RADIOACTIVITY CONCENTRATION OF SOME STAPLE FOOD CONSUMED IN UVWIE AND WARRI SOUTH LOCAL GOVERNMENT AND EFFECT ON HUMAN AFTER CONSUMPTION

CHAPTER TWO

LITERATURE REVIEW

2.1 REVIEW OF RELATED LITERATURE

Delta State is an oil and agricultural Producing state of Nigeria, situated in the region known as the South-South geo-political zone with a population of over 4 million, the capital city is Asaba located at the Northern end of the state with an estimated area of 762 square kilometers (294sqm) while Warri is the economic nerve center of the state and also the most populated in the Southern end of the state. The state has a total land mass of about 18,050km2, of which more than 605 ps land. The state covers approximately between the longitude 5000 and 6045’ east and latitude 5000 and 6030 north. It is bounded in the north and west by Edo state, the east by Anambra, Imo States, South East by Bayelsa state and on the southern flank is the Bight of Benin which covers about 160 kilometers of the state coastline. Delta State is generally low lying without remarkable hills. For the purpose of the research Warri south local government area is the area of concentration One of the three goals of the United Nations for sustainable food security is to ensure that all people have access to sufficient, nutritionally adequate and safe food (Jibiri, et al., 2007).Natural radioactive elements are transferred and cycled through natural processes and between the various environmental compartments by entering into ecosystems and food chains. Vegetables may be subjected to direct and indirect contamination of uranium _series radionuclide. Use of fertilizers leads to elevation of uranium series nuclides in vegetables. Naturally Occurring Radionuclides (NORM) There are significant contribution of ingestion dose and are present in the biotic systems of plants, animals, soil, water and air. Distribution of those radionuclide in different parts of the plant depends on the chemical characteristics and several parameters of the plant and soil (Shanthi, et al, 2009). Olomo (1990) study’s on natural radioactivity in some Nigeria foodstuffs examined, varies and concluded that the major factor that may be responsible include; application of fertilizer, soil type and irrigation pattern.

Arogunjo, et al., (2004) studied the level of natural radionuclide in some Nigerian cereals and tubers using HpGe detector and reported average concentration of 40K, 238U and 232Th as 130±8.12Bqkg-1, 11.5±3.86Bqkg-1, and 6.78±2.13Bqkg-1 respectively, while 137Cs was not detected in any of the food stuffs analyzed. Eyebiokin et al., (2005) also studied the activity concentrations and absorbed dose equivalent of commonly consumed vegetables in Ondo state using Nal(IL) detector and reported that mean effective dose equivalent for Akure, Idanre and Agbabu were 0.59mSvy-1 0.73mSvy-1 and 0.64mSvy-1 respectively. They concluded that the values obtained are lower than the UNSCEAR (1993) recommended value for normal food crop of 1.0 mSvy-1. Ojo and Ojo, (2007) on the radiological study of brackish and fresh water food samples in Lagos and Ondo States, they reported that the average concentration of 50.92±7.04Bqkg-1 238U and 24.60±6.47Bqkg-1 232Th were found to be higher in brackish water while 40K (738.94±84.81Bqkg-1) was found to be higher in food samples got from fresh water.

Mlwilo et al (2007) study on radioactivity levels of staple foodstuffs and dose estimates for most of the Tanzanian population revealed that the average activity concentration of 40K, 232Th and 238U in maize were 48.79±0.11, 4.08±0.01 and 13.23±0.10BqKg-1 respectively and in rice 3.82±0.02, 5.02±0.02 and 24.67±0.03. Bqkg-1 respectively. He concluded that the relatively high average concentrations of the radionuclide in maize compared to rice may be attributed to the extensive use of phosphate fertilizers in maize production in Tanzania and that the total annual committed effective doses due to total 232Th and 238U intakes as a result of consumption of staple food stuff for infants, children and adults were 0.16, 0.29 and 0.36msvyr-1 respectively, which are lower than the annual dose guideline for the general public. Activity concentrations of 226Ra, 228Th, 40K in different food crops from a high background radiation area in Bitsichi, Jos Plateau, Nigeria were studied by Jibiri et al (2007). The activity concentration in the food crops ranged below detection limit (BDL) to 684.5BqKg-1 for 40K from BDL to 83.5Bqkg-1 for 225Ra, and from BDL to 89.8BqKg-1 for 228Th. It was further revealed that activity concentrations of these radionuclide were found to be lower in cereals than in tubers and vegetables. The average external gamma dose rates were found to vary across the farms from 0.50±0.01, to 1.47±0.04 μSvh-1. Because of the past mining activities in the area, it was found that the soil radioactivity has been modified and the concentration level of the investigated natural radionuclide in the food crops has been enhanced but however, the values obtained suggested that the dose from intake of these radionuclide by the food crops is low and that harmful health effects are not expected. Shanthi et al (2009) carried out a study to evaluate the radioactivity concentration in the food crops grown in high-level natural radioactive area (HLBRA) in South–West, India. The calculated daily intakes of these radionuclide isotope (226Ra, 228Ra, 228Th and 40K) using concentrations in south Indian foods and daily consumption of these foods were fund to be 226Ra, 0.001-1.87, 228Ra, 0.0023-1.26, 228Th,0.01-14.09, 40K, 0.46-49.39 Bq/day. It was concluded that the daily internal dose resulting from ingestion of radionuclide in food was 4.92uSv/day and the annual dose was 1.79mSvyr-1.In view of the potentially dangerous effects of radioactive substances, no effort should be spared in their quantitative determination in all the identifiable pathways. Consumption and use of vegetable crops was once very limited in Sudan. Vegetables are usually produced by small farmers in rain-fed areas, irrigated private farms or the big government schemes. The pattern of cereals consumption over the last four decades has shown two major trends: the growth of grain consumption and a shift in the pattern of cereal demand from sorghum, which is the traditional staple, to domestically produces and imported cereals, and wheat for food and feed uses (FAO, 2005).

Delta state is generally self-sufficient in basic foods, albeit with important inter-annual and geographical variations, and with wide regional and household disparities in food security prevailing across the country.

According to national food balance statistics, the food supply, essentially based on cereals, meets population energy requirements. Vegetable foods are complemented with a substantial supply of milk. Nevertheless, national statistics mask large inequalities in access to food in the country. The prevalence of undernourishment is high. Data on actual food consumption are not available. Cereal grains are utilized as food worldwide. In Africa, the majorities of cereal-based foods are consumed in the form of porridge and naturally fermented products (Kabeir. et al., 2004).

Tomato is grown almost all over the country, along the banks of the Nile and other rivers and in the irrigated schemes. Production of tomatoes is concentrated in the cooler winter season. Okra and the leafy vegetables are very popular in delta state.

2.2 FOODSTUFFS RADIOACTIVITY

Wide variety of wild and domestic vegetable, fruit, cereals, and animal products are found in delta state. Ingestion of foodstuffs constitutes an important pathway by which can be transferred to humans. There are many human activities which can enhance the level of naturally occurring radioactivity levels in the environment (Banzi, et al., 2000). Examples are the mining and use of ores containing naturally radioactive substances and the production of energy by burning coal that contains these substances (UNSCEAR, 2000).

The natural radioactivity in food comes mainly from natural isotopes of potassium, uranium and thorium daughter products. The majority of radio nuclides in the environment are present as daughter products of 238U and 232Th isotopes distributed by natural geological and geochemical processes, in addition to the unrelated naturally occurring 40K and 14C (Mlwilo, et al, 2007; Júnior, et al, 2006). The natural occurring radionuclides are also introduced into the environment through the burning of fossil fuels and the uncontrolled mining processes (Banzi, et al., 2000). Naturally-occurring radioactive isotopes can be transferred from the soil to plants (Adam Sam and Åke Eriksson, 1995; Köhler. et al., 2000). As would be expected, these radio nuclides accumulated in arable soil are incorporated metabolically into plants and ultimately find their way into the bodies of animals including humans when contaminated foods are consumed. In addition to root uptake, direct deposition may occur on foliar surfaces, and when this happens, the contaminants may be absorbed metabolically by the plants or may be transferred directly to animals that consume the contaminated foliage. It is known that when radionuclide ingested or inhaled are distributed among body organs (according to the metabolism of the element involved and the organs) normally exhibit varying radio sensitivities (Mlwilo, et al, 2007).

In addition to natural radioactivity, manmade radionuclide may present a potential risk (especially in case of nuclear accidents).For instance, 137Cs is very important because it follows the course of potassium in ecosystems and it persists in the environment for many years due to its relatively long half-life, 30.2years. Furthermore, it is characterized as a potential genetic hazard because it accumulates in many types of human tissues and its penetrating gamma-rays reach all body cells (Mlwilo., et al, 2007). 226Ra is one of the major sources of radioactivity found in food and water (Yücel, et al., 1998). Among the 238U daughter products, 226Ra is one of the most radiotoxic radionuclide with very long half-life (1600 years). Due to its long half-life and radiological effects, it is one of the most important isotopes to be determined among the naturally occurring nuclides in environmental samples (Lawrie, et al, 2000).

There are various techniques for radioactivity analysis in foodstuffs with the ability to determine qualitatively and quantitatively alpha and beta emitting radio nuclides. However, Gamma Ray Spectrometry (GRS) provides a fast, multi elemental and non-destructive method of radioactivity measurement. Both qualitative and quantitative analysis of samples can be easily achieved by gamma-ray spectrometry systems.

2.3 RADIATION

Radiation is the emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles that cause ionization. It is an energy that comes from a source and travels through some material or through space, light, heat, sound are types of radiation. Thus, radiation is the term used to describe energy in motion, radiating or moving away from its source. It is expressed in waves calculated along the electromagnetic spectrum. Some radioisotopes are only found on the earth as a result of human activity, they are Strontium-90 (90Sr) Cesium-137 (137Cs), Iodine-131(131I) and Technetium-99 (99Tc), and some isotope like Potssium-40(40K) is only present due to natural processes, a few isotopes are present as a result of both natural processes and human activities.

There are basically two types of radiation; ionizing and non-ionizing radiation.

2.3.1. IONIZING RADIATION

Ionizing radiation is radiation that carries enough energy to free electrons from atoms or molecules, thereby ionizing them. Ionizing radiation is made up of energetic subatomic particles, ions or atoms moving at high speeds (usually greater than 1 % of the speed of light), and electromagnetic waves on the high-energy end of the electromagnetic spectrum.

2.3.2. NON-IONIZING RADIATION

Non-ionizing radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum (photon energy) to ionize atoms or molecules-that is, to completely remove an electron from a radiation has sufficient energy only for excitation. This non-ionizing radiation is also known as radiofrequency waves (RF). Examples are emission from devices such as mobile phones, microwaves ovens, (G.S.M) base station, radar installations, and telecommunication and broadcast facilities.

2.4. SOURCES OF RADIATION

There several sources of radiation which will be briefly discussed subsequently.

2.4.1 NATURAL BACKGROUND SOURCES

The earth itself is a source of terrestrial radiation. Radioactive materials (including uranium, thorium, and radium) exist naturally in soil and rock. Essentially all air contains radon, which is responsible for most of the dose that is receive each year from natural background sources.

Natural background radiation comes from the following three sources:

• Cosmic Radiation
• Terrestrial Radiation
• Internal Radiation

2.4.2 COSMIC RADIATION

The sun and stars send a constant stream of cosmic radiation to the earth, much like steady drizzle of rain. Differences in elevation, atmospheric conditions and the earth’s magnetic field can change the amount [or dose] of cosmic radiation that we receive.

2.4.3 TERRESTRIAL RADIATION

The earth itself is a source of terrestrial radiation. Radioactive materials including uranium, thorium, and radium] exist naturally in the soil and rock. Essentially all air contains radon, which is responsible for most of the dose that Americans receive each year from natural background sources. In addition, water contains small amounts of dissolved uranium and thorium, and all organic (both plant and animal) contains radioactive carbon and potassium. The dose from terrestrial sources varies in different parts of the world, but locations with higher soil concentrations of uranium and thorium generally have higher doses.

2.4.4 INTERNAL RADIATION

All people have internal radiation, mainly from radioactive potassium-40 and carbon-14 inside their bodies from birth and, therefore, are sources of exposure to others. The variation in dose from one person to another is not as great as that associated with cosmic and terrestrial sources.

2.5. RADIOACTIVE POLLUTION

The radioactive pollution of the environment is majorly from the activities of the solar system (cosmic) and the primordial radionuclide interaction in the earth’s crust (terrestrial) (Turner, 1995). The sources of environmental radiation are both natural and manmade. Many radioactive elements such as radium 224, uranium 235, thorium 232, radon 222, potassium 40 and carbon 14 occur in rocks, soil and water.

Air is radioactively polluted by man’s activities during the atmospheric testing of nuclear bomb, malfunctioning of nuclear plants and the mining of uranium ore (Abalagba, 2011).

1. RADIOACTIVE LAND POLLUTION

The steady rise in the use of Isotopes and nuclear technology in various purposed in human life both agro-industrial military, medical and may increase the chances of radioactive contamination (normal uses or after accidents). That increases the exposure of ionizing radiation (external or internal) which raise awareness in increasing the need to know how to assess that exposure control of imported foodstuffs to ensure that they are not contaminated with radioactive materials is very important.

Studies on radiation level and radionuclide distribution in the environment provide vital radiological baseline information such information is essential in understanding human exposure from natural and man-made resources of radiation and necessary in establishing rules and regulations relating to radiation protection (Ibrahin et al., 2007) Measurements of radioactivity in environment and foodstuffs are extremely important for controlling radiation levels to which mankind is direct and indirectly exposed. Another important fact is that importing contaminated food from any region that suffers a nuclear accident can be indirectly affect people health around can be indirectly affect people health around the world (Melquiades et al., 2004)

The present study aims investigating radioactivity in foodstuff consumed in Delta State. Thirteen (13) staple food stuff has been chosen for the purpose of the study.

2.6. GAMMA RAY SPECTROSCOPY

Gamma ray spectrometry (GRS) provides a fast, multi elemental and non- destructive method of radioactivity measurement. Qualitative and quantitative analysis of samples can be performed by gamma-ray spectrometry systems in which a qualitative measurement identifies the radionuclide of interest from the energies and intensities of the peaks present in the spectrum under investigation. In a quantitative measurement, the activities of radionuclide of interests are determined. Therefore, the accurate determination of the detector efficiency is arguably the most important parameter when gamma-ray spectrometry is used for radionuclide measurement. Gamma ray spectroscopy is an extremely important method in environmental radioactivity (IAEA, 2003).

A gamma-ray photon or an x-ray is uncharged and does not create direct ionization or excitation of material through which it passes. The detection of gamma-rays is therefore critically dependent on causing the gamma-ray photon to undergo an interaction with detector material transferring all or part of the energy to an electron in the absorbing material. Therefore, in order for a detector to serve as a gamma-ray spectrometer, it must carry out two distinct functions. First, it must act as a conversion medium in which incident gamma rays have a reasonable probability of interacting to yield one or more secondary electrons. Secondly, it must function as a conventional detector for these secondary electrons (Mlwilo, et al, 2007).

2.6.1 GAMMA RAY EMISSION

Gamma ray is emitted by excited nuclei while they de-excite to lower-lying nuclear levels. They are produced as a result of beta decay of a parent radionuclide causing a daughter radionuclide in an excited state leading to further decays and the emission of gamma rays. The gamma ray emission caused by de-excitation takes place with energy equal to the change in energy state of the nuclear state. Therefore, gamma ray appears with the half-life of the parent beta decay and with energy that reflects the daughter nucleus energy level structure. Gamma ray sources due to beta decay are generally limited to energies below 2.8 MeV (Knoll, 2000).

Gamma rays are the only member of the electromagnetic radiation spectrum that is caused from the emission of unstable nuclei (Early and Sodee, 1995).

It is possible for gamma ray to produce ionization and penetrate through matter. Theoretically, there is no enough shielding that could be provided to entirely stop any gamma ray (Early and Sodee, 1995).

2.6.2 PRINCIPLES OF GAMMA RAY SPECTROMETRY WITH GERMANIUM DETECTORS

Germanium (Ge) detectors like other semiconductor detectors contain certain levels of impurities. However, the impurity levels in high purity Ge have been largely reduced by techniques such as zone refining in such a way that an impurity level of only about 1010 atoms /cm3 still exists. (If the net concentration favors acceptor atoms (like Al) then the material is mildly p-type called π-type). The coaxial configuration is commonly used in HPGe detector construction (Knoll, 2000).The crystal is cut as a cylinder and a part of the central core is removed. The rectifying contact is fabricated at the outer cylindrical surface by creating a heavily doped layer of the opposite type (n type, called n+). This is formed by evaporation and diffusion of Lithium (Li) or by direct implantation of donor atoms using an accelerator. The n-p junction is reverse-biased and forms the detector active depletion region. It also serves as a blocking contact, since it is difficult to inject electrons from the p side because holes are the majority carrier and electrons are scarce. Without blocking the contacts the steady-state leakage current through the detector bulk material would be so large that discerning the small electric charges created by the radiation would be impossible. The opposite electrode is fabricated in contact with the inner cylindrical surface, which is the blocking contact for majority carriers. Therefore it consists of heavily doped p type layer, produced by ion implantation of acceptor atoms such as boron. HPGe material may be both high-purity p or n type over the whole volume of the crystal until reaches the other electrode and the electric field profile is nearly uniform. When gamma-ray photon interacts with the detector material, electron-hole pairs are produced and their number is proportional to the energy imparted to the detector material. Since the electrostatic field exists in the detector volume, electrons and holes are separated and swept to their respective electrodes. The motion of the charges induces a current to flow in the external circuit. The integration of the current pulse gives the produced charge. In Ge crystal, the hole and electron mobility-lifetime products have similar enough values to ensure that both carriers will reach the electrodes at the majority of events (Mlwilo, et al, 2007).

2.7 GAMMA RAY INTERACTIONS IN THE DETECTOR

The process of detection of radiation is initiated when an incident gamma ray enters the crystal. The energy is totally absorbed under ideal conditions. This is accomplished by three main interactions between gamma ray and the crystal. These interactions are photoelectric effect Compton scattering, and pair production.

2.7.1 PHOTOELECTRIC EFFECT

Photoelectric effect is an interaction between a low energy incident photon and an inner shell orbital electron. The energy of the incident photon is totally absorbed and used to eject the orbital electron from its orbital shell. This electron is known as photoelectron. This happens if the incident photon exceeds the binding energy of the electron. The ejected electron leaves a vacancy in the inner orbital shell causing the emission of characteristic x-rays or auger electron when an outer shell electron full fills this vacancy (Sorenson and Phelps, 1987 and Knoll, 2000).

2.7.2 COMPTON SCATTERING

The interaction process of Compton scattering takes place between the incident gamma ray photon and an electron in the detector material (F. Adams and R. Dams, 1970). The incoming gamma ray photon is deflected through an angle Ө with respect to its original direction and hence the photon transfers part of its energy and momentum to a free electron, creating a recoil electron. Since all angles of scattering are possible, the energy transferred 9 to the electron can vary from zero to a large fraction of the gamma ray energy (Knoll, 2000).

2.7.3 PAIR PRODUCTION

This interaction mechanism occurs in the intense electric field near the protons of the nuclei in the absorbing medium (detector) and corresponds to the creation of an electron-positron pair at the point of complete annihilation of the incident gamma-ray photon. For this process to take place, the gamma ray energy should exceed twice the rest mass energy of the electron (1.022MeV). Both positron and electron disappear and are replaced by two annihilation photons of energy m0c2 (0.511MeV) each emitted back to back (Knoll, 2000).

2.8 SYSTEM CALIBRATION

The calibration of an analytical system is one of the most important tasks required of any analyst. If the calibration is incorrect then all the result produced will be inaccurate. The essential requirements of a calibration are to establish an energy / efficiency / resolution relationship (Sutton, 1993). For gamma ray measurement with germanium detector.

2.9 RADIATION EFFECT ON HUMANS

Every inhabitant of planet earth is constantly exposed to naturally occurring ionizing radiation called background radiation. Sources of background radiation include cosmic rays from the sun and stars, naturally occurring radioactive materials in rocks and soil, radionuclide normally incorporated into our body’s tissue and randon and its product we inhale, we are also exposed to ionizing radiation from man-made sources, mostly through medical procedures like X-ray diagnostics. Radiation therapy is usually targeted only to the affected tissues. Only about 12% of all the cancers that have developed among those among those survivors are estimated to be related to the affected tissues. Ionizing radiation can cause important changes in our cell by breaking the electron bond that holds molecule together. For example radiation can damage our genetic materials (DNA). But the cells also have several mechanisms to repair the damage done to the DNA by the radiation. Potential biological effects depend on how much and how fast a radiation dose is received. An acute radiation dose (a large dose delivered during a short period of time) may result in effects which are observable within a period of hours to weeks. A chronic dose is a relatively small amount of radiation received over a long period of time. The body is better equipped to tolerate a chronic dose than an acute dose as the cells need time to repair themselves. Radiation effects are also classified in two ways namely somatic and genetic effects.

2.9.1 SOMATIC EFFECTS

Somatic effect appears in the exposed person, The delayed somatic effects have a potential for the development of cancer and cataracts. Somatic effects of radiation include skin burns, vomiting, and hair loss. Temporary sterility in men and blood changes, chronic somatic effect include the development of eye cataract and cancers.

2.9.2 GENETIC EFFECT

The second class effects, namely genetic or heritable effects appears in the future generations of the exposed person as a result of radiation damage to the reproductive cells, but risks from genetic effects in humans are seen to be considerably smaller than the risk for somatic effects. Radium causes bone weakening, cranial and nasal tumors. Other diseases caused by radioactivity exposure include lung cancer, pancreas, hepatic, bone, skin, kidney cancers, cataracts, sterility, atrophy of the kidney and leukemia (Tasken et al, 2009)

2.10 GENERAL EFFECTS OF RADIOACTIVITY ON THE HUMAN BODY

Every inhabitant on this planet is constantly exposed to naturally occurring ionizing radiation called background radiation. Sources of background radiation include cosmic rays from the Sun and stars, naturally occurring radioactive materials in rocks and soil, radionuclides normally incorporated into our body’s tissues, and radon and its products, which we inhale. We are also exposed to ionizing radiation from man-made sources, mostly through medical procedures like X-ray diagnostics. Radiation therapy is usually targeted only to the affected tissues. Only about 12% of all the cancers that have developed among those survivors are estimated to be related to radiation. Ionizing radiation can cause important changes in our cells by breaking the electron bonds that hold molecules together. For example, radiation can damage our genetic material (DNA). Although radioactive waste has known negative effects on humans and other animals, no substantial scientific proof of bad effects on the ocean and marine life has been found. Nuclear wastes which are Radioactive, is a potential harbinger of radioactive exposure to humans through many channels. Exposure to radioactive waste may cause serious harm or death. In humans, a dose of 1 Sievert carries a 5.5% risk of developing cancer, and this risk is assumed to be linearly proportional to dose even for low doses. Ionizing radiation causes deletions in chromosomes. If a developing organism such as an unborn child is irradiated, it is possible a birth defect may be induced, but it is unlikely this defect will be in a gamete or a gamete-forming cell. The incidence of radiation-induced mutations in humans is small, as in most mammals, because of natural cellular-repair mechanisms, many just now coming to light. These mechanisms range from DNA, mRNA and protein repair, to internal lysosomic digestion of defective proteins, and even induced cell suicide apoptosis. The routes are direct exposure to materials that are radioactive, inhalation and ingestion of such materials through the air that one breathes or food that one consumes. The quantum of exposure (dose × duration of exposure) decides the deleterious effects that may result. Exposure may occur to particular organs locally or to the whole body. Sufficiently high exposure can lead to cancer.

The radio-toxicity of a particular radionuclide is quantified in terms of what is referred to as ‘potential hazard index’ that is defined in terms of the nuclide availability, its activity, maximum permissible intake annually and its half-life. This depends on a variety of factors like physical half-life, biological half-life, sensitivity of the organ or tissue where the nuclide is likely to concentrate, ionizing power of the radiation from the nuclide that depends on the energy of the radiation emitted from the radionuclide, etc..(Effect of radiation on the human body, Wikipedia, 2009).

Figure 1: The diagram shows different level of radioactive dosage and how they can affect the human body