Introduction

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Humans are naturally exposed to ionizing radiation coming from different sources such as cosmic rays and natural radionuclides in food, air and water. Vegetables is one of the most widespread crops in the world that has great economic significance. Natural radionuclides such as ²³⁸U, ²³²Th and ⁴⁰K can be found almost everywhere; in soil, public water supplies, oil and thereby subjecting human beings to reasonable exposure [1].

The presence of ²²⁶Ra (²³⁸U-series), ²³²Th, and ⁴⁰K in the soil is both a direct and indirect source of radiation exposure for humans. Direct exposure channels can be found in the environment or the construction of dwellings using soil-based bricks. Indirect exposure can also result from soil swallowing during farming operations, airborne dust load during farming, plant uptake, and eventual human ingestion [2]. Human beings inhale or ingest many radionuclide and their radioactive isotopes present all around us, from which they are mainly exposed by outdoor natural terrestrial radiations that originates predominantly from upper 30 cm layer of soil present on earth [3]. The presence of ²³⁸U, ²³²Th and ⁴⁰K radionuclide in soil also leads us to identify the origin and abundance of their daughter elements like radon, thoron and their progenies. Exposure to uranium (U) and radium (Ra) causes several health problems, including chronic lung disorders, acute leucopenia, anemia, and oral necrosis. Exposure to radium causes bone, cranial, and nasal tumors, while thorium causes lung, pancreas, hepatic, bone, and kidney malignancies, as well as leukemia [4]. The activities of radioactive nuclides exist in agricultural soil and irrigation water, and they may transfer to vegetables growing on it.

Food becomes a vital source of radioactivity inside of human bodies, which is significant for radioactivity level measurements [5]. In earlier years, exposures to human beings from background radiation were measured and analyzed at the natural radioactivity levels of surface soil, agricultural soil, farm soil, irrigated soil, environmental soil, residential soil, and uncultivated soil samples in different areas of the world. In the literature, there are various studies from different countries on the measurement of radionuclide concentrations in vegetables, fruits and soil [6-12].

The aims of the study are to determine activity concentration, radium equivalent activity, absorbed dose rate and internal and external hazard index from Akaki Kality, Addis Ababa, Ethiopia using a coaxial HPGe detector in a low background configuration through gamma-ray spectrometry technique. In this study may be provided very important information for the radiological safety of the environment after a radiation fallout that may occur in the future. That is, such information can be used to form the basis for future comparative analysis of high activity concentrations due to human activities in the occupational and residential situation [13].

Experimental Technique

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Study Area

Akaki Kality Sub-cities are among the area where there has been a noticeable increase in the activities of the surrounding industrial zone. Most of the waste products from the industries are released into the environmental vegetable. Vegetables are then contaminated by heavily dosed radioactive from the industry that seeps into them.

Description of the study location

Addis Ababa, is the capital and largest city of Ethiopia, founded in 1886, it is the political and economic capital of the country located on the shewan plateau. Addis Ababa city administration has 10 Sub cities. Akaki Kality is one of the Sub City roughly 10 kilometers from the city center of Addis Ababa. Addis Ababa city administration which is located at 8^o51’’12’’’ to 8^o56’’17’’’N latitude and 38^o45’’19’’’to 38^o50’’23’’’ longitude (Figure 1).

Sample Preparation

Four separate home gardens were selected to serve as the sampling sites for this study. Each garden was dedicated to growing Cabbage, Red Onion, Carrot and Potato as shown in Table 1. Twenty-four samples were collected from each of home garden and composited into one sample for each vegetable. The twenty-four samples, each weighing exactly one kilogram, were crushed, mixed, and sieved to a mesh size of roughly 75µm. After air drying, the samples were placed in an oven for 24 hours at 110˚C. Samples that had been weighed were put into 250 cm³ polyethylene bottles with a 75 µm hole size sieve (Figure 2). To achieve adequate homogeneity around the HPGe detector, the homogenized samples were then moved to General Purpose Marinelli Beakers with constant volume. A computerized weighing balance with great sensitivity was used to evaluate the sample weighting. To enable gamma spectrometry to quantify the radioactive equilibrium between 238U and 232Th and their associated daughters, the Beakers were fully sealed for over a month to allow radioactive equilibrium to be reached between ²³⁸U and ²³²Th and their corresponding daughters to be measured by gamma spectrometry.

Figure 1

Figure 2

Table 1

General purpose Marinelli Beakers having vegetable samples were kept to the detector vicinity to determine the activity concentration of the ²²⁶Ra, ²³⁸U, ²³²Th and ⁴⁰K radionuclides. Gamma spectrometry was used to determine primordial radionuclides present in the vegetable samples.

Measurement of radioactivity using High Purity Germanium (HPGe) detector

Vegetable radioactivity levels were analyzed using a gamma-ray spectrometry setup featuring a High Purity Germanium (HPGe) detector. The device was an n-type coaxial CAMBERRA HPGe detector with a 72.5 mm diameter and 72.5 mm thickness crystal, offering 80% relative efficiency. The detector was connected to a computer-based multichannel analyzer (MCA) system, which utilized Genie 2000 software to quantify characteristic photopeak areas.

Background measurements were conducted monthly using an empty container to monitor counting rate stability. For accurate isotope identification, qualitative assessment, and quantitative analysis, the system underwent energy and efficiency calibration. To ensure that the instrument calibration is impartial, a standard calibration source with radionuclides Am-241, Ce-139, Cs-137, Sn-113, Cd-109, Co-60, Y-88, Sr-85, Co-57 and Cr-51. Energy calibration employed point sources including ²⁴¹Am (59.5 keV), ¹³⁷Cs (661.6 keV), and ⁶⁰Co (1173.2/1332.5 keV), while efficiency calibration used a certified MGS6M315 mixed standard alongside ¹²⁵Sb (176.3/427.9/600.6 keV), ¹⁵⁵Eu (60.0/86.5/105.3 keV), ⁵⁴Mn (834.8 keV), and ⁴⁰K (1460.8 keV). Genie 2000 software calculated absolute efficiency and associated uncertainties, with both samples and background measured for 18,000 seconds.

The activity concentration of ²²⁶Ra was calculated using energy lines of 295.2 keV and 51.9 keV for ²¹⁴Pb and 609.3, 1120.3, and 1238.1 keV for ²¹⁴Bi. The activity concentration of ²³²Th was calculated using 300.1 keV of ²¹²Pb, 277.4 and 860.6 keV of ²⁰⁸Tl, 209.3 and 911.1 keV of ²²⁸Ac, and 723.3 and 785.3 keV of ²¹²Bi gamma lines. The activity of 40K was directly calculated using the 1460.8 keV gamma line.

The net count rates beneath the most prominent photopeaks of all radionuclides were computed by subtracting the respective background count rate from the gross count rate for all radionuclides acquired during the same counting period. The radionuclide activity was then computed using Equation 1.

Calculations of radiological parameters

Determination of Activity

The activity concentrations were calculated using the formula below [14-16].

[

Where:

N = Net counts per second (C.P.S) = (Sample C.P.S – background C.P.S)

Pγ = Intensity of the radionuclide / P is the emission probability of radionuclide.

ε = Efficiency in %

W = Weight of sample in kilograms

Determination of absorbed dose rate

The determination of the absorbed dose rate is the very first step in evaluating the health-related risk. With regards to biological effects, the radiological effects are directly related to the absorbed dose rate in the air at 1 meter above the ground surface [17]. The measured activity concentrations of ²²⁶Ra (²³⁸U), ²³²Th, and ⁴⁰K were converted into doses by applying the conversion factors 0.462, 0.604 and 0.0417 for Uranium, Thorium, and Potassium, respectively.

The absorbed dose rate Dγ(nGy h^-1) due to gamma radiations in outdoor air at 1 m above the ground surface was calculated as follows [17-19].

Where Dγ is the absorbed dose rate in nGy.h^-1, A_Ra, A_Th and A_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively.

Radium equivalent activity

The radiation hazards associated with the radionuclides were estimated by calculating the Radium equivalent activity (Ra_eq). This is weighted sum of activities of ²²⁶Ra, ²³²Th and ⁴⁰K basing on the assumption 370 Bq/Kg of ²²⁶Ra, 259 Bq/Kg of ²³²Th and 4810 Bq/Kg of ⁴⁰K produce the same gamma radiation dose rate.

The formula below defines Radium equivalent [20].

Where A_Ra, A_Th and A_k are the activity concentration of ²²⁶Ra, ²³²Th and ⁴⁰K respectively. The conversion constants for Thorium and Potassium are 1.4286 and 0.07692 respectively.

External and internal hazard indices

A widely used hazard Index (reflecting the external exposure) called the external hazard index (H_ex) is defined as follows [21].

There is also a radiation hazard to respiratory organs due to the ²²⁶Ra decay product ²²²Rn and its short-lived decay products. To account for this hazard, the maximum permissible radium concentration must be reduced to half of the normal limit [22].

The internal hazard index (H_in) is calculated using the following formula [21].

Where A_Ra, A_Th and A_k have the same meaning as in Equation 3 and Equation 4.

Results and Discussion

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Results

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[In the present study, a total of four vegetable samples (Cabbage, Carrot, Red Onion and Potato) were analyzed. The radionuclides detected and corresponding activity concentrations in four different vegetable samples have been summarized in Table 2.

Table 2

BDL - Below detectable limit and ^b Data from United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) [23]

The radionuclides found in the samples were ²¹⁴Pb, ²¹⁴Bi, ²²⁸Ac (due to ²³⁸U and ²³²Th decay) and ⁴⁰K. The activity concentration of ²³²Th was determined from its decay products ²²⁸Ac (911.21 keV and 968.97keV), while the activity concentration of ²²⁶Ra was determined from ²¹⁴Bi (609.31 keV) and ²¹⁴Pb (351.92 keV) and taken to be equal to ²³⁸U activity concentration on the assumption of the prevalence of secular equilibrium in ²³⁸U series. The activity concentration of ⁴⁰K was determined from its 1460.82 keV gamma spectrum. More details of the gamma ray energy levels of radionuclide and their emitter nuclides. are shown in Table 3.

Table 3

The calculated Radium equivalent activities, absorbed dose rate, and external and internal hazard indices in four vegetable samples from Akaki Kality, Addis Ababa, Ethiopia, were calculated as shown in this Table 4.

Table 4

Discussion

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The Activity Concentration of vegetable samples

The highest activity concentrations displayed in Table 2 correspond to the naturally occurring radionuclide ⁴⁰K. The highest concentration of ⁴⁰K was 840 ± 49 Bq/kg which is measured in the Cabbage sample. The spread of measured values is rather large; Red Onion sample was with the lowest concentration of 390 ± 23 Bq/kg. The activity concentration of ⁴⁰K was found to be high in all vegetables in this work; the measured concentration value of ⁴⁰K was higher than the worldwide value 400 Bq/Kg except Red Onion sample. This can be attributed to the use of fertilizers and irrigate with waste water to a large extent affecting the radionuclides concentrations, especially potassium. But the high value of ⁴⁰K may also is due to the soil origin and the nature of some vegetables. In this context, ⁴⁰K is a key biological element in human tissue through metabolic control. As shown in Figure 3 obtained activity concentration of ²²⁶Ra and ²³²Th was 2.13 ± 1.09 Bq/Kg in Carrot and 3 ± 1.3 Bq/Kg in Cabbage which is the highest value of ²²⁶Ra and ²³²Th respectively in this study.

In the rest vegetable samples (Cabbage, Red Onion and Potato) no measured activity concentration of ²²⁶Ra was found and in vegetable samples (Carrot, Red Onion and Potato) no activity concentration of ²³²Th also was found, their values were below minimum detectable limit. In the present study, the measured activity concentrations of ²²⁶Ra and ²³²Th in all vegetable samples are lower than the worldwide values 35 Bq/Kg and 30 Bq/Kg respectively [23-24].

Figure 3

Radium Equivalent Activity (Ra_eq)

Radium Equivalent Activity (Ra_eq) calculated using Eq. 3. The Ra_eq for Cabbage, Carrot, Red Onion and Potato samples were 68.94 Bq/kg, 39.09 Bq/kg, 30.03 Bq/kg and 52.36 Bq/kg respectively. As shown in Table 4 and Bar graph in Figure 4 the maximum calculated value of Ra_eq was 68.94 Bq/Kg in the Cabbage sample, while the minimum calculated value of Ra_eq was 30.03 Bq/Kg in Red Onion sample. These values are far below the allowable limit (370 Bq/kg) (Figure 4) recommended by the International Atomic Energy Agency [23].

Figure 4

Absorbed Dose Rate

[The absorbed dose rate, ADR (nGy h ^{- 1}) depends on the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K natural radioisotopes. The absorbed dose rate was calculated using (Eq.2) on the basis of the UNSCEAR (2000) guideline and is given in Table 4. In the present study, the dose rate due to ²²⁶Ra, ²³²Th and ⁴⁰K in Cabbage, Carrot, Red Onion and Potato samples are 37.28 nGyh^-1, 21.16 nGyh^-1, 16.38 nGyh^-1 and 28.56 nGyh^-1, respectively. As shown in Figure 5 the maximum absorbed dose rates were observed in Cabbage sample, while the minimum absorbed dose rates were obtained in Red Onion sample. The obtained values in all samples are below the world average of 55 nGyh^–1 (Figure 5) [25].

Figure 5

External and Internal Hazard Indices

Hazard Indices (H_ex and H_in) are two indices that represent the external and internal radiation hazards. These indices are calculated using (Eq.4) and (Eq.5). The calculated H_ex and H_in values for all vegetable samples in this study are shown in Figure 6. The calculated external hazard indices for Cabbage, Carrot, Red Onion and Potato in this study were 0.187, 0.112, 0.081 and 0.14, respectively.

Figure 6

As shown in Table 4, the highest calculated external hazard index was 0.187 in the Cabbage sample, while the lowest external hazard index was 0.081 in Red Onion sample. In our study, the vegetable samples H_in values were 0.187, 0.106, 0.081 and 0.14 for Cabbage, Carrot, Red Onion and Potato respectively. As shown in Figure 6, the internal hazard indices (H_in) in Cabbage were greater than Potato, H_in in Potato was greater than Carrot, and H_in in Carrot was greater than Red Onion. Therefore, the highest and the lowest internal hazard indices (H_in) were calculated in Cabbage and Potato respectively. The calculated H_ex and H_in values for all vegetable samples should be below unity, which does not cause harm to the populations of the investigated region in Akaki Kality, Addis Ababa, Ethiopia.

Comparison of activity concentration (Bq/Kg) in this study with different countries

[[[The present study in the Measurement of activity concentration of ⁴⁰K, ²³²Th and ²²⁶ Ra of selected vegetables samples were compared with that were reported in the literature. It may be concluded that the activity concentration of ⁴⁰K in all the same vegetables and the activity concentration Carrot sample are higher than the values from reported in Table 5.

Table 5

Conclusion

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This study aimed to measure the activity concentrations, gamma absorbed dose rates (Dγ), radium equivalent activity (Ra_eq) and hazard indices (H_ex and H_in) of the naturally occurring radionuclides ²²⁶Ra, ²³²Th and ⁴⁰K as well as the artificial radionuclide ¹³⁷Cs in selected vegetable samples (Cabbage, Carrot, Red Onion and Potato) has been assessed due to consumption of various popular vegetables locally grown in Akaki Kality, Addis Ababa, Ethiopia and matched well with world values. There was no 137Cs activity concentration in any of the samples from this location, indicating that artificial radioactive fallout did not occur.

The measured activity concentrations (Bq/kg) of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively in Cabbage sample were BDL, 3 ± 1.3 and 840 ± 49, in Carrot sample were 2.13 ± 1.09, BDL and 480 ± 29, in Red Onion sample were BDL, BDL and 390 ± 23 and in Potato sample were BDL, BDL and 680 ± 34. It was observed that the activity levels of ⁴⁰K in the vegetable samples were not uniform, varying with the types of vegetables. Moreover, the activity of ⁴⁰K exceeded by far the values of both ²²⁶Ra and ²³²Th, being the most abundant radioactive element present in the environment and it also being noted that potassium is used extensively as part of an NPK (Nitrogen,Phosphorous and Potassium) fertilizer in intensive farming activities to promote dynamic growth. In this study, the value of Ra_eq activity was found to be less than 370 Bq/kg, while the absorbed dose rate was less than the worldwide value 55 nGyh^-1 and internal and external hazard indices were found to be less than acceptable limit of unity.This shows that the concentrations of radionuclides found in the surveyed area were nominal and do not pose any potential health hazard to the general public.

Therefore, with an activity concentration ⁴⁰K higher than the worldwide average values recommended by UNSCEAR, regular monitoring is required to identify contamination trends and provide public awareness campaigns promoting safe farming, as well as policy interventions to regulate agricultural inputs, particularly near industrial zones.

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