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ORIGINAL ARTICLE
Year : 2021  |  Volume : 44  |  Issue : 1  |  Page : 28-33  

Study of ambient gamma dose levels at national atmospheric research laboratory, Gadanki, India


1 Department of Physics, Bangalore University, Bengaluru, Karnataka, India
2 Radar and Application Development Group, National Atmospheric Research Laboratory, Tirupati, Andhra Pradesh, India

Date of Submission25-Dec-2020
Date of Decision28-Jan-2021
Date of Acceptance31-Jan-2021
Date of Web Publication07-Jun-2021

Correspondence Address:
Nagaraja Kamsali
Department of Physics, Bangalore University, Bengaluru - 560 056, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.rpe_62_20

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  Abstract 


Simultaneous observations of ambient gamma dose levels, temperature, relative humidity, and atmospheric pressure were carried out at National Atmospheric Research Laboratory (NARL), Gadanki, India (13.459°N and 79.175°E), from November 2011 to May 2014. The results show that about 92% of ambient gamma dose values lie between 150 and 200 nSv/h and best possible fit resulted in Gaussian fit with adjusted R2 of 0.99. A weak Pearson's correlation coefficient was found between ambient gamma dose levels and selected meteorological parameters measured over the location. No seasonal trend was observed in ambient gamma dose levels, but pronounced seasonal variations in temperature, relative humidity, and air pressure were found. A sudden increase in ambient gamma levels was observed during precipitation event (Nilam cyclone) and may be attributed to an additional contribution of precipitation washed 222Rn progeny aerosols within the atmosphere. The mean ambient gamma dose over NARL was 186 ± 4.3 nSv/h and is within world average given by UNSCEAR.

Keywords: Gadanki, gamma dose, meteorological parameters, radon


How to cite this article:
Kumar K C, Prasad T R, Kamsali N. Study of ambient gamma dose levels at national atmospheric research laboratory, Gadanki, India. Radiat Prot Environ 2021;44:28-33

How to cite this URL:
Kumar K C, Prasad T R, Kamsali N. Study of ambient gamma dose levels at national atmospheric research laboratory, Gadanki, India. Radiat Prot Environ [serial online] 2021 [cited 2021 Jun 24];44:28-33. Available from: https://www.rpe.org.in/text.asp?2021/44/1/28/317952




  Introduction Top


Regional monitoring of gamma radiation levels from the radionuclides has been of utmost importance nowadays. Several countries worldwide have felt the need for a network for dedicated radiation monitoring stations to monitor short-term variations in gamma radiation levels and long-term shift in the levels if any.[1],[2],[3] The main natural contributors toward ambient gamma near earth's surface are radiations from the soil, cosmic rays, and gamma-emitting radioactive aerosols suspended in atmospheric air (daughter nuclei of 220Rn and 222Rn). Due to the wide variety of orographic structures, the natural background radiation doses are region dependent. In a particular location, the variations in ambient gamma dose are mainly due to variations in gamma-emitting radioactive aerosols and the soil and cosmic rays' contribution. Hence, long-term gamma dose levels for a location can significantly provide information on local influencing parameters.

Several researchers in India were continuously monitoring the ambient gamma dose levels on various time scales by different techniques and reported that the gamma dose levels are constant in normal conditions over a site.[4],[5],[6] In a few instances, anomalous changes in gamma dose levels, namely enhancement of dose due to the accumulation of 222Rn progenies near the surface, are also reported due to radionuclides' deposition to nuclear disasters (maybe rare case) or due to intensified precipitation events.[7] The scavenging of 222Rn progenies and its attachment to the aerosols to form condensation nuclei within the cloud is one of the most important research problems in cloud physics. It is reported that the droplets containing the aerosols may significantly contribute toward enhancement of ambient gamma dose during precipitation events.[7],[8] In general, air mass transport containing several atmospheric constituents and precipitation events is controlled by local and regional meteorological processes. Hence, more studies on simultaneous ambient gamma dose and meteorological parameters can provide essential evidence for a possible relationship. The present research work aims to analyze the data set of ambient gamma radiation levels at National Atmospheric Research Laboratory (NARL), India, and to study the possible relationship between ambient gamma dose and selected meteorological parameters.


  Methodology and Site Description Top


The continuous ambient gamma dose measurements were carried out using an integrated Geiger-Muller (GM) tube present inside the AlphaGUARD PQ 2000 PRO. AlphaGUARD is a professional radon monitoring instrument based on pulse ionization chamber principle and digital signal processor technology. The sophisticated instrumentation and accurate continuous measurements make it a reference device in calibrating several other passive devices. The GM-tube inside AlphaGUARD is wholly protected, and the measuring range is from 20nSv/h to 10 mSv/h with a resolution of 1nSv/h. AlphaGUARD also consists of temperature, atmospheric pressure, and relative humidity sensors, making it an ideal instrument to monitor radon and ambient gamma dose with less maintenance.[9]

The ambient gamma dose measurements and selected meteorological parameters were carried out at NARL, Gadanki (13.459°N 79.175°E), India. It is an autonomous research institute to carry out advanced research in atmospheric physics and has the facilities such as mesosphere–stratosphere–troposphere radar, Rayleigh Doppler lidar, ionosonde, and Doppler sodar. The geography around NARL is complex with rocky hills and dense vegetation. The influence of both southwest and northeast monsoons is perceived. Within 30 km of radial distance from the observation site, several active land excavation works are influenced by the wind.[10]


  Results and Discussions Top


The simultaneous measurements of ambient gamma dose along with meteorological variables were carried out at NARL. The diurnal variations of meteorological parameters have significant variations, and the behavior of several atmospheric processes was observed. The 24-h variations of ambient gamma dose and temperature, relative humidity, and air pressure for a typical fair-weather day on January 20, 2013, at NARL are shown in [Figure 1].
Figure 1: Ambient gamma dose with temperature, relative humidity, and pressure

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[Figure 1] shows no pronounced diurnal trend in ambient gamma dose. However, meteorological parameters show significant variation.[10] The lower Pearson's correlation coefficients between ambient gamma and measured meteorological parameters indicate a nonlinear relationship of former on latter. This kind of behavior was observed for most of the fair-weather days over NARL and may be attributed to the constant emission of gamma radiation from the soil surface.

The time series of any measured quantity can be utilized to check the regularities within the time series. [Figure 2] shows the time series of ambient gamma dose levels at NARL at the height of 1 m from the surface from October 2011 to May 2014. The general inspection clearly shows no significant variations in ambient gamma dose levels with an average value of 186 ± 4.3 nSv/h. However in the case of temperature, as shown in [Figure 3], a significant monthly variation with highest during the summer period (March–May) and lowest during the winter period (December–February) was observed. The highest values were observed for relative humidity during winter months and the lowest minimum values were observed during the summer season. In air pressure, the highest values were observed during the winter season and the lowest values were observed during southwest monsoon (June–September) were observed. It is due to the frequent occurrence of unstable atmospheric conditions due to convective activities. It is interesting to know that the range of variations in ambient gamma dose levels and air pressure is relatively less than that of air temperature and relative humidity.
Figure 2: Gamma dose levels (gray line for 10 min and solid line for hourly averaged)

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Figure 3: Time series graph of temperature, relative humidity, and pressure

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Over NARL, the seasons are defined as winter from December to February, summer from March to May, southwest monsoon from June to September, and northeast monsoon from October to November. The seasonal variations in ambient gamma dose levels and measured meteorological parameters are shown in [Figure 4]. It is evident that there is no seasonal dependence of ambient gamma dose levels at NARL. In case of temperature variations, the high values were observed during the summer season and low values during the winter season.
Figure 4: Seasonal variations in ambient gamma dose and meteorological parameters

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The relatively higher relative humidity was observed either in NE monsoon or winter and lowest in the summer season. In air pressure, the highest values were observed during winter and lowest during monsoon seasons. The reports on variations in ambient gamma dose on the seasonal scale over NARL are not available, and this study can be utilized to establish the background radiation data for NARL environment. The frequency distribution of ambient gamma dose levels for NARL environment is shown in [Figure 5]. The Gaussian fitting of frequency distribution results in the highest adjusted r2 of 0.99% and 92% of the gamma dose values lie between 150 and 200 nSv/h. It shows that the variations of ambient gamma dose levels are relatively narrower in range over NARL from October 2011 to May 2014.
Figure 5: Frequency distribution of ambient gamma dose levels

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The effect of measured meteorological parameters on the ambient gamma dose levels using the scatter plot is shown in [Figure 6]. More than 85,000 data points were used to study the possible relationship, and it is found that ambient gamma dose has no Pearson's correlation with insignificant coefficients of 0.107 with temperature, negative of 0.048 with relative humidity, and 0.022 with atmospheric pressure. The correlations indicate relatively no influence of measured meteorological parameters on the measured ambient gamma dose during the study period at NARL.
Figure 6: Plots of ambient gamma dose with temperature, relative humidity, and pressure

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Several researchers have reported ambient gamma dose behavior during convective activity and occurrence of rainfall.[7],[8] It was reported that an abrupt change in gamma dose levels is observed during precipitation event, and after rainfall, it regains to its original background trend. During the study period, the precipitation data were available only for 2012, and during this period, two such instances were observed at NARL and analyzed. During the passage Nilam cyclone (from October 29 2012, to November 7, 2012), the simultaneous measurements of ambient gamma dose levels and precipitation were observed. It is observed that during the precipitation period, the gamma dose increased to 218 nSv/h, which is 32nSv/h higher than the total study period mean value of 186nSv/h. This kind of sudden increase was also observed during a rainfall event and is shown in [Figure 7].
Figure 7: Increase in ambient gamma dose during rainfall (October 29–November 7, 2012)

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A similar observation was chronicled during precipitation event from November 1–9, 2012, as shown in [Figure 8]. During this period, the ambient gamma dose increased by 11 nSv/h from the total study period value of 186nSv/h. It is interesting to observe that, after the precipitation event, ambient gamma dose has resumed to its usual trend.
Figure 8: Increase in ambient gamma dose during rainfall (December 1–9, 2012)

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The reports on this kind of ambient gamma dose behavior are not available for NARL and south Indian environment. Interestingly, during weak, moderate rainfall events, no abrupt increase in ambient gamma dose levels was observed at NARL, but only during intensified rainfall events, an abrupt increase in gamma dose levels was observed. This behavior may be attributed to the additional contribution from the 222Rn progeny present on the atmospheric constituents. During the rainfall, the cloud droplets which carry 222Rn progeny are accumulated near the surface. The washout of 222Rn progeny present in the atmosphere significantly contributes to increased ambient gamma dose during rainfall.[7],[8] These results are preliminary and need more measurements during such events to understand the direct impact of rainfall on ambient gamma dose, 222Rn and its progeny activity.

The mean values of ambient gamma dose for individual months are shown in [Figure 9].
Figure 9: Monthly mean values of ambient gamma dose

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The ambient gamma dose levels for different geological locations are summarized and presented in [Table 1]. It was found that the ambient gamma dose levels at NARL are within the world average of as given by UNSCEAR.[11]
Table 1: Ambient gamma dose levels at different locations

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  Summary and Conclusions Top


The continuous measurements of ambient gamma dose, ambient temperature, relative humidity, and air pressure were carried out at NARL, Gadanki, India. The diurnal, monthly, and seasonal variations in ambient gamma dose show no pronounced trend. The scatter plots between ambient gamma and measured meteorological parameters indicate no correlation between the former and the latter. Gaussian fit on the frequency distribution indicates that 92% of gamma dose measurements lie between 150 and 200 nSv/h. Interestingly, a sharp peak in ambient gamma dose was observed during intensified precipitation events at NARL, and it may be attributed to the additional contribution from 222Rn progenies present in atmospheric constituents and cloud particles. Rigorous studies are required to understand the possible effect of precipitation on the ambient gamma dose levels.

Financial support and sponsorship

Indian space research organisation, government of India

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
 
 
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