Project On RADIOACTIVITY
CERTIFICATE This is to certify that of class XII has completed the physics project entitled ‘RADIOACTIVITY’ herself and under my guidance. The progress of the project has been continuously reported and has been in my knowledge consistently.
Radioactivity: Radioactivity is the decay or disintegration of the nucleus of a radioactive element. The radiation emitted is the alpha-particles, the beta-particles and the gamma rays and a lot of heat. This phenomenon was first discovered by a French Physicist, Henri Becquerel in 1896. Other famous people parts of this radioactive era are; Lord Rutherford, and the Curie couple, Marie and Pierre. Radioactive decay is a stochastic (i.e., random) process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay. However, the chance that a given atom will decay is constant over time.
A diagram showing an alpha particle (α) being ejected from the nucleus of an atom. Protons are red and neutrons are blue.
CONTENTS
1. BECQUEREL’S DISCOVERY 2. THE CURIE’S DISCOVERY 3. RUTHERFORD’S CONCLUSION 4. RADIATIONS 5. TYPES OF RADIOACTIVITY 6. UNIVERSAL LAW OF RADIOACTIVE DECAY
7. HALF LIFE 8. IONIZATION 9. HALF LIFE 10. OCCURENCE 11. DETECTION OF RADIATIONS 12. USES OF RADIOCTIVITY 13. HAZARDS OF RADIOCTIVE SUBSTANCES
BECQUEREL’S DISCOVERY
In March of 1896, during a time of overcast weather, Becquerel found he couldn't use the sun as an initiating energy source for his experiments. He put his wrapped photographic plates away in a darkened drawer, along with some crystals containing uranium. Much to his Becquerel's surprise, the plates were exposed during storage by invisible emanations from the uranium. The emanations did not require the presence of an initiating energy source--the crystals emitted rays on their own! Although Becquerel did not pursue his discovery of radioactivity, others did and, in so doing, changed the face of both modern medicine and modern science. He was a member of a scientific family extending through several generations, the most notable being his grandfather Antoine-César Becquerel (1788–1878),
his father, Alexandre-Edmond Becquerel (1820–91), and his son Jean Becquerel. (1878–1953)
THE CURIE’S DISCOVERY
Working in the Becquerel lab, Marie Curie and her husband, Pierre, began what became a life long study of radioactivity. It took fresh and open minds, along with much dedicated work, for these scientists to establish the properties of radioactive matter. Marie Curie wrote, "The subject seemed to us very attractive and all the more so because the question was entirely new and nothing yet had been written upon it." On February 17, 1898, the Curies tested an ore of uranium, pitchblende, for its
ability to turn air into a conductor of electricity. The Curies found that the pitchblende produced a current 300 times stronger than that produced by pure uranium. They tested and recalibrated their instruments, and yet they still found the same puzzling results. The Curies reasoned that a very active unknown substance in addition to the uranium must exist within the pitchblende. In the title of a paper describing this hypothesized element (which they named polonium after Marie's native Poland), they introduced the new term: "radio-active." After much grueling work, the Curies were able to extract enough polonium and another radioactive element, radium, to establish the chemical properties of these elements. Marie Curie, with her husband and continuing after his death, established the first quantitative standards by which the rate of radioactive emission of charged particles from elements could be measured and compared. In addition, she found that there was a decrease in the rate of radioactive emissions over time and that this decrease could be calculated and predicted. But perhaps Marie Curie's greatest and most unique achievement was her realization that radiation is an atomic property of matter
rather than a separate independent emanation. Polish-born French physicist, famous for her work on radioactivity and twice a winner of the Nobel Prize. With Henri Becquerel and her husband, Pierre Curie, she was awarded the 1903 Nobel Prize for Physics. She was the sole winner of the 1911 Nobel Prize for Chemistry. She was the first woman to win a Nobel Prize, and she is the only woman to win the award in two different fields.
RUTHERFORD’S CONCLUSION
In 1911, Rutherford conducted a series of experiments in which he bombarded a piece of gold foil with positively charged (alpha) particles emitted by radioactive material. Most of the particles passed through the foil undisturbed, suggesting that the foil was made up mostly of empty space rather than of a sheet of solid atoms. Some alpha particles, however, "bounced back," indicating the presence of solid matter. Atomic particles, Rutherford's work showed, consisted primarily of empty space surrounding a welldefined central core called a nucleus. In a long and distinguished career, Rutherford laid the groundwork for the determination of atomic structure. In addition to defining the planetary model of the atom, he showed that radioactive
elements undergo a process of decay over time. And, in experiments which involved what newspapers of his day called "splitting the atom," Rutherford was the first to artificially transmute one element into another--unleashing the incredible power of the atom which would eventually be harnessed for both beneficial and destructive purposes.
Taken together, the work of Becquerel, the Curies, Rutherford and others, made modern medical and scientific research more than a dream. They made it a reality with many applications. A look at the use of isotopes reveals just some of the ways in which the
pioneering work of these scientists has been utilized.
RADIATIONS 1. Alpha-particles: This type of radiation is positively charged. It is relatively massive. It has a low penetrating power. It’s about 1-20th as fast as light. It is exactly like the helium atom.
2.Beta-particles: This type of radiation is negatively charged (but can also be +vely charged). It is relatively light. It is about as fast as light. They are high energy electrons. It has a medium penetrating power.
3. Gamma Rays: This radiation is neutral in charge. Has a very high penetrating power. It is at the speed of light. It is an electromagnetic wave with very short wavelength. It is very light.
TYPES OF RADIOACTIVITY
I. NATURAL RADIOCTIVITY This is the type of radioactivity which consists of a spontaneous decay of the radioactive nucleus. The phenomenon is experienced by naturally radioactive substances. The radiation might come out individually or combined and, as always, with a lot of energy. Some radioactive substances are: Americium -241: Used in many smoke detectors for homes and business. To measure levels of toxic lead in dried paint samples. To ensure uniform thickness in rolling processes like steel and paper production and to help determine where oil wells should be drilled. Cadmium -109: Used to analyze metal alloys for checking stock, sorting scrap. Calcium - 47: Important aid to biomedical researchers studying the cell functions and bone formation of mammals. Californium - 252: Used to inspect airline luggage for hidden explosives...to gauge the moisture content of soil in the road construction and building industries...and to measure the moisture of materials stored in silos. Carbon - 14: Helps in research to ensure that potential new drugs are metabolized without forming harmful byproducts.
Cesium - 137: Used to treat cancers. To measure correct patient dosages of radioactive pharmaceuticals. To measure and control the liquid flow in oil pipelines. To tell researchers whether oil wells are plugged by sand. And to ensure the right fills level for packages of food, drugs and other products. (The products in these packages do not become radioactive.) Chromium - 51: Used in research in red blood cell survival studies. Cobalt - 57: Used in nuclear medicine to help physicians interpret diagnosis scans of patients' organs, and to diagnose pernicious anemia. Cobalt - 60: Used to sterilize surgical instruments. To improve the safety and reliability of industrial fuel oil burners. And to preserve poultry fruits and spices. Copper - 67: When injected with monoclonal antibodies into a cancer patient, helps the antibodies bind to and destroy the tumor. Curium - 244: Used in mining to analyze material excavated from pits slurries from drilling operations. Iodine - 123: Widely used to diagnose thyroid disorders. Iodine - 129: Used to check some radioactivity counters in vitro diagnostic testing laboratories. Iodine - 131: Used to diagnose and treat thyroid disorders. (Former President George Bush and Mrs. Bush
were both successfully treated for Grave's disease, a thyroid disease, with radioactive iodine.) Iridium - 192: Used to test the integrity of pipeline welds, boilers and aircraft parts. Iron - 55: Used to analyze electroplating solutions. Krypton - 85: Used in indicator lights in appliances like clothes washer and dryers, stereos and coffee makers. To gauge the thickness of thin plastics and sheet metal, rubber, textiles and paper. And to measure dust and pollutant levels. Nickel - 63: Used to detect explosives. And as voltage regulators and current surge protectors in electronic devices. Phosphorus - 32: Used in molecular biology and genetics research. Plutonium - 238: Has safely powered at least 20 NASA spacecraft since 1972. Polonium - 210: Reduces the static charge in production of photographic film and phonograph records. Promethium - 147: Used in electric blanket thermostats. And to gauge the thickness of thin plastics, thin sheet metal, rubber, textiles, and paper. Radium - 226: Makes lightning rods more effective.
Selenium - 75: Used in protein studies in life science research. Sodium - 24: Used to locate leaks in industrial pipelines. And in oil well studies. Strontium - 85: Used to study bone formation and metabolism. Technetium - 99m: The most widely used radioactive isotope for diagnostic studies in nuclear medicine. Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies. Thallium - 204: Measures the dust and pollutant levels on filter paper...and gauges the thickness of plastics, sheet metal, rubber, textiles and paper. Thoriated tungsten: Used in electric are welding rods in the construction, aircraft, petrochemical and food processing equipment industries. It produces easier starting, greater arc stability and less metal contamination. Thorium - 229: Helps fluorescent lights to last longer. Thorium - 230: Provides coloring and fluorescence in colored glazes and glassware. Tritium: Used for life science and drug metabolism studies to ensure the safety of potential new drugs. For self-luminous aircraft and commercial exit signs. For
luminous dials, gauges and wrist watches and to produce luminous paint. Uranium - 234: Used in dental fixtures like crowns and dentures to provide a natural color and brightness. Uranium - 235: Fuel for nuclear power plants and naval nuclear propulsion systems. Also used to produce fluorescent glassware, a variety of colored glazes and wall tiles. Xenon - 133: Used in nuclear medicine for lung ventilation and blood flow studies.
II. ARTIFICIAL RADIOACTIVITY In this radioactivity, normally unreactive elements are made reactive by bombarding them with radiation. Curie and Joliot showed that when lighter elements such as boron and aluminum were bombarded with α-particles, there was a continuous emission of radioactive radiations, even after the α−source had been removed. They showed that the radiation was due to the emission of a particle carrying one unit positive charge with mass equal to that of an electron. Neutron activation is the main form of induced radioactivity, which happens when free neutrons are captured by nuclei. This new heavier isotope can be stable or unstable (radioactive) depending on the chemical element involved. Because free neutrons disintegrate within minutes outside of an atomic nucleus, neutron radiation can be obtained only from nuclear disintegrations, nuclear reactions, and highenergy reactions (such as in cosmic radiation showers or particle accelerator collisions). Neutrons that have been slowed down through a neutron moderator (thermal neutrons) are more likely to be captured by nuclei than fast neutrons. A less common form involves removing a neutron via photodisintegration. In this reaction, a high energy photon (gamma ray) strikes a nucleus with energy greater than the binding energy of the atom, releasing a neutron. This reaction has a minimum cutoff of 2 MeV (for deuterium) and around 10 MeV for most heavy nuclei. Many radionuclides do not produce gamma rays with energy high enough to induce this reaction.
The isotopes used in food irradiation (cobalt-60, caesium137) both have energy peaks below this cutoff and thus cannot induce radioactivity in the food. Some induced radioactivity is produced by background radiation, which is mostly natural. However, since natural radiation is not very intense in most places on Earth, the amount of induced radioactivity in a single location is usually very small. The conditions inside certain types of nuclear reactors with high neutron flux can cause induced radioactivity. The components in those reactors may become highly radioactive from the radiation to which they are exposed. Induced radioactivity increases the amount of nuclear waste that must eventually be disposed, but it is not referred to as radioactive contamination unless it is uncontrolled.
Universal law of radioactive decay Radioactivity is one very frequent example of exponential decay. The law describes the statistical behavior of a large number of nuclides, rather than individual ones. In the following formalism, the number of nuclides or nuclide population N, is of course a discrete variable (a natural number)—but for any physical sample N is so large (amounts of L = 1023, Avogadro's constant) that t can be treated as a continuous variable. Differential calculus is needed to set up differential equations for modeling the behavior of the nuclear decay.
One-decay process Consider the case of a nuclide A decaying into another B by some process A → B (emission of other particles, like electron neutrinos ν e and electrons e– in beta decay, are irrelevant in what follows). The decay of an unstable nucleus is entirely random and it is impossible to predict when a particular atom will decay. However, it is equally likely to decay at any time. Therefore, given a sample of a particular radioisotope, the number of decay events −dN expected to occur in a small interval of time dt is proportional to the number of atoms present N, that is
Particular radionuclides decay at different rates, so each has its own decay constant λ. The expected decay −dN/N is proportional to an increment of time, dt:
The negative sign indicates that N decreases as time increases, as each decay event follows one after another. The solution to this first-order differential equation is the function:
Where N0 is the value of N at time t = 0. We have for all time t:
Where Ntotal is the constant number of particles throughout the decay process, clearly equal to the initial number of A nuclides since this is the initial substance. If the number of non-decayed A nuclei is:
Then the number of nuclei of B, i.e. number of decayed A nuclei, is
HALF-LIFE
Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. For the case of one-decay nuclear reactions:
The half-life is related to the decay constant as follows: set N = N0/2 and t = T1/2 to obtain
This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from more than 10 years, such as for the very nearly stable nuclide 209Bi, to 10−23 seconds for highly unstable ones. The factor of ln (2) in the above relations results from the fact that concept of "half-life" is merely a way of selecting a different base other than the natural base e for the lifetime expression. The time constant τ is the e -1 -life, the time until only 1/e remains, about 36.8%, rather than the 50% in the half-life of a radionuclide. Thus, τ is longer than t1/2. The following equation can be shown to be valid:
Since radioactive decay is exponential with a constant probability, each process could as easily be described with a different constant time period that (for example) gave its "(1/3)-life" (how long until only 1/3 is left) or "(1/10)-life" (a time period until only 10% is left), and so on. Thus, the choice of τ and t1/2 for markertimes, are only for convenience, and from convention. They reflect a fundamental principle only in so much as they show that
the same proportion of a given radioactive substance will decay, during any time-period that one chooses. Mathematically, the nth life for the above situation would be found in the same way as above—by setting N = N0/n, {{{1}}} and substituting into the decay solution to obtain
OCCURRENCE IN NATURE
According to the Big Bang theory, stable isotopes of the lightest five elements (H, He, and traces of Li, Be, and B) were produced very shortly after the emergence of the universe, in a process called Big Bang nucleosynthesis. These lightest stable nuclides (including deuterium) survive to today, but any radioactive isotopes of the light
elements produced in the Big Bang (such as tritium) have long since decayed. Isotopes of elements heavier than boron were not produced at all in the Big Bang, and these first five elements do not have any long-lived radioisotopes. Thus, all radioactive nuclei are, therefore, relatively young with respect to the birth of the universe, having formed later in various other types of nucleosynthesis in stars (in particular, supernovae), and also during ongoing interactions between stable isotopes and energetic particles. For example, carbon-14, a radioactive nuclide with a half-life of only 5730 years, is constantly produced in Earth's upper atmosphere due to interactions between cosmic rays and nitrogen. Nuclides that are produced by radioactive decay are called radiogenic nuclides, whether they themselves are stable or not. There exist stable radiogenic nuclides that were formed from short-lived extinct radionuclides in the early solar system. The extra presence of these stable radiogenic nuclides (such as Xe-129 from primordial I129) against the background of primordial stable nuclides can be inferred by various means. Radioactive primordial nuclides found in the Earth are residues from ancient supernova explosions which occurred before the formation of the solar system. They are the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present. The naturally occurring shortlived radiogenic radionuclides found in rocks are the daughters of these radioactive primordial nuclides. Another minor source of naturally occurring radioactive nuclides are cosmogenic nuclides, formed by cosmic ray bombardment of material in the Earth's atmosphere or crust. The radioactive decay of
these radionuclides in rocks within Earth's mantle and crust contribute significantly to Earth's internal heat budget.
DETECTION OF RADIATIONS 1. USING A DOSIMETER OR A FILM BADGE: A dosimeter is a device worn by radioactive workers. It is basically a film which darkens on incidence of radiation. It is used to know the level of radiation the worker has been exposed to.
2. A GEIGER COUNTER: This consists of a Geiger-Muller tube (which consists of a wire), a scale/rate meter, and often a loudspeaker. The walls of the container acts as the cathode while the central wire acts as the anode. The radiation enters through a thin window. Each particle or ray ionizes several gas atoms. Ions attracted to the cathode, electrons to the anode. Other atoms are hit on the way creating an avalanche of more ions and electrons. The loudspeaker amplifies a click sound for each pulse showing the randomness of the decay. 3. Pulse (Wulf Electroscope) 4. Cloud Chamber 5. Bubble Chamber 6. Scintillation Counter (for detecting gamma rays)
USES OF RADIOACTIVITY 1. Radiology: This is used for research and study in the medical field.
2. Radiotherapy: This is used in the treatment of diseases, especially cancer. Due to the penetrating power of gamma rays, they are used to collectively and controllably destroy malignant cells.
3. Irradiation: This is the exposure of controlled gamma rays to fruits or vegetables to delay ripening and improve freshness length of the irradiated foodstuffs.
4. Gamma-Radiography: This is the production of a special type of photograph, a radiograph. It is used for quality control in industries. The making of a radiograph requires some type of recording mechanism. The most common device is film. A radiograph is actually a photographic recording produced by the passage of radiation through a subject onto a film, producing what is called a latent image of the subject.
5. Radiocarbon or carbon dating: All living matter contains carbon-14 absorbed from the atmosphere. This radioactive element has a half-life of about 5300 years. The element continues decaying even after death of the living organism. This phenomenon is used to estimate the amount of years the organisms have been in existence. This is very useful to archaeologists and researchers.
6. Tracers are a common application of radioisotopes. A tracer is a radioactive element whose pathway through which a chemical reaction can be followed. Tracers are commonly used in the medical field and in the study of plants and animals. Radioactive Iodine-131 can be used to study the function of the thyroid gland assisting in detecting disease.
7. Nuclear reactors are devices that control fission reactions producing new substances from the fission product and energy. Nuclear power stations use uranium in fission reactions as a fuel to produce energy. Steam is generated by the heat released during the fission process. It is this steam that
7. Other uses of radioactivity: Sterilization of medical instruments and food is another common application of radiation. By subjecting the instruments and food to concentrated beams of radiation, we can kill microorganisms that cause contamination and disease. Because this is done with high energy radiation sources using electromagnetic energy, there is no fear of residual radiation. Also, the instruments and food may be handled without fear of radiation poisoning. Radiation sources are extremely important to the manufacturing industries throughout the world. They are commonly employed by nondestructive testing personnel to monitor materials and processes in the making of the products we see and use every day. Trained technicians use radiography to image materials and products much like a
dentist uses radiation to x-ray your teeth for cavities. There are many industrial applications that rely on radioactivity to assist in determining if the material or product is internally sound and fit for its application.
HAZARDS OF RADIOACTIVE SUBSTANCES
The dangers of radioactivity and radiation were not immediately recognized. The discovery of X-rays in 1895 led to wide spread experimentation by scientists, physicians, and inventors. Many people began recounting stories of burns, hair loss and worse in technical journals as early as 1896. In February of that year, Professor Daniel and Dr. Dudley of Vanderbilt University performed an experiment involving x-raying Dudley's head that resulted in him losing hair under where the tube was placed (reported in the The X-rays Science news supplement). A report by Dr. H.D. Hawks, a graduate of Columbia College, of his suffering severe hand and chest burns in an x-ray demonstration, was the first of many other reports in Electrical Review. Many experimenters including Elihu Thomson at Thomas Edison's lab, William J. Morton, and Nikola Tesla also reported burns. Elihu Thomson deliberately exposed a finger to an x-ray tube over a period of time and suffered pain, swelling, and blistering. Other effects were sometime blamed for the damage including ultraviolet rays and (according to Tesla) ozone. Many physicians claimed there were no effects form x-ray exposure at all.
The genetic effects of radiation, including the effect of cancer risk, were recognized much later. In 1927, Hermann Joseph Muller published research showing genetic effects, and in 1946 was awarded the Nobel Prize for his findings. Before the biological effects of radiation were known, many physicians and corporations began marketing radioactive substances as patent medicine in the form of glow-in-the-dark pigments. Examples were radium enema treatments, and radiumcontaining waters to be drunk as tonics. Marie Curie protested this sort of treatment, warning that the effects of radiation on the human body were not well understood. Curie later died from aplastic anemia, likely caused by exposure to ionizing radiation. By the 1930s, after a number of cases of bone necrosis and death of enthusiasts, radium-containing medicinal products had been largely removed from the market (radioactive quackery).
BIBLIOGRAPHY 1. 2. 3. 4.
NCERT Physics Textbook for class XII www.wikipedia.org www.google.com en.wikibooks.org
