BIBECHANA Vol. 22, No. 2, August 2025, 85-92 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher:Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University)Biratnagar Radiological assessment of the Bagh Bhairav Temple in Kirtipur, Nepal using in-situ gamma ray spectrometry Anita Mishra∗, Raju Khanal Central Department of Physics, Tribhuvan University, Kirtipur, Nepal ∗Corresponding author. Email: anita.745711@cdp.tu.edu.np Abstract In this work, the results of an in-situ radiological survey of the Bagh Bhairav complex in Kir- tipur, Nepal, using a portable gamma ray spectrometer equipped with a GPS and data logger unit is presented. The study aims to assess external exposure by evaluating and mapping external absorbed dose rates and radionuclide activity concentrations in the area. The mea- sured absorbed dose rate in air ranged from 100.330 nGy/h to 170.506 nGy/h, with a mean value of 128.661 ± 14.637 nGy/h. The activity concentrations of the gamma radionuclides 238U, 232Th and 40K, outdoor annual effective dose (AED) and excess lifetime cancer risk (ELCR) were found higher than the world average values. The elevated absorbed dose rate is attributed to the high activity concentrations of gamma radionuclides present in soils, rocks at high altitudes. This study provides baseline radiological data for the Bagh Bhairav area and contributes to the limited radiological data available for Nepal. Keywords Absorbed dose rate, activity concentration, AED, ELCR, in-situ radiological survey, Nepal Article information Manuscript received: October 17, 2024; Revised: January 20, 2025; Accepted: February 9, 2025 DOI https://doi.org/10.3126/bibechana.v22i2.70839 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 1 Introduction Human populations are continually exposed to ion- izing radiation from environmental sources, which contributes to their radiation dose. High radiation doses have been reported in various parts of the world, including India, which borders Nepal [1, 2]. Gabdo et al. measured the mean terrestrial gamma radiation dose rate in Pahang state, Malaysia, to be 176 ± 5 nGy/h, with an annual effective dose rate of 0.22 mSv/y for outdoor exposure [3]. A sur- vey by Taskin et al. revealed the average outdoor gamma dose rate in Kirklareli, Turkey, to be 118 ± 34 nGy/h [4]. Ramola et al. studied the average ex- ternal absorbed dose rates in the Garhwal Himalaya region of India from the radionuclides 226Ra, 232Th and 40K, finding it to be 138 nGy/h [5]. The high concentration of radionuclides in these areas is at- tributed to various rock types and their chemical properties. Rajasthan and Kerala in India have also reported higher dose rates [6, 7]. Radionu- clides present in building materials can further el- evate radiation levels [8]. Bala et al. found that the activity concentrations of 226Ra, 232Th, and 40K in soil and building materials in Una, India, were higher than the world average [9]. In Tereng- ganu state, Malaysia, the mean terrestrial gamma 85 http://nepjol.info/index.php/BIBECHANA anita.745711@cdp.tu.edu.np https://doi.org/10.3126/bibechana.v22i2.70839 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Anita Mishra and Raju Khanal/ BIBECHANA 22 (2025) 85-92 86 radiation dose rates and annual effective dose were measured to be 150 nGy/h and 0.92 mSv/y, respec- tively [10]. Gamma ray spectrometry has also been employed to study environmental gamma radiation in some selected areas of Nepal [11–14]. Recently, gamma ray spectrometry has found diverse appli- cations, including lithological mapping [15], qual- itative analysis of clay minerals [16], investigation of shutdown radioactivity for the Experimental Ad- vanced Superconducting Tokamak (EAST) [17], ra- dioactivity depth distribution analysis in activated concrete [18], and evaluating tropical soil processes and attributes [19]. The Bagh Bhairav complex is an ancient temple built in the 16th century, located in Kirtipur city, a UNESCO tentative site (https://en.wikipedia. org/wiki/Kirtipur), approximately 5 km south- west of the capital city (Figure 1). The temple lies between latitudes 27.6790 - 27.6800 and lon- gitudes 85.2760 – 85.2770 at an altitude of 1294.970 – 1352.410 meter. The complex consists of a large temple surrounded by many smaller temples and statues. The bricks, timber, clay mortar, stones, and clay tiles used for construction may have high concentrations of primordial radionuclides, which can produce gamma radiation and elevate back- ground radiation levels. Pilgrims and visitors regu- larly visit the temple, and the surrounding area is inhabited, making a radiological survey of this area essential to assess any potential radiological haz- ards to the public. The image of the Bagh Bhairav temple featured on a postage stamp issued by the government of Nepal also underscores its historical significance (Figure 2). The aim of this study is to evaluate outdoor external exposure in the complex and its surround- ing area by measuring the absorbed dose rate in air and calculating the annual effective dose (AED) us- ing in-situ gamma ray spectrometry. The absorbed dose is the radiation energy imparted to a unit mass of matter and AED is a measure of the energy de- posited by radiation in organs and tissue per year and it measures the biological effects of radiation to humans. The measured absorbed dose rate in air will be compared with the dose rate calculated from the activity concentrations of terrestrial gamma ra- dionuclides 238U, 232Th and 40K. Additionally, the associated health risks will be assessed by calculat- ing the excess lifetime cancer risk (ELCR). ELCR is the difference between the proportion of people who develop or die from the disease in an exposed popu- lation and the corresponding proportion in a similar population without the exposure [20]. The accept- able range of excess lifetime cancer risk (ELCR) is generally contextual and varies based on regu- latory frameworks and risk management practices. As a general rule, mitigation (or any action) may not be necessary if the risks originate from natu- ral sources and the dose remains below established thresholds, such as the ICRP’s recommended limit of 1 mSv/year for the public. Figure 1: Google map and location of the survey site. Figure 2: Postage stamp of Nepal depicting the Bagh Bhairav temple. 2 Material and Methods The spectrometer (PGIS 2) was carried in a back- pack (maintained at a height of 1 m for consistency in measurements) and walked around the study area with a speed less than 2 km/h. 2.1 Gamma Ray Spectrometry The dose rate is studied using gamma ray spec- trometer (PGIS 2 from Pico Envirotec) which can record 512 channels of data in the energy range 20 keV to 3 MeV. It has auto calibration and real-time spectrum stabilization by the natural gamma photo peaks. The activity concentration of 40K is directly measured from its emission line at 1461 keV while the activity concentration of Uranium and Thorium decay series is measured from the gamma emission of 214Bi at 1764 keV and 208Tl at 2614 keV respec- tively. 2.2 Dosimetry 2.2.1 Annual Effective Dose (AED) The outdoor annual effective dose (AED) is calcu- lated based on the absorbed gamma dose rate by https://en.wikipedia.org/wiki/Kirtipur https://en.wikipedia.org/wiki/Kirtipur Anita Mishra and Raju Khanal/ BIBECHANA 22 (2025) 85-92 87 using the equation [1]: AED (nSv) = D ( nGy h ) ×8760h×OF×CF ( Sv Gy ) (1) where D is the average absorbed dose rate, CF is the conversion factor, 0.7 Sv/Gy (to convert ab- sorbed dose rate in air to effective dose equivalent for human), OF is the outdoor occupancy factor, 20% and 8760 is hours in one year. 2.2.2 Excess Lifetime Cancer Risk (ELCR) The excess lifetime cancer risk (ELCR) was calcu- lated using the equation [20]: ELCR = AED × LE ×RF (2) where LE is the Life Expectancy of peo- ple (66.2 year in Nepal) (http://en.worldstate. info/Asia/Nepal) and RF is the overall fatal risk coefficient, 0.05 per Sv as recommended by ICRP for the purpose of radiological protection. 2.2.3 Absorbed Gamma Dose Rate The absorbed gamma dose rate from the concen- tration of gamma radionuclides in rocks and soils is calculated using the equation [1]: Dcalculated ( nGy h ) = 0.0417AK+0.462AU+0.604ATh (3) where AK , AU and ATh is the activity con- centration (Bq/kg) of 40K, 238U and 232Th respec- tively. 3 Theory The semi-empirical model based on mono-energetic radiation is the simplest approach for modeling gamma ray fields. This model uses a two-layer con- figuration, with the Earth represented as an infi- nite half-space of constant density and radioelement concentration, overlaid by a layer of non-radioactive air with constant density. The observed photo peak intensity, dI, is given by: dI = Aε 4πr2 e−µeree−µaraNdv (4) where Ndv is the number of gamma rays of en- ergy E0 emitted per second by the volume element dv, A is the effective cross-sectional area of the de- tector, ε is the photo peak efficiency of the detector for gamma rays of energy E0, µe and µa are the linear attenuation coefficients for the Earth and air, respectively; and re and ra are the distances that gamma rays travel through the Earth and air, respectively, with R = ra + re. 4 Results The measured absorbed dose rate in air of Bagh Bhairav area was obtained in the range of 100.330 to 170.506 nGy/h with an average of 128.661 ± 14.637 nGy/h. The radiological map of measured absorbed dose rate in air is shown in Figure 3. The dose rates were overlaid on the geo-referenced map to find the spatial variability. The higher value was observed in the surrounding area of the temple (yellow colour) with the highest value (red colour) near the temple wall as building materials are also the contributors of gamma dose. The concentrations of 40K, 238U and 232Th was measured in the range of 1.130 to 5.702%, 0.590 to 20.461 ppm and 3.053 to 40.683 ppm with an average of 3.126 ± 0.749%, 6.903 ± 3.150 ppm and 18.264 ± 6.179 ppm respectively. The activ- ity concentrations of 40K, 238U and 232Th, as cal- culated following IAEA (2003) was found in the range 701.433 to 1422.147 Bq/kg with an average of 978.901 ± 161.189 Bq/kg, 39.805 to 153.454 Bq/kg with an average of 85.340 ± 21.423 Bq/kg and 42.454 to 111.790 Bq/kg with an average of 74.326 ± 13.899 Bq/kg respectively. The calculated absorbed dose rates was ob- tained in the range 90.350 to 167.879 nGy/h with an average of 125.140 ± 18.249 nGy/h and was found nearly equal with the measured dose rate. The dose rate from the particular gamma radionuclides 40K, 238U and 232Th were obtained in the range of 14.748 to 74.423 nGy/h, 3.366 to 116.744 nGy/h and 7.486 to 99.764 nGy/h respectively. The average dose rate from 40K, 238U and 232Th were compared with population weighted average (Table 1). Table 1: Absorbed dose rate compared with population weighted average [1] Absorbed dose rate in air (nGy/h) Gamma radionuclide 40K 238U series 232Th series Population weighted average 18 15 27 Present Study 40.806±9.786 39.386±17.976 44.789±15.153 http://en.worldstate.info/Asia/Nepal http://en.worldstate.info/Asia/Nepal Anita Mishra and Raju Khanal/ BIBECHANA 22 (2025) 85-92 88 Figure 3: Measured absorbed dose rate map of Bagh Bhairav overlaid on (a) an imagery map (b) a topographical map showing altitude contour. The average measured dose rate and calculated dose rate were found to 128.661 ± 14.637 nGy/h and 125.140 ± 18.249 nGy/h respectively and the ratio of them (0.972) inferred no discrepancies in survey data. The variation in calculated dose rate was found more than measured dose rate in the studied area (Figure 5). Figure 4: Contour map of absorbed dose rate in air (a), concentrations of 40K (b), 238U (c) and 232Th (d). The contour maps of measured absorbed dose rate, concentrations of 40K, concentrations 238U and concentrations 232Th with latitude and longi- tude were shown in Figure 4. The variation of dose rate in the area seems smooth though it had high standard deviation. The variation of concentrations of 40K and 232Th is smooth in the area while 238U concentration shows little roughness. 4.1 Outdoor AED and ELCR The outdoor AED was found to be 0.157 mSv which is higher than the world average value of outdoor AED from terrestrial gamma dose. The ELCR was calculated from outdoor AED and was found 0.519 ×10−3 which is also higher than the world average. 5 Discussion The dose rate was found to be higher near the walls of the temples, as the gamma radionuclides 238U, 232Th and 40K present in building materials (mud, clay bricks, and stones) also contribute to gamma radiation. These dose rates are associated with gamma radiation from the Earth’s crust (sedimen- tary rocks of lacustrine deposits) as well as from building materials. The elevated dose rate in the complex is attributed to higher activity concentra- tions of primordial radionuclides in the area. The terrestrial gamma dose rate varies slightly within the complex, reflecting the activity concentrations of natural gamma radionuclides present in the soils and rocks, which have dispersed in the area due to long-lasting geological processes. The activity concentrations of the gamma radionuclides 238U, 232Th and 40K were found to be higher than the world population-weighted averages of 33 Bq/kg, 45 Bq/kg, and 420 Bq/kg, respectively [1]. Figure 5: Variation between measured dose rate and calculated dose rate. The shape and spread of the measured ab- sorbed dose rates and calculated dose rate data show asymmetry, as illustrated in the histogram (Figure 6). The frequency distribution of the con- centrations of 238U, 232Th and 40K was also found to be non-symmetric and right-skewed, with most values shifted toward the left, indicating that the mean is greater than the median (Figure 6). The median, dispersion, and range of dose rates from the concentrations of gamma radionuclides 238U, 232Th and 40K were compared using box-whisker plots (Figure 7), with whiskers extending to 1.5 times the interquartile range (IQR). Regression analysis between the activity concentrations of 238U, 232Th Anita Mishra and Raju Khanal/ BIBECHANA 22 (2025) 85-92 89 and 40K and the measured absorbed dose rate was conducted using a scatter plot (Figure 8). The results of the statistical analysis are presented in Table 2. Hypothesis tests for histogram normal fitting and regression analysis (linear fitting) were performed at a 0.05% significance level. Radiological data for soil and rocks in Nepal is available for the Hetauda region [21], and while some areas have been monitored for various rea- sons, comprehensive data is still lacking. Therefore, the data from the present study will serve as base- line data for Nepal. The study determined that the measured absorbed dose rate in air is higher than the world average. The ratio of the absorbed dose rates (calculated and measured) shows a discrep- ancy of less than 30%, indicating that the survey data is logically representative [1]. The outdoor annual effective dose (AED) cal- culated was found to be higher than the world av- erage background radiation for outdoor terrestrial radiation, which is typically around 0.07 mSv [1]. The outdoor AED for the study area is compared with neighbouring countries such as India (which shares similar geological and geographical condi- tions), China, and others (Table 3). The excess lifetime cancer risk (ELCR) value was also found to be higher than the world average of 0.29×10−3 [1], but the associated risk of developing cancer remains negligible. Figure 6: Distribution of (a) measured, and (b) cal- culated absorbed dose rates, activity concentrations of (c) 40K, (d) 238U, and (e) 232Th. Figure 7: Calculated absorbed dose rates from ac- tivity concentrations of 40K, 238U and 232Th. Figure 8: Correlation between measured absorbed dose rate and activity concentrations of (a) 40K (b) 232Th (c)238U and (d) calculated absorbed dose rate. Anita Mishra and Raju Khanal/ BIBECHANA 22 (2025) 85-92 90 Table 2: Statistics of the survey data Measured dose rate (nGy/h) Calculated dose rate (nGy/h) Conc. of 40K (%) Conc. of 232Th (ppm) Conc. of 238U (ppm) Mean 128.661 124.982 3.126 6.903 18.264 Median 130.195 123.916 3.092 6.538 17.581 First quartile 114.845 107.966 2.605 4.586 13.789 Third quartile 138.406 140.085 3.607 8.757 22.042 Standard error 0.391 0.616 0.02 0.084 0.165 Standard deviation 14.637 23.077 0.75 3.151 6.181 Coefficient of variation 0.113 0.184 0.239 0.456 0.338 Skew 0.239 0.261 0.302 0.755 0.564 Kurtosis -0.466 -0.22 0.103 0.766 0.179 Table 3: Comparative study of outdoor (AED) in different regions of Nepal and the world S.N. Sites Outdoor AED (mSv) References 1 Hetauda, Nepal 0.12 [22] 2 Garhwal, India 0.17 [5] 3 Una, India 0.1 [9] 4 Kirklareli, Turkey 0.144 [4] 5 Kohistan, Pakistan 0.12 [23] 6 Islamabad, Pakistan 0.16 [24] 7 Pahang, Malaysia 0.22 [3] 8 Baotou, China 1.03 [25] 9 Odha, India 0.17 [26] 10 Mrima Hill environs, Kenya 0.86 [27] 11 Tribhuvan University, Kirtipur 0.142 [28] 12 UNESCO Sites, Kathmandu 0.148–0.186 [29] 13 Tarakeshwar, Kathmandu 0.15 [30] 14 Bishnumati Bridges, Kathmandu 0.906 [31] 15 Present Study 0.157 – 16 World average 0.07 [1] 6 Conclusions The in-situ radiological survey of the Bagh Bhairav complex was performed using a portable gamma- ray spectrometer equipped with a GPS and data logger unit. The outdoor absorbed dose rate in air was found to be nearly twice the world average. The absorbed dose rate in air was compared with terrestrial data, and statistical analysis revealed a positive linear correlation between them. The dis- tribution showed a skewed pattern and was corre- lated with the concentrations of terrestrial gamma radionuclides. Additionally, there was a strong agreement between the measured and calculated absorbed dose rates; the ratio of the calculated to measured dose rate was nearly equal to one, indicat- ing no discrepancies in the data. The activity con- centrations of 238U, 232Th, and 40K in the complex were approximately 2.5 times higher than the world average, contributing to the elevated dose rates ob- served. Although the outdoor annual effective dose (AED) and excess lifetime cancer risk (ELCR) were higher than the world averages, the outdoor AED value obtained was below the ICRP-recommended limit for public exposure (< 1 mSv/y). Hence, there is no potential radiological hazard to the public in the complex. Moreover, this study is valuable for establishing baseline data and radiological maps for the area. Acknowledgements We acknowledge the support of the International Atomic Energy Agency (IAEA), Austria, for pro- viding the equipment through the IAEA TC Project NEP0002. We also thank the Ministry of Education, Science and Technology, Government of Nepal, for their coordination with the IAEA. Anita Mishra expresses gratitude to the University Grants Commission, Nepal, for the Ph.D. fellowship award (PhD-74/75-S&T-15). Additionally, we would like to thank Anish Maskey, Atit Deuja, and Denish Poudyal for their support during the survey. Anita Mishra and Raju Khanal/ BIBECHANA 22 (2025) 85-92 91 References [1] UNSCEAR. Sources and Effects of Ionizing Radiation. United Nations, New York, USA, 2000. [2] A.V. Sankaran et al. U, Th and K distribu- tions inferred from regional geology and the terrestrial radiation profiles in India. Techni- cal report, Bhabha Atomic Research Centre, Mumbai, India, 1986. [3] H.T. Gabdo et al. Terrestrial gamma dose rate in Pahang state Malaysia. J. Radioanal. 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