Activity concentrations of 226 Ra , 232 Th , and 40 K in common maize meal consumed in Namibia and their potential radiation hazards

Gamma spectroscopy was performed to determine the concentrations of K, Ra and Th in maize meal samples collected from shop outlets and open markets in Namibia. The activity concentrations and Excess Lifetime Cancer Risk were determined using a high purity germanium (HPGe) detector. The average activity concentrations of K, Ra and Th were found to be 29.98 ± 4.05, 0.99 ± 0.40 and 0.35 ± 0.08 Bq.kg – 1 in the maize meal samples. The result showed the activity concentrations of K significantly higher than the other radionuclides in all the maize meal samples. The average excess life cancer risk varies from 1.33 x 10 – 13 to 6.05 x 10 – 13 for K, 8.76 x 10 -13 to 1.19 x 10 – 12 for Th and 2.43 x 10 -11 to 2.83 x 10 – 11 for Ra. Hence, when compared with internationally acceptable limits, all values fall within the safety limits. Thus, the study concludes that the maize meals consumed in Namibia are radiologically safe for consumption.


Introduction
The earth is continuously exposed to radiation mainly from natural and artificial sources (UNSCEAR, 2008). Natural radioactivity from uranium, thorium, and their progenies and potassium form the largest contributions to internal radiation dose received by humans owing to their wide distribution in the environment, whereas the contributions from anthropogenic sources are minimal (Al-Mastri et al., 2004). The anthropogenic release of radionuclides are localized to specific area through routine and/or accidental release and unplanned disposal of industrial and/or radioactive waste etc., that eventually find their way into the environment and food chain (Al-Mastri et al., 2004;Uwatse et al., 2015). All foods consist of naturally occurring radionuclides with concentrations varying with agricultural practices, geographical location and food type (WHO, 2011). Once radionuclides are ingested they are easily assimilated into the tissues of humans where they ultimate cause biological damages which could be for the entire lifetime (Onjefu et al., 2017).
Worldwide, maize is one among most commonly consumed staple foods. This has necessitated studies on radionuclides concentrations in maize meal samples in different regions around the globe (Nkuba and Sungita, 2017;Changizi et al., 2013;Olatunji et al., 2014). The measurement of natural radionuclides in food is relevant in quantifying radiological risk to humans due to ingestion (IAEA, 1989). This study is therefore aimed at determining the activity concentrations of natural radionuclides in different maize meals sold in both open markets and shop outlets in Windhoek-Namibia and to assess the Excess Lifetime Cancer Risk arising from gamma emitting radionuclides in maize meal consumed by the general public in Windhoek-Namibia.

Sample collection and preparation forpectroscopy
Different maize meal samples were collected from shop outlets and open markets in Windhoek, Namibia to determine the natural radioactivity of the maize meal samples using gamma ray spectrometry; the Hyper Pure Germanium (HPGe) detector with relative efficiency 45% of the energy resolution at 1332.5 keV energy peak of 122 keV was connected to a multi-channel analyzer (MCA). The HPGe detector was placed in the lead shield 64.80 mm thick, coated with tin and copper with a thickness of each 1 mm and 1.6 mm to reduce the effects of background radiation. Gamma ray spectrometry system was calibrated using a mixture of radioactive sources 60 Co, 88 Y, 85 Sr, 57 Co, 113 Sn and 137 Cs.
The concentration of radium, thorium and potassium was measured for each variety of maize meal namely: meme mahangu, namib braai pap, namib super meal (Okuryangava), maize meal, super meal, and top score. The samples were thoroughly crushed. About 500 g of each sample was taken to carry out further procedure. The samples were placed and sealed in a Marinelli beaker and left for 30 days for secular equilibrium to be attained (Onjefu et al., 2017). Initially, samples were laid in the HPGe detector, and then gamma spectrum was obtained using equation 1 (Onjefu et al., 2017): where C is the activity concentration of the radionuclide in the sample in units of Bq.kg -1 , Cn is the count rate of gamma rays associated with a peak per second (cps), is the detector efficiency at the specific -ray energy, is the absolute transition probability of the specific -ray, and is the mass of the sample (kg). The activity of the daughter radionuclide 214 Bi and 214 Pb were obtained from the 609.31 keV and 351.92 keV gamma peak respectively and were chosen as indicators of uranium ( 238 U) while the activity of 212 Bi ( 232 Th) was determined from its 727.17 keV gamma ray peak and was used as an indicator for thorium ( 232 Th). Potassium-40 was determined by measuring the 1460.3 keV gamma rays emitted during its decay (Jibiri and Emelue, 2008).

Calculation of radiobiological impact
The daily intake of radionuclides from the activity concentrations of 226 Ra, 232 Th and 40 K due to the consumption of maize meal was calculated from equation 2 (Nahar et al., 2018).
represents the daily intake of radionuclides (Bq.kg -1 ) by an individual, is the activity concentration of radionuclides (Bq.kg -1 ), is the per capital per year consumption of maize meal where for Namibia is at 44 kg (Shifiona et al., 2016), is the days in a year.
Another radiological parameter called the annual committed effective dose (µSv.y -1 ) to an individual from ingestion of radionuclides in maize meal is defined from the following formula (Asaduzzaman et al., 2015;Nahar et al., 2018) , , and is the annual effective dose (µSv.y -1 ), activity concentration of radionuclides (Bq.kg -1 ), annual intake of maize meal (44 kg) (kg.y -1 ) and ingestion dose conversion factor for radionuclides investigated (2.8 x 10 -7 Sv.Bq -1 for 226 Ra, 2.3 x 10 -7 Sv.Bq -1 for 232 Th and 6.2 x 10 -9 Sv.Bq -1 for 40 K) (16) (IAEA, 2011). The gamma index ( ) was calculated using equation 4 (Senthilkumar & Narayanaswamy, 2016), is the gamma index, activity concentrations of 226 Ra, 232 Th and 40 K (Bq.kg -1 ). The total ingested dose from the consumption of maize meal was determined using the relationship in equation 5 (Asaduzzaman et al., 2015): where f represents a food group, the coefficients and denote the average activity concentration of radionuclides in (Bq.kg -1 ) and the consumption rate per year (kg), respectively and is the dose coefficient for an intake by the ingestion of radioisotope, r given in unit of (Sv.Bq -1 ).
Excess life time cancer risk (ELCR) was obtained from equation 6 (Nahar et al., 2018): is the life cancer risk, should be the annual effective dose ( −1 ), the average lifespan (70 years) and is the risk factor ( −1 ).
The mean activity concentrations were in the order 40 K > 226 Ra > 232 Th. This clearly show that 40 K dominates over 226 Ra and 232 Th because of its relative abundance in terrestrial bodies (Wild, 1993, Dar andEl-Saharty, 2013). The meme mahangu maize meal had the highest activity for 40 K, while namib super meal had the highest activity of 232 Ra. The highest activity of 232 Th was recorded for super meal maize meal. The differences in the activity concentrations of 40 K, 226 Ra and 232 Th in the maize meal samples maybe attributed to differences in soil properties and geographical setting of the soil and the type of fertilizer added to the soil to improve soil fertility (Karunakara et al.,2013).
The obtained mean activities of 40 K, 226 Ra and 232 Th has been compared with those reported from other countries (Table 2). It shows that the activity of 40 K, 226 Ra and 232 Th in maize meal sold in Windhoek, Namibia is lower than the reported maize meal range for the populations of Tanzania, Malaysia and Iran respectively (Olatunji et al., 2014;Nkuba and Sungita, 2017;Changizi et al., 2013).   Nkuba and Sungita (2017) Changizi et al., (2013) Olatunji et al., (2014

Daily intake and Annual Effective Dose
The daily intake of 40 K, 226 Ra and 232 Th and annual effective dose due to the consumption of maize meal was calculated and the results shown in Table 3. The estimated daily intakes of activity concentrations of 40 K, 226 Ra and 232 Th into the human body by ingestion of the maize meal varied from 1.33 to 6.05 Bq.d -1 with an average value of 3.61±0.79 for 40 K, 0.33 to 0.39 Bq.d -1 with an average value of 0.36±0.03 for 226 Ra, and 0.05 to 0.08 Bq.d -1 with an average value of 0.07±0.008 for 232 Th, respectively. The daily intake of 40 K was found to be significantly higher than the other radionuclides in all the maize meal samples. This radionuclide is an essential element for the human body. However, the use of fertilizers for improve crop yield may have informed the high levels of 40 K activity concentrations in all the samples (Jayasinghe et al., 2020). The annual effective dose in µSv.y -1 due to ingestion of maize meal were in the range 3.01 x 10 -06 to 1.37 x 10 -05 for 40 K, 3.38 x 10 -05 to 3.95 x 10 -05 for 226 Ra and 6.45 x 10 -06 for 232 Th respectively. The Annual effective dose for Namib Super Meal was the highest for all radionuclides except for 232 Th. These values are however far below the threshold dose values for safety from ingestion of uranium (6.3 µSv.y -1 ) and thorium (0.38 µSv.y -1 ) radionuclides as reported by UNSCEAR (2000). Table 4 present the gamma index and excess lifetime cancer risk. The calculated values of the gamma Index ranges from 0.01 (Okuryangava Maize Meal) to 0.03 (Super Meal), with a mean value of 0.03±6.66. The values for all maize meal sample were lower than the critical value of unity (UNSCEAR, 2000). The Excess life time cancer risk (ELCR) varied from 1.33 x 10 -13 to 6.05 x 10 -13 for 40 K, 2.43 x 10 -11 to 2.83 x 10 -11 for 226 Ra and 7.99 x 10 -13 to 1.02 x 10 -12 which are lower than the acceptable ELCR limit as set by USEPA (1989). The United States Environmental Protection Agency (USEPA) considers acceptable for regulatory purposes an excess life time cancer risk of between 1 × 10 -6 and 1 × 10 -4 (USEPA, 1989).

Conclusions
Concentration of natural radionuclides in various types of maize meal samples consumed in Namibia has been determined. The mean activity concentrations of 40 K, 226 Ra and 232 Th were found to be 29.98 ± 4.05, 0.99 ± 0.40 and 0.35 ± 0.08 Bq.kg -1 in maize meal samples. The daily intake of radionuclides was found to be in the order 40 K ˃ 226 Ra ˃ 232 Th. From the measured values of the activity concentrations, the effective dose, gamma index and excess lifetime cancer risk were calculated and found to be lower than internationally accepted threshold limits. The study reveals that radionuclides intake from consumption of maize meals pose no radiological threat to members of the public in Windhoek-Namibia. The findings from this study will help in establishing baseline information for radionuclides exposure to members of the public due to consumption of maize meal.