Radioactivity and health risk assessments in cement samples commonly used as construction materials in the case of South Gondar Zone, Ethiopia

. This work deals with the measurement of the radioactivity concentrations from different cement samples collected from the South Gondar Zone, Ethiopia. The measurement of naturally occurring radioiso-topes and radiation health impact indices caused by these nuclides in cement samples are indisputable in the dwellings. Specifically, the average concentrations for 226 Ra, 232 Th and 40 K were estimated as 76.53, 81.67, and 407 Bq kg − 1 , respectively. It was seen that all the measured mean values were greater than the world’s limited levels. External and internal hazard indices are calculated as defined by the European Commission from the corresponding concentrations of 226 Ra, 232 Th and 40 K nuclides. Moreover, the natural radionuclides have variety of concentrations which must be caused by the types of raw materials used in the industries for the production of cement.


Introduction
Knowing of man about naturally occurring radioactivity concentrations and their distribution in the living areas have become a focus of much attention in assessing the human health indices.Naturally occurring radioactive materials are widely spread in the earth's environment and exist various geological formations such as soils, rocks, plants, water bodies, air, and construction materials [1][2][3].Finding of levels for these radioactive elements in building equipment plays the prominent roles in the evaluation of public health indices, as most citizens kill 80% of their time indoors in either of living homes or working offices [4][5][6][7].Naturally occurring radiation sources in construction materials are means of external particle exposure sources in dwellings [3,[5][6][7].Thus, Uranium, Thorium and their daughter products and 40 K are important elements which played the predominant role to contribute a large part of energy dose received by people [8].The radiation is caused by gamma ray originating from the Uranium, Thorium series and from 40 K [8][9][10].
The average levels of these radionuclides in the natural environment are 35, 30 and 400 Bq/kg, respectively [4,5].The accepted values in all parts of the world average due to gamma ray from construction materials is equivalent to 0.4 mSv/y [4].This external radiation exposure caused by those gamma releasing material examining of building equipment with the dose rate modeling.This outdoor dose estimation due to the natural sources is essential as these doses vary depending upon the values of the radioactive materials of 226 Ra, 232 Th and their decay products and 40 K, found in soil and rocks which more depending upon local soil and rock types of each country [13].Since radium is found everywhere in the environment, its essential daughter nucleus radon is found everywhere with varying concentration and its solid daughter products of 218 Po and 214 Po which have a chance to distribute themselves over the aerosols in ambient air [3,[11][12][13][14][15].When we take or inhale radon gas or and its daughters along with normal air, most of radon is exhaled, and its decay products got logged to the inner walls and membranes of our respiratory organ and continue constant change due their alpha ionization power.Different cancers and kidney diseases are so influenced by the intake of solid radon daughter products [5,[16][17][18].
Radiological health disturbance caused by radionuclides due to particle energy exposure of a body by the gamma rays and irradiation of the lung cells from inhalation of 222 Rn is therefore attracted the world researchers for more investigations [16].Thus, radiation sources and radioactivity in building materials have got considerable research interest hence humans are exposed to radiation sources at different levels based on natural radioactive minerals present in each region worldwide [19][20][21][22][23][24].The purpose of this study is to evaluate the naturally occurring radioactivity values of the commonly used construction material called cement in South Gondar zone in the unit of Bq kg −1 and to estimate the health impact caused by these radioactivity.

Location of study area
South Gondar is located in Amhara Region, Ethiopia.This zone is named for the city of Gondar, which was the capital of Ethiopia until the mid-19 th century, and has often been used as a name for the local province.South Gondar Zone is bordered on the south by East Gojjam, on the southwest by West Gojjam and Bahir Dar, on the west by Lake Tana, on the north by North Gondar, on the northeast by Wag Hemra, on the east by North Wollo, and on the southeast by South Wollo; the Abbay River separates South Gondar from the two Gojjam zones [25].

Sampling techniques
The main cement stores are located in Debre Tabor (capital of South Gondar Zone), Nefas-Mewucha and Woreta towns.Hence, it is impossible to get all types of cement in one town, we collected the samples from three distribution areas to get representative cement types.In this regard, for this study, thirty cement samples were taken from Dangote, Derba, and National Cement Factory suppliers found in the study area.Approximately, 1 kg of cement sample was collected from the source using cement bags in places where the required sample types are found in the South Gondar (Fig. 1).

Sample preparation procedure for gamma-ray spectrometry
All the assembled cement samples have been sieved using 400 µm and were dried at a temperature of 100 • C about 16 hours until they reached a constant weight to eliminate moisture and humidity.After moisture removal, the samples were cooled in moisture-free atmosphere at room temperature [26].The dried cement samples were stored into pre-weighting cylindrical plastic container (beaker) of that has a diameter of 6 cm and 7 cm height within the overall volume of 197.82 cm 3 .The cylindrical plastic container were hermetically sealed with vinyl tape around its screw neck to prevent possible escape of radon gases.The default method of radon sealing is done by placing the sample container into an aluminum-lined bag and evacuating it using a Henkovac Mini Eco commercial vacuum packaging machine [27].The closed samples were stored for a minimum period of 30 days to allow for a secular equilibrium between 226 Ra, 222 Rn and its short-lived decay products before gamma spectrometer measurements.
The measurement was carried-out using p-type high purity Germanium detector which has an efficiency of 48% at 1332 keV and resolution of 1.786 keV with 60 Co isotope.The detector was covered by lead at the coat of 10 cm diameter.A used lead shield was with a fixed place at a base and a moving cover to shield the detector from the above to prevent background radiation interactions.The 2 mm thick copper lining was used at the inner side of the shield to protect the detector from unwanted particles produced by the interaction of background with the material around a lead detector.A photo-peak efficiency was determined as a function of the gamma-emission energy, E γ .These efficiencies were fitted to the energy values [7].
where a i is the fitting coefficient, and E γ is the energy expressed in MeV.By focussing attention on the energies emitted from the radionuclides of interest 214 Pb, 214 Bi, 228 Ac and 40 K efficiency versus density was fitted to an exponential decay function, for each gamma-energy, in order to obtain an empirical relationship between both.This fitting function is [8]: where a, b are the fitting parameters and ρ is the sample's density expressed in kg/m 3 .The activity of 226 Ra was measured by the photo-peaks of 609.35 keV and 351.9321 keV from 214 Bi and 214 Pb radionuclides, respectively.The activity of 232 Th was determined through the photo-peak of 238.632 keV, 583.187 keV and 911.204 keV emitted from its short-lived decay products of 212 Pb, 208 Tl and 228 Ac, respectively, while the 40 K activity was directly measured using 1460 keV peak of its gamma-ray energy [26,27].
The choice detector has a high efficiency for typical samples, the high purity germanium detector is the Canberra Broad Energy Germanium (CBEGe) which exhibits lower background than typical coaxial detectors since it is more transparent to high energy gamma from naturally occurring radioisotopes such as 40 K and the Thorium series of 208 Tl [4].The function of any detector is to convert radiations' energy into useful electrical signals which can be used, with suitable signal processing.The correct efficiency calibration requires the correct homogenization of the tracer in the cement, because a heterogeneous distribution would produce an imprecise efficiency determination, that, therefore, would affect negatively the activity values obtained for all the measured samples using such efficiency.Considering the crucial importance of this issue, the homogenization has been verified by measuring the prepared standard for several times and shaking the standard in a strongly way during 10 min before performing each measurement.The standard was measured a total of 13 times, being 2 h the acquisition time, in order to reduce the counting statistical uncertainty below 1% for all the gamma emissions.
The detector was calibrated based on efficiency calibration with a reference geometry: density, volume, radionuclides, summing corrections, and so on.The density, volume and height of sample in the container have been the same as that of the standards (Marinelli beakers used for the standard mixed radionuclide and also for the samples) used for the calibration. 241Am, 133 Ba, 60 Co and 152 Eu IAEA certified radioactive sources were used to determine the efficiency curves.as well the certified activity by IAEA on 01/01/1982 [28].Generally, the efficiency calibration of the HPGe detector available at the Zentrum für Strahlenschutz und Radiooekologie (ZSR) Hannover University is presented.A detailed information of the standards is available at https:/www.eurostandard.cz/products.html.
The quality assurance of the measurement was carried out periodical efficiency and energy calibration and repeated sample measurement.The counting time of each sample was determined to be 36,000 seconds and repeated twice to get the accepted gamma spectrum with special statistics.To find out the background distribution of radiation spectrum in the living environment around the detector, an empty container was measured in the same manner in the same geometry as we did for the samples.The net peak area of gamma rays for the measured radioisotopes were corrected by using the background spectra [29].
The radioactivity values of terrestrial environment sources of radiations in the counted samples in Bk kg −1 were calculated using the relation [5,12]: where W is the net counts of gamma-ray, is the efficiency of a detector, P is the absolute transition of gamma decay, M is mass of a final prepared sample in kg, and t is a counting time in international unit system.

Results and discussions
The radioactivity levels of naturally occurring radioactive materials of 226 Ra, 232 Th and 40 K were evaluated for 30  1 and the radiological health indices for risk assessments are depicted in Table 3.
As could be stated in Table 1, the activity concentrations of natural radionuclides of 226 Ra, 232 Th and 40 K ranges from 20 ± 10.50 to 100 ± 4.36, 30 ± 9.60 to 101 ± 3.59, and 130 ± 51.44 to 550 ± 26.55 Bq kg −1 , respectively.All the calculated average concentrations are higher than the world's written standard values in the naturally living environment of 35, 30, and 400 Bq kg −1 , respectively [4].The maximum values of 226 Ra were observed in the samples which were taken from Derba and National Cement Factories and a minimum value was measured from Dangote Cement industry, while the National Cement and Dangote Cement Factories have recorded the maximum, and Derba Cement Factory has relatively lowest levels of 232 Th and 40 K.Moreover, it is observed that there are different levels of concentrations for the same type of cement.This may be happened due to the composition of cement type.Some experimental values presented for reported data of other countries are given for comparison in Table 2.In this situation, the 226 Ra mean activity concentration for cement samples was higher than the concentration for Riyadh-Saudi Arabia, Turkey, India (2020), India (2016), Tamilnadu India, Iraq and Slovak Republic, while less than the concentrations for China and Greece.All the observed values were less than for 232 Th activity concentrations which investigated in this study.The mean activity concentration value of 40 K was 407 Bq kg −1 .This value was lower than for India, Turkey (2493 Bq kg −1 ), and China, and higher than for all other studies in the table of comparison.According to our results, the distribution of 226 Ra, 232 Th and 40 K in cements were not constant for all samples.The reason why is because the raw materials which are used by the factories have been taken from different geological places and material types.The nonhomogeneous which occurs within exposure to radiation could be defined due to the variety values of radium equivalent activity (Ra eq ) in Bq kg −1 that was generated from the specific activity of materials containing different amounts of 226 Ra, 232 Th and 40 K [5].The radium equivalent activity estimated through the following relation [4,5] where B Ra , B Th and B K are the activity concentrations of radium-226, thorium-232 and potassium-40 in Bq kg −1 , respectively.The mean Ra eq values are tabulated in Table 3 for the different cement types where they were collected.The minimum value (151.65 ± 13.43 Bq kg −1 ) was measured for the samples that were taken from Dangote Cement Facory, while the maximum (276.53 ± 9.67 Bq kg −1 ) value of Ra eq was found in Derba and National Cement Factories cement types, respectively.In general, the average values of Ra eq for the studied cement samples were lower than the limit value of 370 Bq kg −1 recommended by Organization for Economic Cooperation and Development [32].The terrestrial environmental radiation sources of indices dealing with the assessment of the excess gamma radiation arising from construction equipment such as outdoor and indoor hazard indices and gamma radiation indices has been investigated by several investigators [33][34][35][36][37][38][39].For this study, the gamma index (I γ ) was computed using the relation recommended by the European Commission and used by [7,32] where B Ra , B T h and B K are the activity concentrations of radium-226, Thorium-232 and potassium-40, respectively and 300, 200 and 3000 are given in Bq kg −1 .The values of I γ evaluated from the calculated activity concentration of radium-226, Thorium-232 and potassium-40 are presented in Table 3 for different cement types.The values of I γ for the studied samples varied in the range between 1.053 ± 0.1 to 1.963 ± 0.1 for type of cements which were greater the critical value of unity.
At this particular time, several alpha indices have been recommended to assess the exposure amount due to radon inhalation originating from construction materials [7].The alpha index, therefore, was calculated by the following relationship where B Ra is the activity value of radium-226 in Bq kg −1 that assumed in equilibrium with uranium-238.The proposed widely used in all walks of life for lower and upper level of radium-226 activity concentrations in construction equipment are 100 and 200 Bq kg −1 , respectively, as suggested by ICRP [40].When the materials are used for construct the dwellings or towers, the concentrations of 226 Ra never exceeded from the higher recommended value of 200 Bq kg −1 for which I α will have a value equal to unity or greater than 1.While exceeding values results that enough radon could exhalate from these materials which may cause indoor radon value that will exceeded 200 Bq m −3 too.In other words, if 226 Ra has the concentration less than 200 Bq kg −1 , then it resulting indoor radon level which is less than 200 Bq m −3 .Thus, such considerations determined the I α index values which could be less than or greater than unity.For the studied cement samples, I α index values are given in Table 3 for the various types of cement.The estimated concentrations of I α for this investigation, ranged from 0.10 to 0.50, with the average of 0.38.Accordingly, our result shows the mean value calculated for I α index was exactly less than unity.Mainly, of course, the buildings will cause excessive radiation exposures to the human body due to gamma rays released by lead-214 and bismuth-214 which are progeny of radium-226 and thorium-232 decay chains, respectively and the contribution of 40 K to the human body radiation dose is also considered.The intake dose rate in indoor air due to gamma-ray released from radioactivity levels of radium-226 and thorium-232 and potassium-40 was computed using the following formula by applying the factors of 226 Ra, 232 Th and 40 K in The factors listed above were used to estimate the overall received dose in air at 1 m to the top from the ground level as recommended by UNSCEAR [8].It may also be seen that, the inhaled dose in indoor air computed from the computed activities in different cement samples are shown in Table 3 (column 5).The calculated absorbed dose rate for the case of indoor air was found to vary from 68.21 ± 4.50 to 126.77 ± 6.19 nGyh −1 with a mean of 103.05 ± 2.87 nGyh −1 .It was compared that the mean value was higher than the world average recommended value of 84 nGyh −1 .Results tabulated in Table 3 show that the minimum and the maximum absorbed dose rates in air were delivered by cement samples which have basically higher activity concentrations of radium and thorium, respectively.To evaluate the annual effective dose, it must be taken into account that: (a) the conversion coefficient (0.7 Sv Gy −1 ) from absorbed dose in air to effective dose and (b) the indoor occupancy factor is 80% for population specially for mothers and children [8,32].Therefore, the effective dose rate in mSv y −1 unit was calculated by the following formula: AED = D × 8760 hy −1 × 0.7 × 0.8 × 10 −6 . ( The measured values of annual effective dose for the selected cement samples were ranged from 0.3346 to 0.6219 mSvy −1 , with an average value of 0.5055 mSvy −1 . The corresponding annual effective dose rate from all samples were below the dose criterion of 1 mSv y −1 .The external and internal living environment must qualify for healthy family.In order to keep the safety of the human health from the impact from radon and its progeny which may give rise to indoor radiation exposure, indoor hazard index (H in ) was evaluated by using the formula stated by [41].
The values of H in gained are depicted in columns 7 in Table 3.The values are ranged from 0.5177 ± 0.06 to 1.0118 ± 0.04 with the mean of 0.8122 ± 0.02 which are slightly less than unity as required were obtained for H in from all samples except the maximum of 1.0118 ± 0.04.The external values of H ex were also less than unity

Table 1 .
The radioactivity concentrations of natural radionuclides in cement samples in Bq kg −1 .

Table 2 .
Comparison of mean activity concentrations inBqkg −1 with studied results in cement samples from some selected countries.

Table 3 .
Radiological health indices parameters values for different cement types.