Abstract
The areas around Homa and Ruri hills in Homa Bay County in Kenya are associated with high background radiation levels. The activity concentration of the natural radionuclides (226Ra, 232Th, and 40K) in earthen building materials used in the areas of Homa and Ruri hills has been measured using a NaI (Tl) detector in this work. The measured values of radioactivity concentrations are used to estimate the associated radiological risk. The earthen building material samples from Ruri registered relatively high 232Th concentration values averaging 1094 ± 55 Bq/kg, nearly three times those of the samples from Homa. 226Ra level was not significantly different in both regions with Homa reporting 129 ± 10 Bq/kg and Ruri 111 ± 6 Bq/kg. 40K was however higher in the samples from Homa by an approximate factor of 2 relative to those from Ruri where the activity concentration was 489 ± 24 Bq/kg. The radium equivalents for 226Ra, 232Th, and 40K in the samples from Ruri were 111 ± 9, 1564 ± 125, and 38 ± 3 Bq/kg, while in Homa, the values were 129 ± 10, 570 ± 46, and 69 ± 5 Bq/kg, respectively. The calculated value of total radium equivalent in Ruri was 1713 ± 137 Bq/kg which was two times higher than that of Homa. 232Th contributed about 74% and 91% to the total radium equivalent in Homa and Ruri, respectively; thus, it was the one with the largest contribution to radiation exposure in both regions. The average indoor annual effective dose rates were 1.74 ± 0.14 and 3.78 ± 0.30 mSv/y in Homa and Ruri, respectively, both of which were above the recommended safety limit of 1 mSv/y.
1. Introduction
Natural radiation in the environment contributes approximately eighty percent of the total radiation exposure to the general public. The major categories of natural exposure include inhalation of radon and thoron, external exposure from (226Ra, 232Th, and 40K), cosmic radiation, and ingestion of food and water [1]. The major natural contributors to external exposure are the primordial radionuclides 226Ra, 232Th, and 40K which are not uniformly distributed in the environment but occur in varying quantities in rock and soil as characterized by the geology of a region [2]. The average worldwide dose rate as a result of these terrestrial radionuclides is about 60 nGy−1 for areas with normal background. Therefore, it is important to determine their levels in soil and rocks, as well as their individual contributions to the total radiation dose for purposes of radiation protection and management [3]. High background radiation areas (HBRAs) are characterized by abnormally high levels of background radiation; they are distributed throughout the world, e.g., Yangjiang, China; Guarapari, Brazil; Ramsar, Iran; and Kerala, India [4, 5, 6, 7]. In Kenya, some of the HBRAs include Mrima hill in the coastal part of Kenya and Homa and Ruri in southwestern Kenya [8, 9, 10]. Studies carried out, for instance, in high background radiation areas of Ramsar, Iran [11], and Mrima hill, Kenya, have shown that building materials contribute significantly to indoor radiation exposure. Homa and Ruri are both experiencing increased growth in human settlement with readily available soil being used as a building material. Therefore, there exist a potential radiation risk indoors from the soil used as the building material in these regions and the fact that people generally spend more time indoors. Despite this apparent risk, there are no data on radiation exposure as a result of the terrestrial radionuclides inside the local earthen dwellings in Homa and Ruri which this research seeks to determine. This paper reports the activity concentration of the primordial radionuclides in the earthen building materials used in the two regions as well as the risk indices associated with them. The radionuclides responsible for the highest radiation exposure are also determined.
2. Methodology
2.1. Study Area
Homa and Ruri hills are located in Homa Bay County along the shores of Lake Victoria in Kenya.
Homa hill is located between latitude 0° 30′ N and 0o 20′ N and longitude 33o 26′ E and 34o 34′ E. This is mainly covered by a large carbonatite peninsula complex on the eastern shores of Lake Victoria with a series of cone sheets of carbonatite and breccia intrusions in the oldest rock in the Nyanzian series and ijolites [12].
Ruri hill is located at latitudes 0o 30′ S and 1o 00′ S and longitude 34o 30′ E and Lake Victoria shoreline. The altitudes range from about 1000 m to approximately 1800 m at the hilltop. This area is mainly covered by Precambrian metabasalt of the Nyanzian type of rocks composed of ijolites and the nepheline syenites [13]. The hill also has a ring-shaped intrusion of carbonatites of lower tertiary age and monazite and pyrochlore minerals associated with high 232Th levels [14]. Figure 1 shows the map of Homa Bay County and the two hills with the sampling points marked with the dots around each region.
2.2. Sample Collection and Its Preparation
Fifteen soil lumps were chopped off from the earthen walls of randomly selected houses in each region. In the laboratory, each lump was crushed to a fine powder before drying in an oven at a temperature of 110°C for a period of 24 hours. 250 g mass of each soil sample was then sealed in a Marinelli beaker, labelled, and stored for about 4 weeks to attain radioactive secular equilibrium between 226Ra (238U decay chain) series and 232Th series and their daughters [15].
3. Experimental Techniques
3.1. Radionuclide Concentration Analysis Using Gamma-Ray Spectroscopy (NaI (Tl))
The gamma-ray spectrometer used in this work is composed of a 76 mm × 76 mm thallium-activated sodium iodide (NaI (Tl)) single-crystal detector and an Oxford PCA-P multichannel analyzer which is a PC-based plugin PCI card. It consists of an 80 MHz Wilkinson analogue-to-digital converter for spectral data acquisition. The energy calibration of the detector was done using caesium-137 at the energy peak of 662 keV and cobalt-60 at energy peaks of 1170 keV and 1330 keV. The detector efficiency calibration was done using International Atomic Energy Agency (IAEA) standard-certified reference materials RGU-1, RGTh-1, and RGK-1 having the same geometry as the samples, and each was counted for a period of 30,000 seconds. 226Ra and 232Th activity concentrations were determined based on the 214Bi gamma energy peak of 609 keV and 208Tl at the energy peak of 2615 keV, respectively, while for 40K, an energy peak of 1460 keV was used from the spectrum of the background counting. The background counts were then used for the correction of net peak area of gamma rays of the measured standard isotopes. The minimum detectable activity (MDA) for 40K, 232Th, and 226Ra was determined as 1.4, 0.196, and 0.401 Bq, respectively.
The activity concentration was determined by using the following equation [15]:where Ai is the activity concentration of radionuclide i, N is the residual net counts at the peak energy of interest, γ is the emission probability of the gamma ray of interest, m is the mass of the sample in kg, is the detection efficiency of the gamma ray of interest, and t is the acquisition time in seconds.
3.2. Radium Equivalent Activity (Raeq)
Radium equivalent activity is a single value that describes the gamma output from the terrestrial natural radionuclides as determined by the following equation [15]:where ARa, ATh, and AK are the activity concentrations of 226Ra, 232Th, and 40K, respectively. 1.429 and 0.0769 are conversion factors for 232Th and 40K, respectively.
3.3. Absorbed Gamma Radiation Dose Rate (D)
Absorbed gamma radiation dose rate is the dose of ionizing radiation per unit time and is dependent on the concentration of the terrestrial radionuclides in the earthen building materials. The absorbed gamma dose rate D (nGy/h) in air considered 1 m above the ground surface was determined using the following equation [1]:where 0.462, 0.604, and 0.0417 nGyh−1/Bqkg−1 are dose conversion factors for 226Ra, 232Th, and 40K, respectively, and ARa, ATh, and AK are the activity concentrations of 226Ra, 232Th, and 40K in Bq/kg, respectively.
3.4. Indoor Annual Effective Absorbed Dose Rate (AEDR)
Indoor annual effective absorbed dose rate is the measure of biological effect of radiation on humans inside a dwelling made of the soil. It was determined by the following equation [1, 16, 17]:where AEDR is the indoor annual effective absorbed dose rate in mSv/y, D is the absorbed dose rate in nGy/h, 8760 is the time in hours for a whole normal year of 365 days, 0.6 is the rural Kenya indoor occupancy factor [8], and 0.7 Sv/Gy is the gamma dose conversion factor; 1.4 is a factor that accounts for the indoor environment given that gamma dose rates indoor are about 1.4 times higher than outdoors [8, 16, 17].
4. Results and Discussion
The activity concentration of 226Ra, 232Th, and 40K in Homa and Ruri is summarized in Tables 1 and 2, respectively. In both regions, the activity concentration of 226Ra had no significant difference between them with average values of 129 ± 10 Bq/kg and 111 ± 6 Bq/kg. respectively. The average value of 232Th was approximately 60% higher in Ruri compared to Homa which had an average of 399 ± 20 Bq/kg; this was attributed to the ring intrusion of monazite and pyrochlore minerals in Ruri associated with higher thorium concentration [13]. On the contrary, the average activity concentration of 40K was about 40% higher in Homa than Ruri which had an average of 489 ± 24 Bq/kg, which was attributed to the alkaline igneous rocks in Homa associated with higher potassium levels [18]. Average activity concentration of 226Ra was approximately 3 times higher than the world average of 35 Bq/kg in both Homa and Ruri.
The average activity concentration of 232Th was 13 and 36 times higher than the world average of 30 Bq/kg in Homa and Ruri, respectively. The arithmetic mean of 40K was twice that of the world average of 400 Bq/kg in Homa but was nearly equal to the mean value in Ruri [1]. Radium equivalent (Raeq) for 226Ra, 232Th, and 40K and the total radium equivalent in Homa and Ruri are presented in Tables 3 and 4, respectively. The average radium equivalents for 226Ra were more or less the same in both regions given their nearly equal activity concentrations. Raeq for 232Th was 60% higher in Ruri which had an average of 1564 ± 125 Bq/kg, while 40K in Homa was higher by a factor of 2 relative to Ruri which was 38 ± 3 Bq/kg.
Figures 2 and 3 show pie chart representation of the contribution of 226Ra, 232Th, and 40K to total Raeq in Homa and Ruri, respectively. In Homa hill, 232Th contributed 74% as 40K contributed 9% to total Raeq despite 40K having the highest activity concentration, while in Ruri, 232Th contributed 91% to total Raeq, the lowest contributor still being 40K at just 2% of total Raeq. 232Th was therefore the highest contributor to the total radium equivalent and radiation exposure in both regions. The determined average total radium equivalent in Homa was 767 ± 61 Bq/kg which was just 40% of the total average radium equivalent in Ruri.
The indoor annual effective dose rate (D) is determined from the absorbed gamma radiation dose rates in Tables 3 and 4 for Homa and Ruri, respectively, using equation (4). The average annual effective dose rate in Homa and Ruri was 338 ± 30 nGy/h and 733 ± 66 nGy/h, respectively, both of which were above the world average of 84 nGy/h [19]. The average annual effective dose rate in Homa was 1.74 ± 0.14 mSv/y which was about half that of Ruri. Figure 4 shows a bar graph presentation of the percentage contribution of 226Ra, 232Th, and 40K to the total annual effective dose rate. 232Th contributed the highest percentage of about 65% and 85% to the indoor annual effective dose in Homa and Ruri, respectively, compared to 40K and 226Ra. 40K contributed the least to the effective dose in both regions despite it having a high activity concentration. Approximately 80% of the sampled points in Homa had indoor AEDR above the recommended safety limit of 1 mSv/y, while all the sampled points in Ruri were above this limit [19].
The results obtained in this work have been compared with results reported in building materials in other high background radiation areas in Kenya and others around the world as tabulated in Table 5 [8, 11, 20, 21]. 40K was 70% and 50% higher in Homa and Ruri, respectively, compared to Mrima hill, Kenya. On the contrary, 232Th was 60% higher in Ruri compared to Mrima which attributed to monazite and pyrochlore minerals in Ruri which contains higher thorium levels [1].
5. Conclusion
The levels of activity concentration of 226Ra, 232Th, and 40K in earthen building materials used in high background radiation areas of Homa and Ruri have been assessed using the NaI (Tl) detector. The average concentration of 226Ra, 232Th, and 40K was above the world average values of 35 Bq/kg, 30 Bq/kg, and 400 Bq/kg, respectively [1]. The radium equivalent, absorbed gamma radiation dose rate, and indoor annual effective dose rate have also been determined from the measured activity concentrations. 232Th was the highest contributor to the total radium equivalent and indoor annual effective dose rates in both Homa and Ruri; it is therefore the radionuclide responsible for the largest radiation exposure in the two regions attributed to high monazite levels associated with high 232Th levels. The determined average indoor annual effective dose rates were all above the recommended safety limit of 1 mSv/y in both Homa and Ruri [19]. Therefore, the earthen building materials in both hills are not safe for the construction of the dwellings.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors would like to thank the National Commission for Science, Technology and Innovation (NACOSTI) for the research permit they granted them.