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Radiation & Radioactivity PROJECT:- :- RADIOACTIVITY.


DEFINITION OF RADIOACTIVITY :- The phenomenon due to which certain elements spontaneously emit highly penetrating rays made of sub-atomic particles is known as radioactivity. The elements that emit radioactive rays are called radioactive elements. Examples :- uranium, thorium, radium, polonium.

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Solid Sphere Model or Billiard Ball Model proposed by John Dalton Plum Pudding Model or Raisin Bun Model proposed by J.J. Thomson Planetary Model or Nuclear Model proposed by E. Rutherford Bohr Model or Orbit Model proposed by Neils Bohr Electron Cloud Model or Quantum Mechanical Model proposed by Louis de Broglie & Erwin Schrodinger …a little bit of history… …1808 …1909 …1913

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The Bohr’s atomic model consists of a central nucleus composed of neutrons and protons, which is surrounded by electrons which “ orbit ” around the nucleus. By means of Quantum Mechanical Model, proposed by Louis de Broglie and Erwin Schrodinger , the Electron Cloud has been postulated. Protons carry a positive charge, Neutrons are electrically “neutral”, Electrons carry a negative charge. Atoms in nature are electrically neutral so the number of electrons orbiting the nucleus equals the number of protons in the nucleus. Without neutrons, the nucleus would split apart because the positive protons would repel each other. Elements can have nuclei with different numbers of neutrons in them. For example hydrogen , which normally only has one proton in the nucleus, can have a neutron added to its nucleus to from deuterium , or have two neutrons added to create tritium , which is radioactive . Atoms of the same element which vary in neutron number are called isotopes .


DISCOVERY OF RADIOACTIVITY :- The phenomenon of radioactivity was discovered in 1896 in Paris by Professor Henry Becquerel, when he was working on the nature of phosphorescent substances. He conducted certain investigation on uranium salt. They showed that emission of such radiations was not the property of uranium salt but of the element uranium. Later, in 1898, Madam Curie and her husband Pierre Curie found that pitchblende, an ore of uranium, was more radioactive than uranium itself. This was because the ore contained, in addition to uranium, the elements radium, thorium and polonium, all radioactive elements themselves. This gave support to the view that radioactivity was an elemental property.


TYPES OF RADIOACTIVE RADIATIONS :- Alpha ( α ) rays :- These rays consists of positively charged particles called α -particles. They are Helium nuclei each containing two protons and two neutrons, but no electrons, i.e., they have two units positive charge and four amu mass. The velocity of light. The penetrating power of α -particles is not very high. They are only slightly affected by magnetic and electrical fields.

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2. Beta ( β ) rays :- These rays consist of one unit negative charge and have mass equivalent to the mass of an electron. The velocity of β -particles is equal to the velocity of light. They have greater penetrating power compared to the α -particles. They are strongly affected by electrical and magnetic fields. 3. Gamma (y) rays :- These are electromagnetic radiations. They have neither mass nor charge. Their velocity equals to that of light. They have very high penetrating power, i.e. they can penetrate a 30 cm thick iron plate. They are not affected by electrical or magnetic fields.


TYPES OF RADIOACTIVE PHENOMENON :- There are two types of radioactive phenomenon :- Nuclear fission :- Fission means ‘breaking up’. A nuclear reaction in which a heavy atomic nucleus breaks up into two smaller nuclei, with the release of a very large amount of energy, is called nuclear fission. Nuclear fusion :- fusion means ‘to fuse or to join together’. A nuclear reaction in which the nuclei of light atoms join up to form a heavier nucleus, causing the release of a huge amount of energy, is known as nuclear fusion.

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Alpha decay is a radioactive process in which a particle with two neutrons and two protons is ejected from the nucleus of a radioactive atom. The particle is identical to the nucleus of a helium atom . Alpha decay only occurs in very heavy elements such as uranium, thorium and radium. The nuclei of these atoms are very “neutron rich” (i.e. have a lot more neutrons in their nucleus than they do protons) which makes emission of the alpha particle possible. After an atom ejects an alpha particle, a new parent atom is formed which has two less neutrons and two less protons. Thus, when uranium-238 (which has a Z of 92) decays by alpha emission, thorium-234 is created (which has a Z of 90). Because alpha particles contain two protons, they have a positive charge of two. Further, alpha particles are very heavy and very energetic compared to other common types of radiation. Typical alpha particles will travel no more than a few centimeters in air and are stopped by a sheet of paper. Alpha decay

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Beta decay is a radioactive process in which an electron is emitted from the nucleus of a radioactive atom, along with an unusual particle called an antineutrino (almost massless particle that carries away some of the energy). Like alpha decay, beta decay occurs in isotopes which are “neutron rich” . When a nucleus ejects a beta particle, one of the neutrons in the nucleus is transformed into a proton. Since the number of protons in the nucleus has changed, a new daughter atom is formed which has one less neutron but one more proton than the parent. For example, when rhenium-187 decays (which has a Z of 75) by beta decay, osmium-187 is created (which has a Z of 76 ).Beta particles have a single negative charge and weigh only a small fraction of a neutron or proton. As a result, beta particles interact less readily with material than alpha particles. Beta particles will travel up to several meters in air, and are stopped by thin layers of metal or plastic Beta Decay

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Gamma decay After a decay reaction, the nucleus is often in an “excited” state. This means that the decay has resulted in producing a nucleus which still has excess energy to get rid of. Rather than emitting another beta or alpha particle, this energy is lost by emitting a pulse of electromagnetic radiation called a gamma ray. The gamma ray is identical in nature to light or microwaves, but of very high energy. Like all forms of electromagnetic radiation, the gamma ray has no mass and no charge . Gamma rays interact with material by colliding with the electrons in the shells of atoms. They lose their energy slowly in material, being able to travel significant distances before stopping. Depending on their initial energy, gamma rays can travel from 1 to hundreds of meters in air and can easily go right through people. It is important to note that most alpha and beta emitters also emit gamma rays as part of their decay process . However, there is no such thing as a “pure” gamma emitter.

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Properties of Radiation Alpha particles are heavy and doubly charged which cause them to lose their energy very quickly in matter. They can be shielded by a sheet of paper or the surface layer of our skin . Alpha particles are considered hazardous only to a persons health if an alpha emitting material is ingested or inhaled . Beta and positron particles are much smaller and only have one charge, which cause them to interact more slowly with material. They are effectively shielded by thin layers of metal or plastic and are again considered hazardous only if a beta emitter is ingested or inhaled. Gamma emitters are associated with alpha, beta, and positron decay . X-Rays are produced either when electrons change orbits within an atom, or electrons from an external source are deflected around the nucleus of an atom. Both are forms of high energy electromagnetic radiation which interact lightly with matter . X-rays and gamma rays are best shielded by thick layers of lead or other dense material and are hazardous to people when they are external to the body. Neutrons are neutral particles with approximately the same mass as a proton. Because they are neutral they react only weakly with material . They are an external hazard best shielded by thick layers of concrete .

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Neutron-Induced Fission Bombardment with a neutron resulting in splitting the nucleus into two parts (fission fragments), neutrons, and gamma rays . Fusion Cold Fusion Neutron Capture Coulomb Excitation Particle Transfer Pair Production A collision process for gamma rays with energies greater than 1022-keV (two electron masses) where an electron /positron pair is produced . A heavy nucleus must be present for pair production.

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Photoelectric effect Collision process between an x-ray or gamma rays and a bound atomic electron where the photon disappears, the bound electron is ejected, and the incident energy is shared between the ejected electron and the remaining atom. The photon energy must be greater than the atomic binding energy . Positron Annihilation Positron decay in matter by annihilation with an electron . Usually and "atom" of positronium (e+e-) forms which annihilates to produce two 511-keV photons. Occasionally, the positron will annihilate in flight to produce on or more photons sharing the total rest mass and kinetic energy of the positron and electron.

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Half-life is the time required for the quantity of a radioactive material to be reduced to one-half its original value. All radionuclides have a particular half-life, some of which a very long, while other are extremely short. For example, uranium-238 has such a long half life, 4.5x10 9 years, that only a small fraction has decayed since the earth was formed. In contrast, carbon-11 has a half-life of only 20 minutes. Since this nuclide has medical applications, it has to be created where it is being used so that enough will be present to conduct medical studies.

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When given a certain amount of radioactive material, it is customary to refer to the quantity based on its activity rather than its mass . The activity is simply the number of disintegrations or transformations the quantity of material undergoes in a given period of time . The two most common units of activity are the Curie and the Becquerel . The Curie is named after Pierre Curie for his and his wife Marie's discovery of radium . One Curie is equal to 3.7x10 10 disintegrations per second. A newer unit of activity if the Becquerel named for Henry Becquerel who is credited with the discovery of radioactivity . One Becquerel is equal to one disintegration per second. It is obvious that the Curie is a very large amount of activity and the Becquerel is a very small amount. To make discussion of common amounts of radioactivity more convenient, we often talk in terms of mille and micro Curies or kilo and MegaBecquerels .

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Common Radiation Units – SI Gray (Gy) - to measure absorbed dose ... the amount of energy actually absorbed in some material , and is used for any type of radiation and any material ( does not describe the biological effects of the different radiations) Gy = J / kg (one joule of energy deposited in one kg of a material) Sievert (Sv) - to derive equivalent dose ... the absorbed dose in human tissue to the effective biological damage of the radiation Sv = Gy x Q (Q = quality factor unique to the type of incident radiation) Becquerel (Bq) - to measure a radioactivity … the quantity of a radioactive material that have 1 transformations /1s Bq = one transformation per second, there are 3.7 x 10 10 Bq in one curie. __________________________________________________________________________________ Roentgen (R) - to measure exposure but only to describe for gamma and X-rays , and only in air . R = depositing in dry air enough energy to cause 2.58E-4 coulombs per kg Rad (radiation absorbed dose) - to measure absorbed dose Rem (roentgen equivalent man) - to derive equivalent dose related the absorbed dose in human tissue to the effective biological damage of the radiation. Curie (Ci) - to measure radioactivity . One curie is that quantity of a radioactive material that will have 37,000,000,000 transformations in one second. 3.7 x 10 10 Bq

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Since we cannot see, smell or taste radiation, we are dependent on instruments to indicate the presence of ionizing radiation. The most common type of instrument is a gas filled radiation detector. This instrument works on the principle that as radiation passes through air or a specific gas, ionization of the molecules in the air occur. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to the negative side of the detector (the cathode) and the free electrons will travel to the positive side (the anode). These charges are collected by the anode and cathode which then form a very small current in the wires going to the detector. By placing a very sensitive current measuring device between the wires from the cathode and anode, the small current measured and displayed as a signal. The more radiation which enters the chamber, the more current displayed by the instrument. Many types of gas-filled detectors exist, but the two most common are the ion chamber used for measuring large amounts of radiation and the Geiger-Muller or GM detector used to measure very small amounts of radiation. The second most common type of radiation detecting instrument is the scintillation detector. The basic principle behind this instrument is the use of a special material which glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube. The first part of the photomultiplier tube is made of another special material called a photocathode. The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage. When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger than it was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by a special instrument. Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and as laboratory instruments.


HARMFUL EFFECTS OF RADIO-ACTIVITY :- Radioactive radiations can :- cause harmful gene mutation, i.e. abnormal change in the genes of living organisms. cause skin and other types of cancer. lead to the birth of deformed babies. Cause unimaginable destruction when the are uncontrolled.


USES OF RADIOACTIVE MATERIALS :- Radioactive materials are used in the treatment of cancer (radiotherapy) because nuclear radiations kills cancer cells. They are used also to detect any disorder in our body system (radio tomography). Radioactive rays are used to sterilize food, drugs, etc., at normal temperature. They are used also to determine the age of rocks, fossils, mineral deposits, historical findings, etc.


NUCLEAR ENERGY :- The energy that is stored in the atomic nuclei of the elements is known as nuclear energy. Nuclear energy can be obtained in the form of heat and light by :- The nuclear fission of heavy elements like uranium, polonium, etc. The nuclear fusion of lighter elements like hydrogen. Nuclear Nuclear Fission. Fusion.


NUCLEAR POWER PLANT :- Nuclear power plant are set up to generate electricity. Several nuclear power plants are functioning in India. They are at Tarapur, Kalpakkam, Kota, Narora, etc. A nuclear power plant consists of :- A nuclear reactor, where nuclear fission takes place. A turbine and a generator for conversion of nuclear energy into mechanical and electrical energy respectively. A Nuclear power plant.

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