphere, as well as that which has entered the troposphere from the stratosphere.[21] Moreover, no correlation between wind speed and air kerma rate could be found.{Figure 1}

On the other hand, since the sampling site is located in a region where there are strong seasonal variations in the rainfall rate, a negative correlation was observed, as expected, between concentration of 7Be and variables such as relative humidity (r = −0.33) and precipitations (r = −0.11). This negative correlation could indicate the importance of washout of the atmospheric aerosol that carries the 7Be.[21] However, the correlation coefficients are not strong enough to confirm the previous hypotheses.

 Conclusions



Seasonal variations in the concentration of 7Be at ground level were observed as expected, according to the literature. These changes in 7Be concentration can be attributed to the variations and dynamic of air exchange between the stratosphere and the troposphere and to the variations of the tropopause height. To fully understand and correctly interpret the meaning of these seasonal variations in the concentrations of atmospheric 7Be, active atmospheric processes must be taken into account.

According to the measurements carried out, contribution of 7Be concentration to the natural background radiation dose is negligible.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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