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Introduction |
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Atoms contain three fundamental particles namely electrons, protons and neutrons. Protons and neutrons constitute the nucleus whereas electrons are present in the extra nuclear region of the atom. |
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The Nucleus |
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Rutherford and his co-workers, performed series of scattering experiments and concluded that the entire mass and the positive charge is concentrated in a very small region at the centre of the atom, known as nucleus. The radius of the nucleus is about 10-15m i.e., 10-5 times that of the atom. |
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Discovery of Radioactivity and Nature of Radiations |
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In 1896, the phenomenon of radioactivity was accidentally discovered by French scientist Henri Becquerel. He was performing experiments with uranium compounds. He accidentally placed a crystal of potassium uranyl sulphate over photographic plate wrapped with usual black paper. |
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Emission of alpha, beta and gamma Rays |
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Emission of a and b-rays bring some changes in the nucleus of the atom and the new element formed as a result of these charges is called daughter element. |
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Group Displacement Law |
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Soddy and Fajan summarised the a and b - particle emission in the form of the law known as Group Displacement Law. |
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Radioactive Decay Series |
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The emission of a or b - particles from a radioactive element results in the function of new element called daughter element. If the daughter element still has unstable nucleus, it further disintegrates by emitting a or b - particle and produces a new daughter element. This process of successive disintegration continues till the end product, is an element with stable nucleus. This series of spontaneous changes that take place starting from a radioactive element till the formation of a non-radioactive end product is called radioactive disintegration series. |
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Nuclear Stability and Neutron/Proton Ratio |
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It has been found that the stability of nucleus depends upon its neutron to proton (n/p) ratio. A plot of N (the neutron number) against Z (atomic number or number of protons) show that for stable
nuclides upto Z = 20, N = 20 i.e., 
the relationship can be represented by a line with a slope of 45o. Thus maximum stability is attained when N = Z. |
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Nuclear - Binding energy |
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The sum of the individual masses of various particle in the nucleus must be equal to the nuclear mass. But this is not so in actual practice. The nuclear mass is somewhat less than the sum of the individual masses of various nuclear particles. The difference between the actual nuclear mass and the expected nuclear mass (sum of the individual masses of nuclear particles) is referred to as mass defect. The mass defect can be converted into equivalent energy by means of Einstein equation (DE = Dmc2). |
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Rate of Radioactive Decay |
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The spontaneous breaking up of the nucleus is known as radioactive disintegration or radioactive decay. |
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Rate of Disintegration |
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Suppose an element A disintegrates into another element B.
Let number of atoms of A when t = 0 is N0. |
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Half-Life Period |
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The time required for the decay of the radioactive element to one half of its original amount is called Half-Life Period. |
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Artificial Nuclear Reactions |
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Radioactivity is broadly classified into two categories:
a) Natural radioactivity and
b) Artificial or Induced radioactivity. |
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Synthetic Elements Including Transuranics |
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Nuclear reactions involving the bombardment technique by different particles have been used to synthesise artificial elements such as Technetium, Astatine and Transuranium. (Z > 92) which follow uranium in the periodic table. |
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Nuclear Fission |
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Two consequences of nuclear reactions are the phenomena of nuclear fission and nuclear fusion. Both are important sources of nuclear energy, which may be used for peaceful or destructive purposes. |
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Nuclear Reactors |
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An equipment in which the nuclear chain reaction is carried out in a controlled manner is called a nuclear reactor. The energy liberated in a controlled manner is used to produce steam, which can run turbines and produce electricity. In nuclear reactors, the nuclear fission is controlled by controlling the number of neutrons released during the fission. |
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Breeder Reactors |
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Natural uranium contains very little (0.72%) of its fissionable isotope 235U needs to be enriched in the latter to be useful as a fuel in nuclear reactor. A breeder reactor is one that produces more fissionable nuclei than it consumes. |
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Nuclear Fusion |
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The fission of heavy nuclei is accompanied by mass losses resulting in the liberation of large amounts of energy. Similarly, the fusion of light nuclei is accompanied by mass losses and the evolution of large quantities of energy. Nuclear fusion is a process in which lighter nuclei fuse together to form a heavier nucleus with the evolution of large amounts of energy due to the mass defect that occurs. |
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Applications of Radioactivity and Radioisotopes |
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Radioisotopes find numerous uses in different areas such as medicine, chemistry, biology, archaeology, agriculture, industry and engineering. |
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Radio Carbon Dating |
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Radiocarbon 
dating of historical wooden - derived objects is based on the knowledge that the cosmic ray intensity (responsible for 14C production) has been practically constant for thousands of years. 14C is formed in the upper atmosphere by the action of cosmic radiation on 14N. |
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Summary |
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Emission of alpha particle reduces the atomic number by 2 and the mass number by 4. b- emission advances the atomic number by one unit without changing the mass number. Emission of a - rays affects neither the mass nor the atomic number. |
