Introduction
In chapter one, the researcher introduced the reader to the proposed study. Several aspects of the proposed study were highlighted. These included the research design, the research questions to be addressed and such other things. In this chapter, the researcher will provide the reader with a critical review of literature that exists in this field. 8 articles were identified by the researcher and the findings as well as the methodology adopted will be critically analyzed. The link between the studies reported in these articles and the proposed study will be analyzed.
In spite of the care and precaution that is taken by human beings to stay away from radiation and radioactive material, majority of the people are not aware of the fact that human beings are always exposed to naturally occurring radioactive materials in the environment. This radiation is referred to as natural radiation and is presented by particles both within and without the earth’s atmosphere.
Natural radiation is present and predominant in the earth’s atmosphere. It is to be found in various elements that we come into contact with in our daily environment. According to studies conducted by scholars such as Ibrahim and others in 1993, Aly Abdo and others in 1999, Malance and his colleagues in 1996, Myrick and his colleagues in 1983, Maul and Ohara in 1989 and Pimpl and colleagues in 1992 (and cited in Singh, Rani & Mahajan 2005), elements such as the soil, rocks, plants, water and even air all have some form of natural radiation emitting from them. Zaidi, Arif, Ahmad, Fatima & Qureshi (1999) cites the UNSCEAR as saying that natural radiation is the largest source of radiation in the world today.
From lecture notes 2 (n.d), it was noted that Gamma rays from these radiations are particularly a source of concern. Exposure to these rays on the part of the human body is dangerous and a cause of several diseases and radiation- associated problems. This is for example the irradiation of lung tissue arising from the inhalation of radon and related radio nuclides. The analysis and assessment of the dosage of gamma radiation to humans from these everyday natural sources is therefore necessary and of grave concern. From these measurements, critical precautionary steps can be taken especially if the dose of gamma rays from natural radiation in a particular area is found to be above the normal or recommended limits (Kannan, Rajan, Iyengar & Ramesh, 2002). In this review, the researcher will examine the gamma spectroscopy technique of analyzing natural radiation. Some of the experiments undertaken by several groups on the extent of natural radiation are briefly illuminated.
Estimation of Natural Radioactivity Levels by Gamma Spectrometry Technique
In order to estimate the terrestrial gamma dose rate for an outdoor setting, it is important to estimate the natural radioactivity in soils. The natural radioactivity in soils is usually determined from the 226Ra, 232Th and 40K content of the soil sample (OECD 79 as cited in Lecture 1, n.d). Since almost 98.5% of uranium radiological effects are produced by radium and its daughter isotopes, contribution from 238U and 226Ra precursors are not taken into consideration in many occasions (Zaidi et al., 1999).
Uranium is found in trace quantities in almost all kinds of soils, rocks, sand and even water. It displays unique dissolving and precipitation qualities that help it in forming deposits within the earth where conditions favour this deposition. This particular study was objective and aimed at determining the radioactivity concentrations in soil samples for the 238 U, 226Ra, 232Th and 40K isotopes collected from different regions. It was also aimed at observing the variations between different regions. This is how the experiment proceeded.
Soil samples were collected from several undisturbed regions. These were then prepared by getting rid of organic material and stones and finally heated in an oven for approximately two hours at an average temperature of 1000C before being crushed. The samples were then properly sealed and packed for at least 4 weeks before counting by gamma spectrometry to ensure the daughter products has attained equilibrium in relation to their respective parent radio- nuclides. The gamma ray spectrometer was calibrated to about 3MeV using different disc-type reference standard sources. The counting time was then set at 12000s for each sample in order to reduce statistical errors. The radioactivity of 226Ra was then determined and analyzed. From this experiment, several by- measurements were attained using different mathematical computations in figures derived from the experiment. These included the air absorbed dose rates, external hazard index (Kannan et al., 2002), the annual effective dose and the uranium content.
Using the values obtained from this experiment, the researchers were able to determine several factors about the natural radioactivity experienced in a region. The high level areas were also identified and several explanations as to the level variations can be offered. The major one is explained by relating it to the tectonic forces and the distribution of tectonic plates.
Natural Radioactivity in Building Materials
While natural radioactivity may be generally conceived as originating from objects that are outside the environment, it has been proven that radioactivity is actually present even in our homes. We are constantly exposed to radiation when within our normal dwelling houses where we tend to feel safe or in offices where we hide from the outside environment. It is noted that natural radioactivity is taking place every minute of the day. Building materials contain several naturally occurring radio- nuclides including 40K, 238U, 232Th and their daughter products (Papachristodoulou, Assimakopoulos, Patonis & Ioannides, 2003). The level of these products in the materials used in building and a measure of the radioactive exposure they are causing to the users and inhabitants of the buildings is essential in assessing the safety of the people in a nation. Many countries- both developed and developing- have decided to undertake nation-wide surveys to assess this radioactive exposure. This is in a bid to try and identify possible hazards that need to be corrected before manifesting and also to ensure safety measures are put in place when it comes to building material standards and specifications (Papachristodoulou et al., 2003).
While the radioactive content and activity of several materials- such as industrial waste and mineral industries- are closely monitored and observed, several governments including that of Pakistan have made little or no efforts to study the radioactive elements in building materials. The Pakistan Environment Protection Agency is yet to define clear values and standards regarding acceptable levels of radioactivity in building materials. Using the same approach of gamma spectrometry technique to determine natural radioactivity, a study was undertaken to measure the concentration of 238U, 232Th and 40k in three main building materials in the regions of Rawalpindi, Islamabad (Zaidi et al., 1999).
The process was fairly simple and short. Different samples of sand were collected from three different locations which are the main source of sand used in the region. The samples were then mixed and homogenized and a representative sample for the three regions was then prepared. Three common cement brands were also randomly selected for the study. Ten samples of each brand were treated similarly as the sand samples. Three brick varieties were finally identified and five samples of each variety were collected. Representative samples were then prepared for the varieties by crushing, pulverizing and homogenizing them.
A cup was filled with weighed quantities of all these materials and then sealed and stored for a month to allow the sample to attain radioactive equilibrium (Zaidi et al., 1999). The gamma ray spectra for the samples ware then measured for twenty four hours and activities of radio- nuclides calculated.
A lot of calculations were carried out in the process of conducting the study and thought provoking questions posed regarding the significance of the amount of radioactivity from building material. The researchers also tried to analyse what can be done to correct the situation should anything bad happen. 40K content in bricks and sand used for building was found to be about three times higher than that in cement. The study also revealed that bricks contain higher amounts of uranium and thorium as opposed to other building materials (Zaidi et al., 1999). This can be attributed to the presence of these minerals in the soil which is the raw material for bricks. The study also tried to make comparisons between radioactive component levels in building materials from different areas and countries around the world.
The reliability of the gamma spectrometry technique in identifying these levels of natural radiation is further verified by analyzing IAEA CRMs soil -7 and lake sediment (SL-1) for uranium and thorium. The values indicate that there is close relationship between the two. The gamma spectrometry and INAA complement each other perfectly. The results made from these experiments are indeed useful in assessing the exposure of natural radiation to the population. Further, the method can be generalised and used to analyse several other building materials to improve public safety. This is by ensuring that the materials meet the safety standards recommended by worldwide agencies. These are materials like flexi-glass, glass, aggregate stone and even marble.
The Gamma Ray and other Naturally Occurring Radioactive Elements
The interaction between cosmic ray particles in the earth’s atmosphere sometimes produces natural radioactive particles called cosmogenic radio- nuclides. These are nuclides such as 3H, 7Be, 14C, and 22Na. The other forms of radio- nuclides are referred to as primordial radio- nuclides and are formed by the process of nucleo- synthesis in stars (Singh et al., 2005). The absorbed ‘dose rate’ of cosmic radiation in air is 30 nGy h-1 (Tzortzis, Tsertos, Christofides & Christodoulides, 2003). Sedimentary rocks are usually related to lower radiations while igneous rocks such as granite portray higher terrestrial radiation patterns. There is however some variation such as in shale and a few other different rock types.
Traditional methods of determining radioactivity in low- level natural radioisotopes such as α spectrometry, liquid scintillation counting and even mass spectrometry have always proven indispensible for detecting low nuclide concentrations (Lecture Notes 3, n.d). However, in cases where low detection limits are not a specification, high resolution gamma spectrometry provides an alternative. This alternative is often non-destructive, variedly elemental in application and certainly consumes less time to conduct it. The reliability of gamma ray spectroscopy has often been illustrated during its use in the quantitative analysis of uranium, thorium and their decay products in environmental samples (Lecture notes 1, n.d).
Using the representative sample that was computed, a high purity germanium detector was used to identify the activity of uranium in soil samples that were collected from military regions of Greek peacekeeping troops in Kosovo (Papachristodoulou et al., 2003). The concern was the health of the soldiers due to the depleted aluminium ammunition in the area. The use of hyper germanium gamma ray spectroscopy is critical in identifying and estimating the isotopic composition of uranium in soils accurately. The measurement and analysis of several different spectra present in the soils is useful when it comes to evaluating and identifying the different nuclides. In this particular experiment, the results showed that uranium isotopes were in their natural abundance states. The typical accepted level of 238U ranges from 48 to 112 Bq kg -1 and an average value of 90 plus or minus 22 Bq Kg-1 (Papachristodoulou et al., 2003).
In general, gamma spectroscopy provides the most appropriate method of measuring, identifying and illustrating the presence of natural radiation which is to be found in the natural environment within which we are living. Using this technique, the experiments cited above plus others documented in the literature in this field are able to indicate the presence of these radiations in the soil, in the air, in our building materials and even in the water we use on a daily basis. The importance of conducting such experiments and identifying the level of radiation the population is usually exposed to is not only a form of research but it also acts a strategy in tackling several potential problems before they manifest themselves (Lecture notes 1, n.d).
It is surprising to note that as agriculture thrives to sustain human race and support our livelihoods, natural radiation from products such as fertilizer could lead to problems if not carefully regulated. Measurements made locally in the environment and as regularly as possible should be supported to ensure that several elements and the radiation they expose the population to are kept in check. The commissioning of several private research experiments in these fields has proven beneficial to the immediate environment and community in several occasions. It should however be noted that experiments involving the use of radioactive materials is in many occasions dangerous. Proper care should be taken at all times and the right materials obtained before attempting to carry out such experiments.
In conclusion, it is noted that gamma ray spectroscopy has the ability to clearly identify what was earlier ignored by scientists as background counts, proving to be essentially useful. Experiments like the few discussed in this review go a long way in identifying and explaining the importance of being familiar with these external factors which may be affecting our lives every single day.
Bibliography
Kannan, V., Rajan, M., Iyengar, M., & Ramesh, R ‘Distribution of natural and anthropogenic radionuclides in soil and beach sand samples of Kalpakkam (India) using hyper pure germanium (HPGe) gamma ray spectrometry’. Applied Radiation and Isotopes, vol. 57, 2002, pp. 109-119.
Lecture Notes 1 ‘Environmental aspects: An introduction to radioactivity in the environment’. Retrieved from *client insert their university and department*.
Lecture Notes 2 ‘Environmental aspects: Radiation all around us.’ Retrieved from *client insert their university and department*.
Lecture Notes 3 ‘Environmental aspects, low level counting.’ Retrieved from *client insert their university and department*.
Papachristodoulou, C., Assimakopoulos, P., Patronis, N., & Ioannides, K ‘Use of HPGe γ-ray spectrometry to assess the isotopic compositiion of uranium in soils’. Journal of Environmental Radioactivity, vol. 64, 2003, pp. 195-203.
Singh, S., Rani, A., & Mahajan, R ‘226Ra, 232Th and 40Kanalysis in soil samples from some areas of Punjab and Himachal Pradesh, India usinggammaray spectrometry’. Radiation Measurements, vol. 39, 2005, pp. 431-439.
Tzortzis, M., Tsertos, H., Christofides, S., & Christodoulides, G ‘Gamma-ray measurements ofnaturally occurring radioactive samples from Cyprus characteristic geological rocks’. Radiation Measurements, vol. 37, 2003, pp. 221-229.
Zaidi, J., Arif, M., Ahmad, S., Fatima, I., & Qureshi, I ‘Determination of natural radioactivity in building materials used in the Rawalpindi/Islamabad area by g-ray spectrometry and instrumental neutron activation analysis’. Applied Radiation and Isotopes, vol. 51, 1999, pp. 559-564.