Gamma Ray Spectroscopy Analysis of Environmental Samples: a Literature Review

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Introduction

Radiation is the process through which particles or waves containing energy travel through matter and vacuums that are not necessary for them to be propagated. The waves and particles may be from natural or man-made sources. For this reason, there are many forms of radiation. Many people associate radiation with nuclear and atomic energy. Others tend to consider radiation as a form of radioactivity. What many do not know is that radiation is a form of energy that is present everywhere at any particular time. Every individual encounters some form of radiation in their daily life, either knowingly or unknowingly. For instance, signals from radio and television stations are a form of radiation that travels from their source of transmission without the use of any medium.

A Brief History of Radiation

Radiation is an evolving discipline in physics. Antoine Becquerel, a French physicist, is said to have discovered radioactivity, along with Pierre Curie and Marie Curie. The discovery saw the three win the 1903 Nobel Prize in physics. Becquerel was born on 15th December 1852, in Paris. His family is famous for having produced four consecutive generations of scientists, starting from his grandfather, César Becquerel. He was selected as the head of the physics department at the National Museum. His tenure in the department commenced in 1892. He was the third member of his family to take up the position. Two years later, he was promoted to work as the chief engineer in the department of bridges and highways.

Becquerel’s discovery was a result of his endless efforts to discover the physics behind phosphorescence, the emission of light of a particular color after a body is exposed to the light of a different color. His work was further inspired by Wilhelm Conrad Röntgen, who discovered X-rays in 1896. In his experiments, Becquerel was able to establish those phosphorescent substances, when exposed to sunlight or high intensity, produced penetrating radiations, similar to those observed in X-rays. He used Uranium salt as his test phosphorescent material.

Marie Curie was also instrumental in the discovery of radioactivity. She was born on 7th November 1867, in Poland. Her career was based in France, where she worked as a physicist and chemist. She is remembered to have pioneered research touching on radioactivity. Her active involvement in physics saw her become the first woman to be awarded a Nobel Prize, and the only woman to win in two scientific fields. In addition, her contributions to physics made her the first woman professor to teach at the University of Paris.

Radiation occurs in many different forms. Visible light and sound waves are examples of radiation. In addition, it occurs in other complex forms, such as ultraviolet and infrared radiations. Radiations also come from a variety of sources. The sources may be natural or man-made. The earth itself is a natural source of radiation. Cosmic rays are a good example of radiation emanating from the earth. The sun is another natural source of radiation. Ultraviolet rays, commonly associated with skin cancer, originate from the sun. Other forms of radiation result from man’s activities.

Scientific advancements have led to the increased application of radiation in the world today. The rising demand for electricity and cheaper sources of energy has prompted governments and companies to seek alternative sources of power. Nuclear energy is identified as one such source, a situation that has seen the establishment of nuclear reactors. The reactors put into use knowledge on neutron fission. Man-made radiation is modified to fulfill the intended purposes. For instance, radiotherapy has become an important field in medicine today. The procedure involves the use of radiation waves to diagnose and treat various medical conditions, such as cancer. Radiation has been used for research purposes. The carbon dating technique is, for instance, used to analyze ancient materials and objects. The knowledge obtained from this field is important, especially to individuals who wish to study historical events.

Types of Radiations

Radiations are classified into two broad categories. The two are ionizing and non-ionizing radiations. The former displace electrons contained in an atom, charging them in the process. As a result, the structure and composition of an atom are changed. Subsequently, the chemical properties of the atom are also changed. Ionizing radiations are used in many medical imaging techniques as a result of this property. Examples of imaging technologies that use ionizing radiations include X-rays and diagnostic computerized tomography. The radiations produced by this medical imaging equipment change the chemical structure of particles that make up biological tissues. Ionizing radiations alter the chemical composition of these tissues, even without the generation of heat. Radioactive materials known to be sources of ionizing radiations should therefore be properly disposed of to ensure they do not pose risks to individuals living or working in areas where they are used.

Nuclear reactors used for the production of nuclear energy use ionizing radiation. The radiations are also used in coal-fired power stations. Ionizing radiations are used in these fields as a result of their ability to displace electrons from the atoms that previously contained them. The displacement of electrons is the major principle behind the generation of electricity. Individuals working in such power production plants are, therefore, exposed to the risk of medical defects brought about by exposure to ionizing radiations. Such individuals are prone to diseases, such as cancer.

Non-ionizing radiations are not as powerful as ionizing radiations. Although they can change the position of an atom, they are incapable of changing the structure and chemical composition of the atoms. As a result, non-ionizing radiations are less harmful to body tissues.

Both forms of radiation travel through an object or body without leaving a trace. Prolonged exposure to these radiations is harmful to biological tissues. Research has shown that such exposure brings about molecular damage to tissues, leading to cancer. Birth defects among children may also be a result of the parents having been exposed to radiation, which affected the genetic materials in their primary sex organs. For this reason, medical practitioners are required to carefully regulate the radiation dosage used in diagnosing and treating patients.

Radioactive Isotopes

Radioactive isotopes are good sources of radiation. Since they are unstable, the isotopes emit radiation during their decay process. The decay process in radioactive isotopes is spontaneous and reduces over time. The reduction stabilizes the radioactive isotope. Stable isotopes no longer emit radiations. Therefore, only unstable atoms generate ionizing energy. Radioactivity only comes down to zero when all the atoms of the radioactive substance become stable.

The radiological half-life is denoted as t½. It is the amount of time that a radioactive isotope requires to achieve 50% decay. Each radioisotope has its own unique t½. Some of the highly radioactive isotopes may have a t½ that is as short as a fraction of a second, while others, which are less radioactive, may have a t½ that runs into billions of years. An isotope with a short t½ is considered to be more radioactive than that with a long t½ since they emit more radiation within a short period of time.

There are three types of radioactive decay. They include alpha, beta, and gamma decays. The three vary depending on the type of particles given off after the decay process. Alpha decay occurs when the particles given off from the nucleus are a pair of neutrons and a pair of protons. Such a form of decay brings about a reduction in the atomic number by a unit of two. At the same time, the mass of the atom reduces by a unit of 4. There are various radioactive isotopes that undergo this form of decay. They include uranium and radium.

Beta-decay involves the conversion of a neutron into a proton and an electron before being expelled out of the nucleus. During this form of decay, the atomic number of the radioisotope reduces by 1, while only a slight decrease in mass is noted. Carbon-14 is a good example of a pure beta decay isotope. Other examples include sulfur-35, strontium-90, and tritium.

Gamma is another form of decay. It occurs when beta and alpha decays leave behind some energy in the radioactive isotopes. It can also take place following neutron capture, a reaction that takes place in nuclear reactors. Unlike other forms of decay, gamma decay neither affects the atomic number nor the mass of the atoms constituting the radioactive isotope. Examples of radioisotopes that undergo this form of decay include iodine-131 and radium-226.

Sources of Radiation

Human beings are exposed to a wide range of radiation. Our bodies have, over time, adapted to these radiations, protecting us from their adverse effects. Most radioisotopes occur naturally and are believed to have come into existence during the creation of the solar system. Such isotopes have a t½ of billions of years. Radioactive isotopes are also thought to have resulted from the interaction between cosmic rays and atmospheric molecules. A good example of a radioisotope formed as a result of these interactions is tritium. As a result of these naturally occurring radioisotopes, the earth experiences a constant natural form of radiation. The natural ionization of the earth is referred to as background radiation. The United Nations indicates that there exist four major sources of natural radiation. They include cosmic radiations and terrestrial radiations. The others involve exposure by means of inhalation and ingestion.

Cosmic radiations originate from the sun and other divine events of the universe. The radiation takes place as a result of continuous bombardment of the earth’s outer atmosphere by fast-moving particles. Cosmic waves may occur in form of protons, waves of energy, or other forms of particles. Some of the ionizing radiations make it to the earth’s atmosphere, where they get into contact with human beings, thereby exposing them to natural radiation.

Terrestrial radiation results from naturally occurring deposits of radioisotopes, such as uranium and thorium. The isotopes constitute a considerable portion of the materials that make up the earth’s crust. Terrestrial radiation is brought about by the natural decay of the radioisotopes. It is the major source of natural radiation. During natural decay, small amounts of ionizing radiations are released. Since the naturally occurring radioisotopes are found almost everywhere on the earth’s crust, all human beings are naturally exposed to this form of radiation.

Exposure to natural radiation can also take place using inhalation. It takes place when individuals breathe in gases released by radioactive minerals occurring naturally in rocks and soils. Radon, a radioactive gas resulting from the decay of uranium, is regarded as one of the major causes of this form of radiation. Thoron is another example of a radioactive gas emitted by decaying thorium. When released into the environment, the two gases are generally harmless. However, there are some instances where these gases are trapped in buildings, leading to their accumulation. When left to accumulate, the gases pose health risks to those who inhale them. Uranium miners are at a high risk of being exposed to this form of natural radiation.

Humans are also exposed to natural radiation by means of ingestion. Small amounts of radioisotopes are present in food, including drinking water. A high percentage of radioactive substances ingested by humans are from the soil and groundwater. The process leads to internal exposure to radiation. There is a link between natural radioisotopes and non-radioactive isotopes. For example, the two have identical chemical properties. As a result, they perform similar biological functions. Such radioisotopes include carbon-14 and potassium-40.

Nuclides generated from the natural decay of long-lasting radioisotopes are radioactive and, as such, decay again. Such radioisotopes include uranium-238 (t½=4.5 billion years), uranium-232 (t½=14 million years), and uranium-235 (t½=0.7 billion years). Decay chains occur as a result of continuous and recurring decay processes. The chains are only terminated when a nuclide that is not radioactive is formed. For instance, the uranium-radium chain of decay begins from uranium-238 to lead-206, which is considered stable.

Radiations used in medical diagnosis and treatment are man-made. The radioisotopes used in this technology are usually by-products of nuclear reactors. Radioisotope generators are also important sources of man-made radioisotopes. The radioisotopes are used in agriculture, manufacturing industries, and biochemistry.

Medical exposure is the most common form of man-made radiation. Most individuals exposed to this form of radiation include those undergoing X-rays. The X-ray machine is used in health facilities to locate broken bones and dislocated joints. With the technological advancement in the field of medicine, X-ray machines are also used in the diagnosis of various diseases. Through the use of gamma radiations, a gamma camera is used in medical diagnosis. Radiations are also used to sterilize equipment.

Individuals are also exposed to man-made radiation through a number of industrial appliances. Nuclear gauges, for instance, are used in road construction. Density gauges are also installed in factories to determine the rate at which substances flow through pipes. Some of the elements glow in the dark. As a result, they may be used as signs to guide individuals through dark exits. Radiations are also used to prospect for minerals and natural resources. For instance, radiations have been used to estimate the reserves of such resources as oilfields.

Radioisotopes are used in nuclear power plants. Radium is used to generate chain reactions, which lead to the production of heat. Water is then subjected to the heat generated, leading to the production of steam that is then used to drive turbines. Electricity is produced in the process. Nuclear power plants release small amounts of radioisotopes and radiation to the environment. The activities carried out in fuel manufacturing plants, uranium mines, and waste facilities must, therefore, be regulated to protect the public from irresponsible handling of radioactive materials.

Atmospheric testing is another common source of man-made radiation. Exposure through atmospheric testing mainly occurs as a result of testing atomic weapons. Fallouts in form of radioactive substances are released into the air before settling on the ground and becoming part of the environment. The radioisotopes that ‘fall out’ on the ground continue to decay, emitting ionizing radiation to the environment.

Properties of Gamma Rays

The photoelectric effect is the term used to refer to the interaction between a gamma ray and an orbital electron expelled from the atom. The interaction makes the rays lose all their energy. The difference between the energy of the incident gamma and the electron’s binding energy is similar to the energy contained in the electron that has been ejected. The interaction helps in the detection of this type of rays given that the ejected electron exhibits a complete energy peak when observed via an energy spectrum. The photon energy of the medium and its atomic number has a great bearing on the photoelectric effect.

The Compton scattering effect brings about the partial loss of the initial energy by a gamma-ray during its interaction with an electron. The interaction brings about the ejection of the electron, while the gamma-ray is scattered. The incident energy contained in the gamma-ray is shared between the electron and the scattered rays as kinetic energy, based on the scattering angle. The kinetic energy of the ejected electron is equal to the energy differences between the initial and the scattered gamma.

Pair production takes place when the energy contained in the gamma-ray is more than two times that of the electron’s rest mass. An electron’s rest mass is usually denoted as 1.02MeV. Following this interaction, an electron and positron are yielded, while the gamma-ray disappears. The energy that is above 1.02MeV is shared between the electron and the positron as their kinetic energy. After having lost its kinetic energy, the positron meets with another electron and is obliterated from the media within a very short time. The obliteration process gives rise to two gamma rays that travel in opposite directions. The gamma rays produced are 511keV.

Project to Determine the Radioactivity of Radioisotopes Contained in Soil Samples Obtained from North West England

North West England is made up of five counties. The five include Cumbria, Lancashire Cheshire, Merseyside, and Greater Manchester. In 2011, the region had a population of 7,052,000. As a result, the region is considered to be the third most populous region in England after London and the southeast regions. The region borders the Peak and Pennines districts to the west and the Irish Sea to the east. North West England is an extensive region, spanning from the Scottish border to the north to the West Midlands region on its southern side. The region also borders North Wales to the southwest. North West England is renowned for its physiographical features, such as the Cheshire plain and the Lake District. The region is home to the highest peak in England, the Scafell Pike, with a height of 3,209 feet, which is approximately 978 meters. The North West England region is also endowed with natural beauty, which includes the Arnside and Solway coasts, as well as the Bowland forest.

Germanium Detectors

Germanium is highly preferred when it comes to gamma-ray spectroscopy of a high resolution as a result of the high-quality crystals and large atomic numbers that it forms. The rays used contain energy ranging from just a few keV to 10 MeV. High resolution and efficiency can be achieved by increasing the sensitivity of the detectors used. Germanium detectors, however, cannot be used at room temperatures. The detectors must be cooled down before use with the help of liquid nitrogen to temperatures of about 77K. Cooling is done to protect the detectors from the current generated at high levels of temperature. The current would result in a small bandgap of 0.7eV, thus generating noise that would have implications on the resolution of the detectors. Preamplifiers are cooled down in an attempt to lessen electronic noise. The n- and p-type detectors are in use with a less than 1010 cm-3 concentration of dopant impurity.

Selecting the type of detector to be used for a particular application will depend on a number of factors. The factors include cost, count rate performance, suitability for the experiment being carried out, efficiency, energy range, and resolution of interest. Both scintillation and semiconductor detectors use a wide range of materials. Semiconductor detectors, for instance, use such materials as germanium and silicon. Scintillation detectors also use a variety of materials, which include sodium iodide and bismuth germanium oxide. The germanium semiconductor detectors are most suitable for detecting gamma radiations. The detectors are highly efficient, yet they come at a reasonably lower price compared to other types of detectors in the market today. In addition, germanium also has a higher resolution compared to other materials of high atomic number. The sodium iodide semiconductor detectors are, however, the most commonly used since they are available in the market at an affordable cost.

Compound or elemental materials of a single crystal are used to make semiconductor detectors. Silicon, germanium, and other compounds are used. The bandgap ranges from 1eV to 5eV. As such, this type of detector offers better resolution compared to the others. Semiconductor detectors are based on the fact that charge signals can be produced using very small amounts of energy. The signal produced by the detectors is, therefore, relatively large compared to that produced by other types of detectors.

In the project carried out to determine the radioactivity of radioisotopes contained in soil samples obtained from North West England, a number of steps should be followed. To begin with, a literature survey should be carried out to obtain the necessary background information for the completion of the project. Proposal of the experiments to be carried out, as well as detailed explanations of how the project is to be conducted, should then be made. One should then embark on learning how the germanium detector is used to carry out measurements in high-resolution gamma-ray spectroscopy. The necessary efficiency and energy calibrations should be made on the system. The system should then be tested to ascertain that it is functioning efficiently. The environment within which the study on a series of samples is to be carried out is then carefully selected. Data is to be carefully collected and analyzed. The final step involves a discussion of results obtained for the purposes of coming up with final conclusions.

Procedures

Samples were completely dried in a laboratory oven at temperatures of 107C for twenty-four hours. Drying was done since activity per unit mass calculations are highly sensitive to the mass of the sample. Moisture should be removed from the samples to avoid variations. Each sample was then ground before being placed in Tupperware for purposes of improving detector symmetry. The mass of all the samples was determined using a digital scale. The mass of the Tupperware was subtracted for every sample. Each beaker was then labeled with the site and other information.

Results of activity from across eleven sites in North West England were calculated manually. The highest activity recorded was from potassium-40 radioisotope occurring singly and naturally in the soil. The high level of activity for this radioisotope may be attributed to the very long t½ of the material, which is usually around 140 years. The lowest activities recorded were from uranium-238 and thorium-232. Radioactive isotopes from man-made sources were also detected. The main man-made isotope detected was cesium-137, whose most likely source is nuclear accidents and industries.

Conclusion

Human beings are exposed to radiation from various sources. The sources may be either natural or man-made. The nature of the source determines the form of radiation generated. There are 2 major classes of radiation. The two are ionizing and non-ionizing radiations. Ionizing radiations are the most useful form of radiation compared to the others. The radiations contain enough power to change the position, chemical composition, and other properties of the particles they travel through. The radiations are, for instance, important in medical diagnosis and treatment since they can alter the composition of intruding microbes and abnormal cells, such as those responsible for cancer.

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