Human exposure to ionizing radiation is one of the scientific subjects that attract public attention over the time since the natural radioactivity from primordial origins is the vast majority to detect on the earth and is responsible for most of the total radiation exposure of the mankind (UNSCEAR, 2000). Among these 226Ra is the most abounded progeny of 238U (99%) that exists in the geo-surface media, generally in soil, in water and/or in the earth-born materials with appearing activity . The radioactive substance of thorium occurs naturally in the earth’s crust in combination with other minerals such as silica, and almost 99% of natural thorium exists in the form of 232Th . Underground mine rocks usually contain a more intense form of thorium. Wind, water action and other natural weathering break these rocks, and make the thorium and all other components of the rocks become a part of the soil . Moreover, the soil containing a high amount of mud materials shows elevated contents of thorium . 39K (93.2581%) and 41K (6.7302%), the two natural isotopes of potassium are compulsory for living species; however, various health hazards are cause of the strong gamma radiation (1460 keV) emitted from the primordial 40K (0.0118%)  . Different physicochemical forms of the aforementioned radionuclides trigger up their mobility and biological uptake. For instance, radium especially bonds to organic matter and has an affinity for hard tissue because of its chemical similarity to calcium . While potassium and thorium favourably bond to clays, and hence these are more radioactive than the other sedimentary rocks. Thorium enters the body mainly through inhalation of contaminated dust and swallowed in water and/or with food. Most of its portion ejects out, if some portion remains it will be deposited to bones, where it may remain for many years. On the other hand, the potassium ingested through the food chain and finally deposited on muscular cells.
Chattogram, the commercial capital of Bangladesh is famous for its industrial affluence. Our study region, Nasirabad Industrial area has around 25 industries of a wide variety, like steel industries, metal & plastic processing industries, food processing industries, paint & chemical production factories, oil refining plants, silver recycling, woodcutting and textile mills . These factories are illegitimately discharging a huge amount of pollutants to the ambient continuously.
Despite being the industrial area, around 100 thousand inhabitants live permanently in the span of 8 square kilometre of this study area . Following the radiological health hazards particularly from the external exposure via gamma radiation, the concentrations of NORMs and contamination due to anthropogenic radionuclides in this area should be the subject of periodic monitoring. This study has been conducted to investigate the level of natural radionuclides (226Ra, 232Th, and 40K) and artificial radionuclides (137Cs) in the soils of Nasirabad Industrial area due to its importance in commerce and radiological assessment to the working and living population.
The results of this study are expected to contribute to the development of environmental regulatory frameworks, and raising public awareness about radiation health hazard.
2. Experimental Procedure
2.1. Sample Collection and Preparation
Nasirabad Industrial area is in the northern side of Chattogram City Corporation. The investigated samples of soil and solid wastes were collected from the area lies between the longitudes of 91˚48.534'E - 91˚49.249'E and the latitudes of 22˚22.465'N - 22˚23.660'N (Figure 1). All the samples were collected in the mid of winter so that it’s expected to get the maximum contamination level, apart from washout of surface contaminant. All the labelled samples were brought to Atomic Energy Centre, Chattogram for ensuing preparation and investigation. To make moisture free for achieving proper matrix, each sample was oven-dried at about 105˚C until getting the constant weight following the removal of unwanted objects. Dried samples were blended by using a domestic blender and a porcelain mortar pestle and then sieved with 500 µm pore size to make homogenized. Later, around 200 g of each sample was kept in a zip lock bag and stored in desiccators for further removal of moisture before taken into individual mini-Marinelli (~250 ml) beakers. The container was simply shaken by hand to settle down the dry powder sample homogeneously. For allowing the secular equilibrium between 232Th and 226Ra and their progenies, the prepared samples
Figure 1. Sampling location.
containers were sealed tightly and wrapped with wide vinyl tapes around their screw necks to ensure airtight condition and were stored for four weeks before the measurement of radioactivity .
2.2. Measurement by Gamma-Ray Spectrometry
The activity concentrations of NORM’s and TENORM’s in the samples were detected using a liquid nitrogen-cooled coaxial p-type HPGe gamma ray detector (BSI, GCD 40190, S/N 2465-17) having a sensitive area of 63.3 mm diameter and 61.3 mm depth, operating on +2200 Volt bias voltage. The detector having relative efficiency for energy 1332 keV to NaI(Ti) is 43.1% with Peak to Compton ratio 71:1, an energy resolution of 1.74 keV FWHM at the 1332 keV peak of 60Co, and capability to detect photon energy ranging from 40 keV to 10,000 keV was enclosed in a cylindrical lead (206Pb) shield of 10 cm thick jacketed by a 5 mm steel outer housing with 1mm of tin to lower the background. This shielding arrangement is like a fixed bottom and removable cover type to place the sample on the top of the detector.
The counting system was connected to a 16 k (although for our study it was set to 8 k, as it’s better efficient at 8 k with HPGe for full range gamma spectra analysis) digital multi-channel analyser (MCA 527, GBS Elektronik GmbH) with associated electronics for data acquisition of photo-peak areas. SpectraLineGP software was used to analyse the gamma rays emitted from the samples. Energy and efficiency calibration were performed using a standard (Code: 8501-EG-SVE, Eckert & Ziegler Analytics), certified multi-nuclide gamma ray source (241Am, 109Cd, 57Co, 139Ce, 113Sn, 85Sr, 137Cs, 88Y, 60Co) having homogeneously distributed activity, maintaining the same geometry and density as the beakers containing the samples. In order to minimize the statistical counting error, the samples were counted for a period of 15,000 s. An empty container was counted under the same conditions to determine the background counts and was deducted from the counts of each sample to obtain the net counts as well as the specific activities.
For spectrum analyses, the single transition gamma ray line 1460.822 keV was used to determine the activity concentrations of 40K. The gamma ray photo-peaks of 295.221 keV and 351.922 keV from 214Pb, and 609.32 keV, 1120.31 keV, and 1764.551 keV from 214Bi were used to determine the activity concentrations of 226Ra. The activity concentrations of 232Th were determined using the net counts under the 238.63 keV and 300.087 keV photo-peaks from 212Pb, 911.205 keV, and 968.97 keV photo peaks from 228Ac, and 583.19 keV and 2614.533 keV from 208Tl. For the evaluation of 226Ra and 232Th activity, a weighted mean approach  was applied using the aforementioned gamma lines.
The activity concentration of each radionuclide of interest in each sample was evaluated using the following equation :
where “A” the activity concentration of the radionuclide in the sample, in Bq/kg, “N” the net counts under the corresponding photo-peak, “ ” the detection efficiency corresponding to each specific gamma ray, “I” the intensity of the corresponding gamma ray energy, “ ” the counting time in seconds and “w” the mass of the sample in gram.
The radioactivity of each sample is being reported with the counting error of one sigma. The analysis software Spectra Line GP calculates the standard deviation (SD) from the several values of uncertainty impute by the operator and their average for a radionuclide using Equation (2).
where, “n” is the total number of samples, “ ” is the arithmetic average and “ ” indicates individual samples value.
3. Results and Discussion
In this study, 37 samples of soils and industrial wastes were analysed to detect the level of radiation contamination and NORM’s. Among them, six samples are throw-out solid (mud, slug, and rust) waste collected near the metal processing industries. Table 1 shows the distribution of detected radionuclides and their specific activity (in Bq∙kg−1) on dry-weight basis. Among them, 10 samples have the 226Ra bellow the detection limit, whereas only in 4 samples 232The was undetected and apart from this, 40K was present as dominating amount over the other radionuclides in every sample.
Table 1. Activity concentration of detected radionuclides.
-bellow detection limit.
In sampling site s-05, it shows that the activity of 226Ra and 232Th are 131 ± 18.33 and 133 ± 15.96 Bq∙kg−1 respectively, which is quite elevated and almost four times higher than the world average , while the average activity of these two particular primordial radionuclides for all samples is still below with compared to the world average, according to UNSCEAR (2000) report . The most reasonable cause for much raised up activity in the aforementioned site is unauthorized disposal of the iron ore residue from the nearest metal processing factory. The distribution of 232Th over all samples is quite precise, as it shows a low standard deviation and almost 78% values are in the first sigma variance. For 226Ra the precision was also found in quit good, as almost 51% values are in first sigma, while 68% samples give the concentration of 40K within standard deviation. No 137Cs or other man-made isotopes have been detected in our studied sample, which indicates the sampling area is free from anthropogenic radionuclide contamination.
The biological effect of these ionizing radiation can be estimated by the means of absorbed dose. The measured activity concentration of 226Ra, 232Th and 40K are converted into doses rate (D, nGy∙h−1) presenting in Table 1 is done by applying Equation (3), which has been extracted from UNSCEAR, 2000 report.
were, , and are the activity concentration (Bq∙kg−1) of 226Ra, 232Th, and 40K respectively.
Ten samples from all studied data show the higher value of dose rate compared to world average (57 nGy∙h−1) reported by UNSCEAR with the maximum value of 157 nGy∙h−1 in sample s-05. This implies that the area of the collected sample will not be a good choice for living.
Annual effective dose equivalent (AEDE) received by an individual was also estimated from the dose rate. In this case, the quotient of effective dose rate to absorbed dose rate in air is taken as 0.7 Sv∙Gy−1 and 20% occupancy for outdoor while 80% for indoor as per suggestion of UNSCEAR report for environmental exposure to gamma rays of moderate energy .
Most of the samples have AEDE (outdoor) bellow the world average (0.07 mSv∙y−1), except ten samples having the higher. On the other hand, only the sample S-05 and S-17 exhibits the AEDE (indoor) higher than world average (0.41 mSv∙y−1) mentioned in UNSCEAR 2000. But in combined annual effective dose (indoor + outdoor), sample S-05 (0.963 ± 0.129 mSv∙y−1), S-17 (0.576 ± 0.090 mSv∙y−1) and S-24 (0.492 ± 0.072 mSv∙y−1) show the value higher than the world average (0.48 mSv∙y−1), whereas the average annual dose (0.29 ± 0.05 mSv∙y−1) for all samples is still below the world average. Table 2 listed the examined value of absorbed dose rate, annual effective dose equivalent for indoor and outdoor.
Radium equivalent activity (Raeq) concentration in Bq∙kg−1, is the widely used hazard index which implies the comparison of activity concentration of radium, thorium and potassium presented in the samples. It is assumed that 370 Bq∙kg−1 of 226Ra, 259 Bq∙kg−1 of 232Th and 4810 Bq∙kg−1 of 40K produced the same gamma-ray dose. Based on this, Raeq was calculated by using Equation (5) for this study .
Table 2. Dose rate (D), annual effective dose equivalent, radium equivalent activity ( ) and activity concentration index ( ) distribution among the studied samples.
where, , and are the activity concentration of 226Ra, 232Th and 40K in Bq∙kg−1 respectively. The Raeq results for the samples are found in the range of 9.24 ± 3.70 to 351 ± 46.53 Bq∙kg−1 with an average of 100.67 ± 17.97 Bq∙kg−1, which is still below the world average 370 Bq∙kg−1 if the materials are intended to use as building materials .
In order to assess the safety level of radiation, the European Commission proposed an index called gamma activity concentration index ( ). is calculated by using the following formula, Equation (6). 
where, , and are the activity concentration of 226Ra, 232Th, and 40K in Bq∙kg−1. If the samples are used as building materials and/or land development, the activity concentration index shall not exceed the following values shows in Table 3 depending on the dose limit criterion .
The distribution of the value of for the collected samples from Nasirabad industrial area, that are used for land development and house construction was studied and presented in Table 2. The average for soil samples is 0.40 ± 0.07 and for solid waste is 0.29 ± 0.07, and for all samples is 0.38 ± 0.07, which are <1. Nonetheless, the soil sample S-05 having the highest value, 1.23 ± 0.16. Therefore the annual effective dose delivered by the all other sample is smaller than the annual effective dose constraint of 1 mSv∙y−1. Hence, except sample S-05, all others can be allowed for land development and building materials for household construction.
External (Hex) and internal (Hin) hazard index are another two dimensionless parameter to estimate the level of hazard due to radiological constituent. The prime objective of this Hex and Hin is to estimate and take preventive measures for limiting the annual permissible dose for the public (1 mSv∙y−1) suggested by IAEA . Equation no (7a) and (7b) were used to calculate the aforementioned indices and the deduced value is presented in Figure 2.
Table 3. Reference level of for different dose criterion.
Figure 2. External (Hex) and internal (Hin) hazard index with compared to allowable limit.
where, , and are the activity concentration of 226Ra, 232Th, and 40K in Bq∙kg−1 respectively. The Hex must not exceed the limit of unity for the radiation hazard to be negligible. As well as, the Hin must also be less than unity to have minimal hazardous effect of radon and its progeny to the respiratory organs . In our study all of the samples from both soil and solid waste category having the Hex bellow the unity and only one soil sample S-05 shows the Hin value 1.30 ± 0.181, which is higher than unity. Nevertheless, the average of Hex and Hin is still within limit. Figure 2 is representing the analysis and result of these indices.
The soil and solid waste are nearby the industries in Nasirabad industrial area. Chattogram was collected and investigated through determining the activity concentration of terrestrial radionuclides (226Ra, 232Th, and 40K) using HPGe gamma ray spectroscopy. No trace of anthropogenic radionuclide (137Cs) was detected in any of the 37 samples i.e., this area is free from unwanted radioactive contamination. The solid waste samples are containing those primordial radionuclides in a noticeably low amount compared to other soil samples. This may due to the source of origin of these two category samples. Besides this, the dose rate from the samples was also determined, few samples show a slightly higher value compared to the world average. The indoor and outdoor annual effective dose equivalent was also studied. In this case, it was clearly found that the average result was below the safety limit except three samples having elevated value. Average of Radium equivalent activity concentration was found below the recommended level, which sounds good. The external, internal hazard index, and activity concentration index value were also measured for further evaluation. It’s seen that the averages of these indices are lower than the allowed safety level. The studied soil and solid waste samples from a growing industrial area will help to establish a baseline radiological data for further environmental safety precaution.
Authors are thankful to staff and authority of Atomic Energy Centre, Chattogram for their undisputable support to complete this study.
 UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation (1993) Sources and Effects of Ionizing Radiation. Report to the General Assembly with Scientific Annexes, United Nations, New York, 34-54.
 Ramola, R.C., Choubey, V.M., Prasad, G., Gusain, G.S., Tosheva, Z. and Kies, A. (2011) Radionuclide Analysis in the Soil of Kumaun Himalaya, India, Using Gamma Ray Spectrometry. Current Science, 100, 906-914.
 Harmsen, K. and de Haan, F.A.M. (1980) Occurrence and Behaviour of Uranium and Thorium in Soil and Water. Netherlands Journal of Agricultural Science, 28, 40-62.
 Peterson, J., MacDonell, M., Haroun, L., Monette, F., Hildebrand, R.D. and Taboas, A. (2007) Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas. Argonne National Laboratory Environmental Science Division, Human Health Fact Sheet, 38-39.
 Farai, I.P., Obed, R.I. and Jibiri, N. (2006) Soil Radioactivity and Incidence of Cancer in Nigeria. Journal of Environmental Radioactivity, 90, 29-36.
 Jeffree, R.A. and Simpson, R.D. (1986) An Experimental Study of the Uptake and Loss of Ra-226 by the Tissue of the Tropical Freshwater Mussel Velesunio angasi (Sowerby) under Varying Ca and Mg Water Concentrations. Hydrobiologia, 139, 59-80.
 Schotzig, U. and Debertin, K. (1983) Photon Emission Probabilities per Decay of 226Ra and 232Th in Equilibrium with Their Daughter Products. The International Journal of Applied Radiation and Isotopes, 34, 533-538.
 Khandaker, M.U., Uwatse, O.B., Khairi, K.A.B.S., Faruque, M.R.I. and Bradley, D.A. (2019) Terrestrial Radionuclides in Surface (Dam) Water and Concomitant Dose in Metropolitan Kuala Lumpur. Radiation Protection Dosimetry, 185, 343-350.
 Barua, T., Bhuian, A.K.M.S.I., Hossain, S., Deb, N., Ahmed, M., Rashid, M. and Khandaker, M.U. (2019) The Presence of Radioactive and Metal Contaminants in Wild Mushrooms Grown in Chattogram Hill Tracts, Bangladesh. Journal of Radioanalytical and Nuclear Chemistry, 322, 173-182.
 UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation (2000) Sources and Effects of Ionizing Radiation. Report to General Assembly with Scientific Annexes, United Nations, New York, 91-125.
 Abedin, M.J., Absar, N., Saiful, A., Bhuian, A.K.M.S.I., Kamal, M., Karim, M., Hazrat, M. and Miah, A. (2017) Determination of Radionuclide Concentration and Estimation of Dose Rate in Industrial Wastes of Selective Industrial Area of Chittagong and Sediments of Karnapuli River Near the Industrial Zone, Chittagong, Bangladesh. International Journal of Scientific and Engineering Research, 8, 1491-1503.
 European Commission (1999) Radiological Protection Principles Concerning the Natural Radioactivity of Building Materials. Environment, Nuclear Safety and Civil Protection, Radiation Protection 112, 8.
 Uddin, M.S., Barua, B.S., Shariff, M.A., Hasan, M.M., Rashid, M.A. and Kamal, M. (2014) Investigation of Elemental and Radiological Contamination of Soils in Two Shipyards in Chittagong, Bangladesh. Radiochimica Acta, 102, 8.