Ogun river serves three states (Oyo, Ogun and Lagos) greatly in terms of economic and social importance. In areas of high population density the river is used for domestic purposes such as bathing, washing and drinking. Fishing is also carried out in major part of the river.
Artisanal fisheries are major activities in lower Ogun river. At Isheri-Olofin, lower Ogun river receives effluents from ‗Kara‘Abattoir which was established in 1984. An average of 200 cows are slaughtered and butchered at the abattoir on daily basis (Ikenweiwe et al., 2011). Meat and milk production are done around the river banks. The effluents being discharged into the river chiefly contain the gut contents of the slaughtered and butchered cows, therefore the river acts as sink to most organic wastes from abattoirs located along its course. The source of income of most people are based on this river because sanding and escavations are done day and night.
The sediments obtained from the river is used to build houses where people live.
Importantly, the three states through which the river passes, are heavily industrialised cities:
most especially, Lagos and Sango – Ota(ogun state) for instance, about six major industries including Vitabiotics, Nestle, Glaxos, Smith kline, Sona Breweries and Nigerian German chemicals discharge their wastes into the river (Farombi et al., 2007). The study area is an area where there is diversification of trade. There is a dam, Ikere Gorge, situated on Ogun River, about 8 km east of Iseyin around Ojubo Sango in Oyo state. There is also a basin, which lies between latitudes 6o33N and 8o58N and between longitudes 2o 40E and 4o10E with total area of about 23,700 km2.Different water uses, including domestic, commercial, industrial and agricultural takes place within the basin (Ojekunle et al., 2011). Farming activities completed with the use of fertilizer to facilitate good crops, most especially cassava
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plantation, hunting, mat making, fishing, cloth dyeing have been the chief occupations of the local people for many decades. However, commerce and industry are other major human activities within the area, which include the Planet Plastic industries in Mile 12, sawmill (plank) industries, as well as the popular food market at Mile 12 (Tejuoso, 2006). Human and industrial activities are higher at the middle and toward the lower parts than at the upper part of Ogun river.
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CHAPTER THREE
RADIATION DETECTION TECHNIQUES
3.1 Interaction of Electromagnetic Radiation With Matter
Gamma rays can be defined as an electromagnetic radiation with both particle and wave-like characteristics. They travel at the speed of light (c), and have a discreet energy (E), frequency (f), and wavelength (λ). These are related by:
hf hc
E (3.1)
where h = Planck‘s constant 6.6261x10-34 Js; c = velocity of light.
Gamma rays are produced by transitions between nuclear energy states. The gamma –ray energies are typically ~100 keV to ~5 MeV (Knoll, 2000). Although there are many possible interaction mechanisms of gamma rays with matter, only three most important processes play a significant role in radiation detection measurements. These are the photoelectric absorption, Compton scattering and pair production. In photoelectric absorption process, the incident photon transfers all of its energy to a bound
electron. Photon disappears and electron is ejected from the atom (mainly from the k shell).
The vacancy shell will be filled quickly through capture of a free electron from other shells of the atom. Therefore, characteristic X-ray photons may also be generated.
The photoelectron appears with an energy given by Eb
e h
E (3.2)
Where h and Eb represent the energy of the photon and the binding energy of the photon respectively. The photoelectric effect is generally the dominant attenuation mechanism for incident photon energies 200 keV, i.e photoelectric effect is the predominant absorption process at low gamma energies. The probability of photoelectric absorption (m2) varies approximately with the atomic number (Z) of absorber material and photon energy (E ), according to the equation (Knoll, 2000),
5 .
tan 3
E t Z Cons
n
(3.3)
Where n varies between 3 and 4 and E is the -ray energy. The reason why high atomic number materials such as lead are used to shield radiation detectors from X-ray sources is
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because of the strong influence of atomic number on the probability of photoelectric absorption. For the Compton effect, it is the elastic scattering of gamma photons by free or loosely bound electrons. In this process, the photons imparts parts of its energy to a free or loosely bound electron and the gamma photon is deflected as though the electrons were at rest with rest mass m and an elastic collision had taken place. This elastic scattering is characterised by the kinetic energy exchange between the colliding particles without loss of energy in excitation and without transition of energy into the bound state. During this process, the incident photon with energy hν is deflected and scattered through an angle with respect to its original direction and its original energy reduced from h to h ′. The frequency is changed and its wavelength increased from λ to λ'. The probability of Compton effect occuring is inversely proportional to the atomic number of the absorbing material and proportional to the gamma ray energy. Compton scattering predominates at moderate gamma-ray energies typical of environmental radioactive materials (Knoll, 1989).
Pair Production is the conversion of photon energy to mass in the vicinity of the nucleus of the atom. This is only possible if E > 1.022 MeV, the rest mass of the pair. It is the process in which high energy photons interact with matter. Here, the gamma photon interacts with the coulomb field surrounding a nucleus or an electron and it disappears with the creation of an electron and positron pair whose total energy is equal to the energy of the initial photon. Both the positron and electron produced lose their kinetic energies by excitation of the absorbing crystal. Positron then annihilates with an electron to produce two photons, each with 0.511MeV energy in approximately 1800 to conserve momentum which may in turn undergo either photoelectric effect or Compton effect. The dominance of the above effects as a function of -ray energy and the atomic number of the absorber is given in Figure 3.1. For the energy range of the natural radionuclides, the Compton effect is most important. When a beam of -ray photons passes through matter, it is attenuated by the interaction processes in the material. The probability per unit path length that the -ray photon is removed from the beam (Knoll, 2000) is expressed in the linear attenuation (absorption) coefficient σ (cm-1):
e x I
I
O
(3.4)
where I is the intensity of the beam of photons with energy E after attenuation, I0 is the
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Fig. 3.1: The interaction of gamma rays with matter (Sakanoue, 1994).
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initial intensity and x the traversed distance in the material. The linear attenuation coefficient depends on -ray energy and density ρ of the absorber. The attenuation coefficient can also be expressed in the form of the mass-attenuation coefficient σ/ρ (cm2 g-1), which depends on -ray energy but is almost independent of the state of the material.