Assessment of radioactivity concentration for building materials used in Babadogo Estate, Nairobi City County, Kenya

Abstract

Natural radioactive materials in certain conditions can get to hazardous radiological level. The aim of the present work was to evaluate the natural activity concentration from sampled building materials collected from different locations in Babadogo Estate within Nairobi City County. The analysis done using gamma ray spectrometer, which was put into action for spectral data acquisition and then analysis. The activity concentration levels of ²³⁸U, ²³²Th and ⁴⁰K for the selected samples of building materials was measured by the use of gamma ray spectrometry method. The analyzed data compared with the standard acceptable values. The activity concentration in ⁴⁰K varied from 55 ± 3 to 2647 ± 132 Bq kg⁻¹, giving an average (sum of all values divided by 33) value of 831 ± 42 Bq kg⁻¹; ²³⁸U varied from 39 ± 2 to 3602 ± 180 Bq kg⁻¹, giving average figures of 378 ± 19 Bq kg⁻¹ and ²³²Th ranged from 5.000 ± 0.300 to 4213 ± 211 Bq kg⁻¹, giving average figure of 290 ± 15 Bq kg⁻¹. The calculated average figures for activity concentration surpassed the world average values of 420, 33 and 45 Bq kg⁻¹ in ⁴⁰K, ²³⁸U and ²³²Th, respectively.

Introduction

Natural radioactivity has been in existence since the onset of the universe owing to long half-life in the natural radioactive elements found in the earth’s lithosphere. The radionuclides in ²³⁸U, ²³²Th and ⁴⁰K are native to all types of gravels, granites, sand and gypsum from which building materials are composed⁽¹⁾. Building is an essential part in human life, owing to the fact most of lifetime is spent at our homes or in offices. Indoor gamma radiation mostly is as a result of building materials likewise terrestrial and cosmic radiation⁽²⁾. The activity concentration of the radionuclides in ²³⁸U, ²³²Th and ⁴⁰K used in construction materials, that primarily builds upon on geological and geographical situations, in addition to the geochemical distinctive principle of the building materials⁽³⁾. Although information on the activity concentration levels in building materials from Kenya is hardly accessible. This, therefore, called for a need to do a research study on building materials used in Babadogo Estate, as it might be regarded to be having superior activity concentration of naturally occurring radioactive materials. Natural activity concentration data in building materials are thusly imperative in the evaluation of the desirable radiological hazards. The sand for buildings is mainly brought from Machakos County⁽⁴⁾. Presence of phosphate and granite rocks in Machakos County and Nairobi are due to the neighboring Machakos County, which has contributed to the elevated levels of activity concentration. According to study⁽⁵⁾, naturally occurring radioactive materials in the environment is enhanced from these rocks. Building materials contribute ~0.008 microsievert (μSv) per hour⁽⁶⁾. The results from this study provide valuable information and statistics on activity concentration of radionuclides. Risks from radiation exposure cannot be eradicated absolutely but can be confined. Deterministic and stochastic are the two side effects from the exposure of radiation to human health since dose rate or risks of radiation-induced health hazard are neither nonexistent nor too small to be observed⁽⁷⁾.

From the research work therefore, statistics and information about the activity concentration in sand, in clay bricks and in concrete blocks used for building in Babadogo Estate will be produced. This will help scientific committees, International Institutions like United Nation Scientific Committee on Effect of Atomic Radiation, Radiation protection Board and others mandated to manage the environment.

Method

Random samples of building materials for sand, clay bricks, as well as concrete blocks were collected from construction site, in Babadogo Estate Figure 1. The sampling method employed here was random sampling, to enhance statistical sensitivity of the sampling⁽⁸⁾. Thirty-three samples of clay bricks, concrete blocks and some sand of approximately between 1.5 and 2 kg were collected first from each site then placed in carton boxes. All the samples were then ground, crushed into powder form, sieved then lastly dried in an oven at 110°C as long as 24 hours. To obtain uniform particle sizes, a 1 × 1 mm meshed sieve was used. Standard plastic Marinelli beakers accurately weighed and labeled was used to pack the dry samples; the beaker together with the samples was ~450 ± 1 g. The hermetically sealed samples were stored for a period of 4 weeks prior to counting to secure radioactive stability among 226-Ra and its short-lived progeny⁽⁹⁾. Again storing for 4 weeks was to enhanced secular equilibrium between ²³⁸U as well as ²³²Th and their respective daughters.

Figure 1.

Map of Nairobi of county where sampling was done.

NaI(T1) gamma ray spectrometer

Determination of energy distribution by gamma rays discharged by the nuclei is called gamma spectroscopy. When the energies of the gamma ray photons are discharged from the nuclei, gamma ray spectrometer determines it as well as the count rate. The detailed analysis of the spectrum obtained from the Multichanel Analyzer determines the character as well as the quantity in the gamma ray emitters present in the headstream. Radioactive elements decay to emit different radiations, which include alpha particles radiation, beta particles radiation as well as gamma rays. Gamma ray decay is where nucleus goes from an excited state to a state of lower energy. In this processes the difference in energies between the two states is what liberates to form a photon. Stable Nickel-60 isotope is obtained when Cobalt-60 undergoes the process of beta decay.

Calibration

The three standard materials acquired from International Atomic Energy Agency were used for system calibration. This was done at Kenyatta University Physics laboratory prior to spectral data acquisition from the samples. At the start of every measurement session energy calibration was carried out to cater for the weather condition variation, vibrations as well as heating up of the detector. Samples placed in a sealed Marinelli beakers were put in the detector one at each time then counted for ~30 000 seconds. Before these counts, background counts were recorded first under similar factors of those of the samples measured later on the figures (background) were subtracted at the end of the counts of all the 33 samples in order to find net counts for the samples.

Calculation of activity concentration levels

Comparison method was used for the calculation of activity concentration of the radionuclides in all the 33 samples, as in the Equation (1)

(1)

In Equation (1), represents the summation of the masses, A represents the activity concentration of the actual sample, while Mi represents the mass of the sample and Ii represents the intensity and i is the sample number.

Results and discussion

The study intended to assess the activity concentration levels for the naturally occurring radionuclide for building materials used in Babadogo Estate, Nairobi City County. The use of NaI(Tl) detector came in handy for the research work. Activity concentrations for the selected building materials samples were measured. These building materials used for construction in Babadogo Estate are known to come from various regions or Counties surrounding Nairobi County. The sand used is mainly from Machakos County⁽⁴⁾. The presence of phosphate and granite rocks, which undergo the process of weathering as a result of change of weather pattern in Machakos County, enhances the formation of sand in this region with elevated levels of radioactive materials, the (NORM) in the environment⁽¹⁰⁾. Heavy rain falls in the regions neighboring Nairobi County, such as Kiambu, Machakos and Kajiado causes the weathering of rocks that forms traces of radionuclides to be deposited in river banks and later collected with the sand for building. In addition, the weathered particles form surface soil, which is used in the manufacturing of clay bricks.

The measured values of the activity concentrations in radionuclides for ⁴⁰K, ²³⁸U and ²³²Th are presented in Table 1. Average value of activity concentration in radionuclides ⁴⁰K was higher than that for ²³⁸U and ²³²Th, as a result of potassium being the major contributor of the naturally occurring radionuclide. The measured activity concentration for ⁴⁰K varied from 55 ± 3 to 2647 ± 132 Bq kg⁻¹ translating to average value of 831 ± 42 Bq kg⁻¹. Activity concentration for ²³⁸U varied from 39 ± 2 to 3602 ± 180 Bqkg⁻¹ giving the average value of 378 ± 19 Bqkg⁻¹ and that of ²³²Th had minimum of value of 5 ± 0.03 Bq kg⁻¹ while the maximum value was 4213 ± 211 Bq kg⁻¹ and the average value was 290 ± 15 Bq kg⁻¹. The maximum activity concentration for ⁴⁰K,²³⁸U and ²³²Th in the selected building material samples used in Babadogo Estate were: 2648 ± 132 Bq kg⁻¹, 3603 ± 180 Bq kg⁻¹ as well as 4214 Bq kg⁻¹ at the same time the minimum activity concentration for radionuclides are: 55 ± 3, 40 ± 2 and 6 ± 1 Bq kg⁻¹.

Table 1.

Activity concentrations values for the natural radionuclides ⁴⁰K, ²³⁸U and ²³²Th measured for the research work in Babadogo Estate building materials, units in Bq kg⁻¹.

Sample site	K40 Bq kg  −1	U238 Bq kg  −1	Th232 Bq kg  −1	Building material	County of origin of the materials
A1	1556 ± 78	100 ± 5	230 ± 12	Sand	Machakos
A2	57 ± 3	187 ± 9	29 ± 1	Sand	Machakos
A3	1744 ± 87	671 ± 34	29 ± 1	Sand	Kiambu
A4	1063 ± 53	180 ± 9	17 ± 1	Sand	Kajiado
A5	637 ± 32	108 ± 5	12 ± 1	Sand	Kajiado
A6	1248 ± 62	108 ± 5	151 ± 8	Sand	Makueni
A7	350 ± 18	210 ± 11	243 ± 12	Sand	Makueni
A8	398 ± 20	360 ± 18	95 ± 5	Sand	Makueni
A9	2648 ± 132	1019 ± 51	45 ± 11	Sand	Kiambu
A10	612 ± 31	174 ± 9	24 ± 1	Sand	Kiambu
A11	1050 ± 53	123 ± 6	23 ± 1	Sand	Narok
A12	1626 ± 81	3603 ± 180	11 ± 1	Sand	Narok
A13	670 ± 34	1243 ± 62	3081 ± 154	Sand	Narok
A14	485 ± 24	110 ± 6	10 ± 1	Sand	Narok
A15	1056 ± 53	63 ± 3	4214 ± 210	Brick	Kiambu
A16	1181 ± 59	144 ± 7	20 ± 1	Brick	Kiambu
A17	56 ± 3	163 ± 8	155 ± 8	Brick	Kiambu
A18	1045 ± 53	40 ± 2	6 ± 1	Brick	Kiambu
A19	55 ± 3	61 ± 3	11 ± 1	Brick	Kiambu
A20	699 ± 35	59 ± 3	11 ± 1	Brick	Kiambu
A21	466 ± 23	181 ± 9	11 ± 1	Brick	Kiambu
A22	992 ± 50	408 ± 20	29 ± 1	Brick	Kiambu
A23	849 ± 42	168 ± 8	90 ± 5	Brick	Kiambu
A24	202 ± 10	140 ± 7	78 ± 4	Block	Nairobi
A25	906 ± 45	179 ± 9	125 ± 6	Block	Nairobi
A26	331 ± 17	421 ± 21	124 ± 6	Block	Nairobi
A27	153 ± 8	523 ± 26	111 ± 6	Block	Nairobi
A28	59 ± 3	130 ± 7	24 ± 1	Block	Nairobi
A29	77 ± 4	284 ± 14	25 ± 1	Block	Nairobi
A30	358 ± 18	273 ± 14	64 ± 3	Block	Nairobi
A31	1724 ± 86	511 ± 26	218 ± 11	Block	Nairobi
A32	1653 ± 83	439 ± 22	172 ± 9	Block	Nairobi
A33	1427 ± 71	98 ± 5	99 ± 5	Block	Nairobi
Max	2648 ± 132	3603 ± 180	4214 ± 210
Min	55 ± 3	40 ± 2	6 ± 1
Average	831 ± 42	378 ± 19	290 ± 15

The measured activity concentration value for the radionuclides ⁴⁰K, ²³⁸U and ²³²Th for my research work.

The average activity concentrations for the radionuclide of the recommended average values are 420, 33 and 45 Bq kg⁻¹ for ⁴⁰K, ²³⁸U and²³²Th, respectively⁽⁶⁾ as in Table 2.

Table 2.

Average activity concentration of radionuclide in building materials from Babadogo Estate compared with the world average. It is an extract from Table 1.

Radionuclide	Study(Bq kg  −1) Average values	World average values   (Bq kg  −1) (UNSCEAR,2000)
40 K	831 ± 42	420
238 U	378 ± 19	33
232 Th	290 ± 15	45

Figure 2 shows the specific activity distribution for ²³⁸U, ²³²Th as well as ⁴⁰K for the selected sample sites in Babadogo Estate for the study area. From Table 1 and Figure 1, the highest activity concentration values for ²³⁸U,²³²Th as well as ⁴⁰K were in sites, which were labeled A’s ^A12^{, A}15 and ^A9, respectively. Activity concentration values in 40K were more advance in all samples collected from Babadogo Estate except for samples ^A2^{, A}7, ^A8 and ^A19, which were less than world average of 420 Bq kg−1. The activity concentration values for 238U were greater than the world average value of 33 Bq kg⁻¹ for all samples collected from study area. The Activity concentration values in ²³²Th were only higher from these seven samples: A1^{, A}6^{, A}7^{, A}8, ^A13^{, A}15 and ^A17, which were above the world average value of 45 Bq kg−1⁽¹⁰⁾. The high level of activity concentration from almost all the sample collected from Babadogo Estate can be attributed to the origin of the source of building materials as indicated in Table 1.

Figure 2.

Activity concentration in natural radionuclides ²³⁸U, ²³²Th and ⁴⁰K.

Conclusion

The study aimed at measuring natural activity concentration levels for the naturally occurring radionuclides in building materials; sand, clay bricks and concreate blocks used in Babadogo Estate in Nairobi City County, Kenya using NaI(T1) detector. The gamma spectrometry technique was employed in measuring the activity concentration of 33 samples collected randomly arising out of Babadogo Estate in Nairobi City County, Kenya. The dominance of assessing the natural activity concentration levels as a prerequisite to estimate the radiological parameters for the purpose of evaluating the ionizing radiation health effects of human exposure to gamma radiation as a result of randomly sampled building materials. The gamma spectrometry technique was used in measuring the activity concentration of 33 samples collected randomly arising out of Babadogo Estate in Nairobi City County, Kenya. Of the study, the calculated range and the corresponding average values of the activity concentration for the potassium-40,uranium-238 and thorium-232 in building materials were found: (55 ± 3–2647 ± 13) and 831 ± 42 (40 ± 2–3602 ± 80) and 378 ± 19 and (6 ± 1–4213 ± 210) and 290 ± 15 ^{Bq kg−1}, respectively. The study revealed that the average determined values for the samples ⁴⁰K, ²³⁸U and ²³²Th were above and beyond the world average figures.