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4/29/2016 Nuclear Physics & Energy The atom 1 4/29/2016 The basics of atomic nucleus - 1 •The atomic nucleus was discovered by Earnest Rutherford in 1911. •A nucleus of an element is represented as ZXA •17Cl35 •The nucleus consists of the elementary particles, protons and neutrons which are known as nucleons. •A proton has positive charge of the same magnitude as that of electron and its rest mass is about 1836 times the mass of an electron. •The charge of a nucleus is due to the protons present in it. •Each proton has a positive charge equal to 1.6 × 10−19 Coulomb. The basics of atomic nucleus - 2 •Atomic mass unit = one twelfth of the mass of carbon atom 6C12 •1 amu = 1.66 × 10−27 kg •The mass of a proton, mp = 1.007276 amu •The mass of a electron, me = 0.000549 amu •mass of the hydrogen atom = 1.007825 amu •The mass of a neutron, mn = 1.008665 amu 2 4/29/2016 The energy equivalence of one amu under e = mc2 m = 1 amu = 1.66 × 10−27 kg c = 3 × 108 ms−1 ∴ E = 1.66 × 10−27 × (3 × 108)2 J Since, 1 eV = 1.6 × 10−19 joule ∴ E= 931 × 106 eV or 931 million electronvolt = 931 MeV Thus, energy equivalent of 1 amu = 931 MeV Nuclear mass defect and binding energy Mass defect: From the measurement of mass by mass spectrometers, it is found that the mass of a stable nucleus (m) is less than the total mass of the nucleons. This difference in mass is denoted by ∆m and called the mass defect. The binding energy : When the protons and neutrons combine to form a nucleus, the mass that disappears (mass defect, ∆m) is converted into an equivalent amount of energy (∆mc2). This energy is called the binding energy (BE) of the nucleus. Binding energy maximum of 8.8 MeV at A=56; 26Fe56 Hence, iron nucleus is the most stable. 3 4/29/2016 Nuclear Fission - 1 In 1939, German scientists Otto Hahn and F. Strassman discovered that when uranium nucleus is bombarded with a neutron, it breaks up into two fragments of comparable masses with the release of energy Nuclear Fission - 2 235 92U + 0n1 → 56Ba141 + 36Kr92 + 3 0n1 + Energy In this fission reaction: As, 1 amu = 931 MeV, energy released in a fission = 0.225303 × 931 ≅ 200 MeV 4 4/29/2016 Nuclear Fission - 3 If we can fission all the nuclei in 1kg of U235 we would release 4.9x1013J of energy. Put in every day terms, this is sufficient meet the power demands of a house with a 150 kW (150 unit) daily consumption continuously for more than 300 years. Chain reaction 1. Uncontrolled chain reaction: Explain 2. Controlled chain reaction: Explain 5 4/29/2016 Critical size and critical mass Critical size of a system containing a fissile material is defined as the minimum size in which at least one neutron is available for further fission reaction. The mass of the fissile material at the critical size is called critical mass. (about 15 kg for pure U235) Artificial transmutation conversion of one element into another by artificial methods 7N 14 + 2He4 → 8O17 + 1H1 6