Raashid Shah

  Nuclear Hazards and Safety Issues   Recently there has been much apprehension about the dangers inherent in nuclear plants—fears of radiation hazard, waste disposal, disastrous accidents. While some of the hazards are real, nuclear scientists point out that many of them are not based on scientific facts and unbiased observation. Radiation Hazard: There is no doubt that radiation causes damage to living cells—but this depends on the intensity of radiation and the time of exposure. When an atom of a complex organic cell is exposed to radiation, ionization takes place and molecules disintegrate, adversely affecting the biological system, sometimes even destroying the cell.While high doses are fatal, low doses may have cumulative effect and cause cancers, especially of the skin, and leukemia. It may affect lymphatic tissues, the nervous system, and the reproductive organs. However, die adverse effects take place after considerably high and constant doses of radiation.The release of radioactivity into air and water from reactors does take place, but it is kept well within the limits prescribed by the AERB. The earth is being constantly bombarded by cosmic ray nuclear particles (65 per cent of natural radiation experienced by a human being is due to this).Background radiation from terrestrial and extra-terrestrial sources is much higher than radiation from nuclear plants. In the circumstances, the radiation exposures from nuclear plants is of a negligible quantity. The fear of radiation arises because most people are unwilling to believe in any “safe level” for radiation exposure.Hazard from Nuclear Waste:Another aspect of nuclear hazard is waste management. The general technique of dealing with radioactive wastes is to concentrate and contain as much radioactivity as possible, and discharge to the environment only effluent of as low a concentration level as is possible.At inland sites like Narora and Rawatbhatta, low level liquid wastes are discharged into the environment at a minimum level. At coastal sites such as Tarapur and Chennai significant dilution in the sea is possible. For solid wastes, different types of containments are used and located at sites selected on the basis of geological and geohydrological evaluation.The fissioning of U-235 produces many radioactive isotopes, such as strontium 90, caesium 137, and barium 140. These wastes remain radioactive and dangerous for about 600 years because of the strontium and caesium isotopes. If these get into food or water supplies, they can be taken into people’s bodies where they can cause harm.The body is unable to distinguish between radioactive strontium and calcium, for instance. The plutonium and other artificially created elements in the wastes remain radioactive for thousands of years. Even in small amounts, plutonium can cause cancer or genetic (reproductive) damage in humans.Larger amounts can cause radiation sickness and death. Safe disposal of these wastes is one of the problems involved in nuclear power production. The wastes are carefully managed by incorporating them in inert solid matrices and placing them in canisters which are kept under cooling till the radioactivity comes to desired level. Finally, the canisters are stored in suitable geological media. However, the problem is not entirely resolved. Effects of a Nuclear Explosion: The effects that a nuclear explosion has on people, buildings, and the environment can vary greatly, depending on a number of factors. These factors include weather, terrain, the point of explosion in relation to the earth’s surface, and the weapon’s yield.The weapon’s explosion would produce four basic effects: (i) Blast Wave: The explosion begins with the formation of a fireball, which consists of a cloud of dust and of extremely hot gases under very high pressure. A fraction of a second after the explosion, the gases begin to expand and form a blast wave, also called a shock wave.The blast wave and wind probably would kill the majority of people within 5 kilometers of ground zero and some of the people between 5 and 10 kilometers from ground zero. Many other people within 10 kilometers of groupd zero would be injured. (ii) Thermal radiation: This consists of ultraviolet, visible, and infra¬red radiation given off by the fireball. The ultraviolet radiation is rapidly absorbed by particles in the air, and so it does little harm. However, the visible and infrared radiation can cause eye injuries as well as skin burns called flash burns.Between 20 and 30 per cent of the deaths of Hiroshima and Nagasaki resulted from flash burns. Thermal radiation also can ignite such highly flammable materials as newspapers and dry leaves. The burning of these materials can lead to large fires. (iii) Initial nuclear radiation: This is given off within the first minute after the explosion. It consists of neutrons and gamma rays. The neutrons and some of the gamma rays are emitted from the fireball almost instantaneously. The rest of the gamma rays are given off by a huge mushroom-shaped cloud of radioactive material that is formed by the explosion. Nuclear radiation can cause the swelling and destruction of human cells and prevent normal cell replacement.Large doses of radiation can cause death. The amount of harm a person would suffer from initial nuclear radiation depends in part on the person’s location in relation to ground zero. Initial radiation decreases rapidly in strength as it moves away from ground zero. (iv) Residual Nuclear Radiation: This comes later than one minute after the explosion. Residual radiation created by fission consists of gamma rays and beta particles. Residual radiation produced by fusion is made up primarily of neutrons. It strikes particles of rock, soil, water, and other materials that make up the mushroom-shaped cloud. As a result, these particles become radioactive. When the particles fall back to earth, they are known as fallout. The closer an explosion occurs to the earth’s surface, the more fallout it produces.Early fallout consists of heavier particles that reach the ground during the first 24 hours after the explosion. These particles fall mostly downwind from ground zero. Early fallout is highly radioactive and will kill or severely damage living things.Delayed fallout reaches the ground from 24 hours to a number of years

Radioactive Dating   Radiocarbon dating is a process used to determine the age of an ancient object by measuring its radiocarbon content. This technique was devel¬oped in the late 1940s by Willard F. Libby, an American chemist.Radiocarbon atoms, like all radioactive substance, decay at an exact and uniform rate. Half of the radiocarbon disappears after about 5,700 years. Therefore, radiocarbon has a half-life of that period of time.After about 11,400 years, a fourth of the original amount of radiocarbon remains. After another 5,700 years, only an eighth remains, and so on.The radiocarbon in the tissues of a living organism decays extremely slowly, but it is continuously renewed as long as the organism lives. After the organism dies, it no longer takes in air or food, and so it no longer absorbs radiocarbon. The radiocarbon already in the tissues continues to decrease at a constant rate. This steady decay at a known rate—a half- life of about 5,700 years—enables scientists to determine an object’s age.After scientists measure an object’s radiocarbon content, they compare it with the radiocarbon in tree-rings whose ages are known. This technique enables them to compensate for small variations of radiocarbon content in the atmosphere at different times in the past. By doing so, scientists can convert an object’s radiocarbon age to a more precise date.Radioisotopes with very long half-lives are used for dating rock specimens such as Uranium-238. Uranium-235 which becomes lead 207; thorium 232, which becomes lead 208; rubidium 87, which changes into strontium 87; and potassium 40, which changes into argon 40 are radio¬isotope which can be used to calculate the age of rocks.    

Radioisotopes   Different forms of radiation originate in the nuclei of radioactive atoms. There are three kinds of radioactive radiation: alpha particles, which were first identified by Becquerel; beta rays, identified by Ernest Rutherford; and gamma rays, identified by Marie and Pierre Curie. Emission of alpha or beta rays causes transmutation, but gamma radiation does not result in transformation.One element can be changed into another artificially. Ail artificial radioisotope is produced by making stable isotopes radioactive—i.e., unstable, their nuclei breaking apart to release small particles and energy (radioactivity). Every element with atomic number greater than that of lead (82) is radioactive.Artificial radioisotopes may be produced by bombarding atoms with particles and rays emitted by radioactive elements in a nuclear reactor. They can also be produced by smashing atoms in particle accelerators such as the cyclotron. The fact that radioactive materials can be detected by their radiation makes them useful in many fields.Radioactive isotopes are effectively used as tracers for diagnostic purposes in medicine. Arsenic-74 is used to detect tumours. Sodium-24 is used to detect blood clots in the circulatory system. Iodine-131 (1-131) is used to determine the activity of the thyroid gland. Cobalt-60 is used in the treatment of cancer; also in use are iridium-192, and caesium-137.The production of radioisotopes in India began in 1956 with the commissioning of the research reactor Apsara at Trombay. The radioiso¬tope production capability was augmented in 1963 when the 40MWt Cirus became operational at Trombay. In 1985 with Dhruva being made operational by BARC, India came to the fore as a major producer of wide spectrum of radioisotopes.The research reactors at Trombay produce a variety of radioisotopes for various uses. Power reactors are also equipped to produce cobalt-60 radioisotope.The variable energy cyclotron at VECC is also used for manufacture of radioisotopes, which are processed for medical applications. Radiation and radioisotope based products and services offered by DAE through BARC and BRIT include radio sources and industrial radiography equip¬ment; radiotracer technologies in leak detection, silt movement, and applications in hydrology; radiation processing, radiation polymerisation, soil-salinity and others.BRIT has been entrusted with the responsibility for processing a variety of radioisotopes and their derived products and supply of industrial radiography equipment and gamma irradiation equipment for applications of this technology.BARC’s Radiation Medicine Centre (RMC) at Mumbai, a premier centre in the country in the field of radio diagnosis and radiotherapy, is a regional referral centre of the World Health Organisation (WHO) for South-East-Asia.The activities of the centre cover the fields of nuclear medicine and allied services, clinical diagnosis and treatment, in-house development of radiopharmaceuticals, RIA technology for thyroid hor¬mones and tubercular antigen and antibodies, etc.Radioisotopes for medical applications are also manufactured by using the variable energy cyclotron at Kolkata. The regional radiation medicine centre (RRMC) meets the radio diagnostic and radiotherapy requirements of the eastern region of the country. CAT at Indore has developed lasers for medical applications.In India, radiation has been in use for decades for sterilisation of medical products. A commercial radiation sterilisation plant (ISOMED) at Trombay provides sterilisation services to the medical industry. A large radiopharmaceuticals laboratory named ISOPHARM has been set up at Vashi, Mumbai.Plants similar to Isomed have been working at Bengaluru, New Delhi and Jodhpur. For use in blood banks and hospitals, BRIT has developed a blood irradiator equipment which is an important import substitute. Uses of Radioisotopes: In Industry Gamma rays can be used to examine metallic castings or welds in oil pipelines for weak points. The rays pass through the metal and darken a photographic film at places opposite the weak spots. Manufacturers may place a radio-isotope that emits beta particles above a sheet of material.A beta-particle detector on the other side measures the strength of the radiations coming through. If the sheet thickness increases, fewer particles reach the detector. The detector can control rollers and keep the sheet at desired thicknesses. Gamma radiation may be used in pest control, especially ingrain stores. Irradiated food has a longer shelf life.In Research Scientists use radio-isotopes as tracers, to determine how chemicals act in the bodies of plants and animals. All isotopes of an element are chemically the same, so the radio-isotope can be used in the same way as the ordinary isotopes.For example, to trace the course of phosphorus in a plant, a botanist may mix radioactive phosphorus with the ordinary phosphorus. To learn when the phosphorus reaches a leaf, he may place a Geiger counter, which detects radioactivity, on the leaf. To find where the phosphorus lodges in the leaf, he may place the leaf on a photographic plate. In the developed plate, called an autoradiograph, darkened regions show the position of the radio-isotope. In Medicine: The use of radio-isotopes is part of a speciality called nuclear medicine. The chief use of radio-isotopes is to study the function of various body organs. To accomplish this, a doctor administers a radio¬isotope attached to a carrier substance. The carrier substance accumulates in the organ that the doctor wants to study.For example, if the doctor wishes to study a patient’s kidney function, a radio-isotope will be attached to a carrier substance that accumulates in the kidneys. As the radio-isotope breaks down, it emits gamma rays. Some of the rays are picked up by a device called a scanner. The doctor “reads” the image on the scanner to determine if the kidneys are working properly.Radioisotopes are also used to treat cancer. Radiation in large doses destroys living tissues, especially cells undergoing division. Because cancer cells divide more frequently than do normal cells, radiation kills more cancerous cells than normal ones. A doctor may take advantage of this fact by administering a radio-isotope that accumulates in a cancerous organ.For example, a radio-isotope of iodine, 1-131 may be used to treat cancer of the thyroid gland, because this gland accumulates iodine. As the radioactive iodine transforms, it gives off radiation that kills the cancerous cells. Cobalt-60 is also used in treatment of cancer. Arsenic- 74 is employed to detect tumours. Blood clots in the

Nuclear Weapons   Nuclear weapons may be of the fission type (atomic weapons) or the fusion type (thermonuclear or hydrogen weapons).Fission weapons get their destructive power from the splittings of atomic nuclei. Only three kinds of atoms are known to be suitable for fissioning in such weapons. These atoms are of the uranium (U) isotopes U-235 and U-238 and of the plutonium (Pu) isotope, Pu-239. An accelerating uncontrolled chain reaction occurs when, for example, two pieces of U-235 come together and exceed the critical mass.Thermonuclear weapons get their power from the fusion atomic nuclei under intense heat. The nuclei fused in thermonuclear weapons are of the hydrogen isotopes, deuterium and tritium. Fusion reactions require temperatures equal to, or greater than, those found in the sun’s core.The only practical way to achieve such temperature si by means of a fission explosion. Thus, thermonuclear explosions are triggered by an implosion-type fission device. (In the implosion method, a subcritical mass is made supercritical by compressing it into a smaller volume.)The first nuclear weapons were two fission bombs used by the United States during World War II (1939-1945). In the war, one was dropped on each of Japanese cities of Hiroshima and Nagasaki.Nuclear explosive devices can have a wide variety of yields. Some older bombs had yields of about 20 megatons, or 1,540 Hiroshima bombs. A megaton is the amount of energy released by 907,000 metric tons of TNT. Today, because of the higher accuracy of missiles, most nuclear devices have yields of less than 1 megaton.    

Nuclear Fusion   Nuclear fusion occurs when two lightweight nuclei fuse (combine) and form a nucleus of a heavier element. The products of the fusion weigh less than the combined weights of the original nuclei. The lost matter has therefore been changed into energy. Fusion reactions that produce large amounts of energy can be created only by means of extremely intense heat. Such reactions are called thermo-nuclear reactions. Thermonuclear reactions produce the energy of both the sun and the hydrogen bomb. A thermo-nuclear reaction can occur only in plasma, a special form of matter which has free electrons and free nuclei. Normally, nuclei repel one another.But if a plasma containing lightweight atomic nuclei is heated many millions of degrees, the nuclei begin moving so fast that they break through one another’s electrical barriers and fuse. Problems of Controlling Fusion: Scientists have not yet succeeded in harnessing the energy of fusion to produce power. In their fusion experiments, scientists generally work with plasmas that are made from one or two isotopes of hydrogen. Deuterium is considered an ideal thermo-nuclear fuel because it can be obtained from ordinary water. A given weight of deuterium can supply about four times as much energy as the same weight of uranium.To produce a controlled thermo-nuclear reaction, a plasma of deuterium or tritium or of both isotopes must be heated many millions of degrees. Bui scientists have yet to develop a container than can hold superhot plasma.Most experimental fusion reactors are designed to contain superhot plasma in “magnetic bottles” twisted into various coil-like shapes. The walls of the bottles are made of copper or some other metal. The walls are surrounded by a magnet.An electric current is passed through the magnet and creates a magnetic field on the inside of the walls. The magnetism pushes the plasma away from the walls and toward the centre of each coil. This technique is called magnetic confinement All the fusion devices thus far developed; however, use much more energy than they create.The most successful fusion reactor, called a tokamak, was originally designed by Russian scientists. Tokamak means strong current in Russian. Like other experimental fusion reactors, a tokamak uses a magnetic field to push plasma away from its containing walls. It also passes a strong current through the plasma. The current acts with the magnetic field to help confine the plasma. India has developed a tokamak Aditya, for research purposes at the Institute of Plasma Research, Ahmedabad. Another experimental method to achieve fusion uses beams of laser to compress and heat tiny pellets of frozen deuterium and tritium. This process creates miniature thermo-nuclear explosions that release energy before the pellets reach the containing walls. But all experiments with this method have not yet produced usable amounts of energy.    

Nuclear Reactor   A nuclear reactor is the central component of a nuclear power station that generates nuclear energy under controlled conditions for use as a source of electrical power.Power reactors generally consist of three main parts. They are (1) the reactor, or pressure, vessel; (2) the core; and (3) control rods. The reactor vessel holds the other reactor parts. It is installed near the base of the reactor building. The vessel has steel walls at least 15 centimetres thick,. Steel pipes lead into and out of the vessel to carry water and steam.The core contains the nuclear fuel and so is the part of the reactor where fission occurs. The core is near the bottom of the reactor vessel. It consists mainly of the nuclear fuel held in place between an upper and a lower support plate.Control rods are long metal rods that contain such elements as boron or cadmium. These elements absorb free neutrons and thus help control a chain reaction. The control rods are inserted into the core or withdraw to slow down or speed up a chain reaction. Moderators and Coolants: Reactor operations also depend on substances called moderators and coolants. A moderator is a substance, such as water or carbon, that slows down neutrons which pass through it. Reactors require a moderator because the neutrons released by fission are fast neutrons. But slow neutrons are needed to cause a chain reaction in the mixture of U-238 and U-235 that reactors use as fuel.A coolant is a substance, such as water or carbon dioxide, that conducts heat well but does not easily absorb free neutrons. The coolant carries heat from the chain reaction. By doing so, the coolant servers both to prevent the reactor core from melting and to produce steam.Many power reactors are light water reactors, which use light (ordinary) water as both the moderator and the coolant. Heavy water reactors use deuterium oxide, or heavy water, as both the moderator and the coolant. Graphite is another moderator. Indian reactors (except the one at Tarapur) use heavy water.Fuel Preparation:The uranium used in light waters reactors must be enriched—that is, the percentage of U-235 must be increased. Free neutrons then have a better chance of striking a U-235 nucleus.Steam Production:The reactor achieves criticality when a chain reaction in the fuel has been induced to provide, on an average, one more reaction for every fission reaction.The light water reactors are of two main types. One type, the pressurised water reactor, produces steam outside the reactor vessel. The other type, boiling water reactor, makes steam inside the vessel.Most nuclear plants use pressurised water reactors. These reactors heat the moderator-water in the core under extremely high pressure. The pressure allows the water to heat past its normal boiling point of 100 °C without actually boiling. The chain reaction heats the water to about 320 °C. Pipes carry this extremely hot, though not boiling, water to steam generators outside the reactors. Heat from the pressurised water boils water in the steam generator and so produces steam.In a boiling water reactor, the chain reaction boils the moderator- water in the core. Pipes carry the steam produced from the reactor to the plant’s turbines.In India, the standard reactor type is the pressurised heavy water reactor.The fuel rods have to be removed and reprocessed from time to time to separate radioactive waste products and small amounts of plutonium-239 from unused uranium. Plutonium-239 is produced in the reactor when uranium-238 absorbs fast fission neutrons; like uranium-235, it undergoes fission and is used in fast-breeder reactors and to make nuclear weapons.Experimental Breeder Reactors:The most important type of experimental breeder uses the plentiful uranium isotope-U-238 as its basic fuel. The reactor changes the U-238 into the isotope plutonium 239 (Pu- 239) by radioactive decay. Like U-235, Pu-239 can create a chain reaction and so can be used for energy production.Another breeder uses the natural element thorium as its basic fuel. It changes the thorium into the isotope U-233, which can also produce a chain reaction. India has developed an experimental breeder reactor at Kalpakkam, Chennai, using mixed carbide fuel and sodium as coolant.      

Nuclear Fission   Nuclear fission is the process whereby an atomic nucleus breaks up into two or more major fragments with the emission of two or three neutrons. It is accompanied by the release of energy in the form of gamma radiation and the kinetic energy of the emitted particles.Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors. However, the process can also be induced by bombarding nuclei with neutrons because a nucleus that has absorbed a neutron becomes unstable and soon splits. The mass defect is large and appears mostly as k.e. of the fission fragments. These fly apart at great speed, colliding with surrounding atoms and raising their average k.e., that is, their temperature. Heat is therefore produced.If the fission neutrons split other uranium-235 nuclei, a chain reaction is set up. In practice, some fission neutrons are lost by escaping from the surface of the uranium before this happens. The ratio of those escaping to those causing fission decreases as the mass of uranium-235 increases.This must exceed a certain critical mass for a chain reaction to start. Critical mass is thus the minimum mass of fissile material that can undergo a continuous chain reaction. Above the critical mass, the reaction may accelerate into a nuclear explosion if uncontrolled.The U-238 isotope would make an ideal nuclear reactor fuel because it is abundant in nature. But U-238 nuclei usually absorb free neutrons without fissioning. An absorbed neutron simply becomes part of the nucleus. The scarce uranium isotope U-235 is the only natural material that nuclear reactors can use to produce a chain reaction. Uranium with an abundant amount of U-235 is called enriched uranium.    

Nuclear Energy   Nuclear Energy is energy derived from nuclear reactions either by the fission of heavy nuclei into lighter ones or by fusion of light nuclei into heavier ones. In principles, the binding energy of a system of particles forming an atomic nucleus is nuclear energy.It results from changes in the nucleus of atoms. Scientists and engineers have found many uses for this energy, especially in producing electricity. But they do not yet have the ability to make full use of nuclear power. If nuclear energy were fully developed, it could supply all the world’s electricity for millions of years.A nucleus make up most of the mass of every atom and this nucleus is held together by an extremely powerful force. A huge amount of energy is concentrated in the nucleus because of this force.Scientists first released nuclear energy on a large scale at the University of Chicago in 1942, three years after World War II began. This achievement led to the development of the atomic bomb. It is since 1945 that nuclear energy has been put to peaceful uses such as production of electricity.Einstein pointed out that if the energy of a body changes by an amount E, its mass changes by an amount m given by the equation, E = mc2. The implication is that any reaction in which there is a decrease of mass, called a mass defect, is a source of energy.The energy and mass changes in physical and chemical changes are very small; those in some nuclear reactions, such as radioactive decay, are millions of times greater. The sum of the masses of the products of a nuclear reaction is less than the sum of the masses of the reacting particles. This lost mass is converted into energy.    

Radioactivity   Ordinary hydrogen has one proton and no neutrons, so it has mass number 1. Heavy hydrogen, or deuterium, has mass number 2, because it has one proton and one neutron.A radioactive form of hydrogen, tritium, has mass number 3. It has one proton and two neutrons. Ordinary hydrogen, deuterium, and tritium are isotopes of hydrogen. All isotopes of an element have the same chemical properties. The uranium nucleus has 92 protons.The most plentiful isotope of uranium has 146 neutrons. Its mass number is therefore 238 (the sum of 92 and 146). Scientists call this isotope uranium 238 or U-238. The uranium isotope that almost all nuclear reactors use as fuel as 143 neutrons, and so its mass number is 235. This isotope is called uranium 235 or U-235.A nuclear reaction involves changes in the structure of a nucleus. As a result of such changes, the nucleus gains or loses one or more neutrons or protons. It thus changes into the nucleus of a different isotope or element. If the nucleus changes into the nucleus of a different element, the change is called transmutation.Radioactivity is the process by which atoms emit radiation, or atomic particles and rays of high energy, from their nuclei (cores). Of more than 2,300 different kinds of known atoms, more than 2,000 are radioactive. Only about 50 radioactive types exist in nature. Scientists make the rest artificially.Antoine Henri Becquerel of France discovered natural radioactivity in 1896. He found that uranium compounds emitted radiation which affected a photographic plate even when they are wrapped in black paper; they also ionised a gas. Soon afterwards, Marie Curie discovered an even more strongly radioactive substance, namely radium.Every element with an atomic number greater than that of lead (82) is radioactive. The nuclei of some of these elements can decay by splitting in two: this is Spontaneous fission.Natural radioactivity occurs in nine of the lighter elements also. Of these the most important are146C (carbon) and 4019K (Potassium). The isotope was probably formed when the earth was created.Its present existence is due to its long half-life of 1.25 x 109 years; though it only constitutes 0.01% of natural potassium, its presence makes living tissue appreciably radioactive. It may decay either by b-emission or electron capture. It is produced continuously from the action of the neutrons in cosmic rays on atmospheric nitrogen, by a nuclear reaction.Of the seventh row elements, only five are round in nature; radium, actinium, thorium, protactinium and uranium.Emission of Radiation:Different forms of radiation originate in the nuclei of radioactive atoms. There are three kinds of radioactive radiation: alpha particles, which were first identified by Becquerel; beta rays; identified by Ernest Rutherford of New Zealand; and gamma rays, identified by Marie and Pierre Curie of France. Emission of alpha or beta rays causes transmutation, but gamma radiation does not result in transformation.Alpha particles have a positive electrical charge. They consist of two protons and two neutrons, and are identical with the nuclei of helium atoms. Alpha particles are emitted with high energies, but lose energy rapidly when passing through matter. These are stopped by a thick sheet of paper; in air they have a range of a few centimetres, being eventually brought to rest by collisions with air molecules.They cause intense ionisation in a gas (by attracting electrons out of their molecules) and are deflected by electric and very strong magnetic fields. All alpha-particles emitted by a particular radioactive substance have the same speed, about one-twentieth of the speed of light. Americium emits alpha-particles only.Alpha radiation occurs in 238U, an isotope of uranium. After losing an alpha particle, the nucleus has 90 protons and 144 neutrons. The atom with atomic number 90 is no longer uranium, but thorium. The isotope formed is 23490Th.Beta rays are electrons. Some radioactive nuclei emit ordinary electrons, which have negative electrical charges. But others emit positrons, or positively charged electrons. For example, an isotope of carbon, 146C , gives off negative electrons. Carbon 14 has eight neutrons and six protons.When its nucleus transforms, a neutron changes into a proton, an electron, and an antineutrino. After emission of the electron and antineutrino, the nucleus contains seven protons and seven neutrons. Its mass number remains the same, but its atomic number 7 is nitrogen. Thus, 146C changes to 147N after emission of a negative beta particle.A carbon isotope, 116C , emits positrons. Carbon 11 has six protons and five neutrons. When it emits a positron, one proton changes into a neutron, a positron, and neutrino. After emission of the positron and the neutrino, the nucleus contains five protons and six neutrons. The mass number remains the same, but the atomic number drops by one.The element of atomic number 5 is boron. Thus,116C changes into 115B after emission of a positron and a neutrino. Strontium emits beta particles only. Beta particles travel with almost the speed of light. Some can penetrate 13 millimetres of wood.Gamma radiation may occur in several ways. In one process, the alpha or beta particle emitted by a nucleus does not carry off all the energy available. After emission, the nucleus has more energy than in its most stable state. It rids itself of the excess by emitting gamma rays. Gamma rays have no electrical charge. They are similar to X-rays, but they usually have a shorter wavelength.Whereas X-rays are due to energy changes outside atomic nuclei, as are all forms of electromagnetic radiation, gamma-rays, like alpha- and beta-particles, come from inside atomic nuclei. These rays are photons (particles of electromagnetic radiation) and travel with the speed of light. They are much more penetrating than alpha and beta particles.Radium emits alpha-, beta- and gamma-rays. Cobalt is a pure gamma source.Radioactive Decay and Half-Life:Radioactive decay is the process by which a nucleus spontaneously (naturally) changes into the nucleus of another isotope or element. The process releases energy chiefly in the form of nuclear radiation. The decay process happens of its own accord and cannot be controlled; it is unaffected by temperature changes,

Electromagnetic waves    Electromagnetic waves are waves which can travel through the vacuum of outer space. Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 m/s (a speed value commonly represented by the symbol c). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 m/s. This is depicted in the animation below.The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorption of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 108 m/s) through the vacuum of interatomic space, the absorption and reemission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorption and reemission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.Waves come in many shapes and forms. While all waves share some basic characteristic properties and behaviors, some waves can be distinguished from others based on some observable (and some non-observable) characteristics. It is common to categorize waves based on these distinguishing characteristics. Longitudinal versus Transverse Waves versus Surface WavesOne way to categorize waves is on the basis of the direction of movement of the individual particles of the medium relative to the direction that the waves travel. Categorizing waves on this basis leads to three notable categories: transverse waves, longitudinal waves, and surface waves.A transverse wave is a wave in which particles of the medium move in a direction perpendicular to the direction that the wave moves. Suppose that a slinky is stretched out in a horizontal direction across the classroom and that a pulse is introduced into the slinky on the left end by vibrating the first coil up and down. Energy will begin to be transported through the slinky from left to right. As the energy is transported from left to right, the individual coils of the medium will be displaced upwards and downwards. In this case, the particles of the medium move perpendicular to the direction that the pulse moves. This type of wave is a transverse wave. Transverse waves are always characterized by particle motion being perpendicularto wave motion. A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction that the wave moves. Suppose that a slinky is stretched out in a horizontal direction across the classroom and that a pulse is introduced into the slinky on the left end by vibrating the first coil left and right. Energy will begin to be transported through the slinky from left to right. As the energy is transported from left to right, the individual coils of the medium will be displaced leftwards and rightwards. In this case, the particles of the medium move parallel to the direction that the pulse moves. This type of wave is a longitudinal wave. Longitudinal waves are always characterized by particle motion being parallel to wave motion. A sound wave traveling through air is a classic example of a longitudinal wave. As a sound wave moves from the lips of a speaker to the ear of a listener, particles of air vibrate back and forth in the same direction and the opposite direction of energy transport. Each individual particle pushes on its neighboring particle so as to push it forward. The collision of particle #1 with its neighbor serves to restore particle #1 to its original position and displace particle #2 in a forward direction. This back and forth motion of particles in the direction of energy transport creates regions within the medium where the particles are pressed together and other regions where the particles are spread apart. Longitudinal waves can always be quickly identified by the presence of such regions. This process continues along the chain of particles until the sound wave reaches the ear of the listener. Waves traveling through a solid medium can be either transverse waves or longitudinal waves. Yet waves traveling through the bulk of a fluid (such as a liquid or a gas) are always longitudinal waves. Transverse waves require a relatively rigid medium in order to transmit their energy. As one particle begins to move it must be able to exert a pull on its nearest neighbor. If the medium is not rigid as is the case with fluids, the particles will slide