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9.1NuclearPhysics_000 - Properties of the nucleus Consists...

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1 9.1 Nuclear Physics Properties of nuclei Binding Energy Radioactive decay Natural radioactivity Properties of the nucleus Consists of protons and neutrons Z = no. of protons (Atomic number) N = no. of neutrons (Neutron number) A = Z+N (Mass number) Notation :For element X with mass no. A and Atomic no. Z A Z X Example He 4 2 Isotopes Isotopes are nuclei that have the same no. of protons but different no. of neutrons. The chemical properties are the same but the nuclear properties are different. i.e. some isotopes may be unstable and are radioactive. eg. 1 1 H 2 1 H 3 1 H Hydrogen - stable Deuterium - stable Tritium - radioactive Stable Nuclei N Z Plot of N vs Z for stable nuclei Outline shows unstable nuclei Max Z=83 Unstable Nuclei Some radioactive nuclei 14 6 C 32 15 P 131 53 I 238 92 U Size of the nucleus From scattering experiments with alpha particles He ++ KE PE =kq 1 q 2 /r For gold nuclei Closest approach closest r < 3x10 -14 m This is smaller than the size of atoms r ~ 10 -10 m Size of the nucleus Radius varies as the cube root of A r= r o A 1/3 where r o = 1.2x10 -15 m example For Uranium 238, A=238 15 1/ 3 15 r 1.2x10 (238 ) 7.4x10 m = = 238 92 U
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2 Forces in the nuclei Coulomb forces The protons repel each other with Coulomb forces. These are enormously large due to the small size. Nuclear forces The nucleus is held together by the nuclear force. This force acts only at short range (~10 -15 m) and is independent of charge (i.e. acts between proton-proton, proton-neutron and neutron-neutron). +Z Equivalence of mass and energy A famous result from Einstein’s Special Relativity Theory E= mc 2 mass can be converted into energy Energy equivalent of an electron mass E=mc 2 = (9.1x10 -31 kg)(3x10 8 m/s) 2 = 8.2x10 -14 J =5.1x10 5 eV= 0.51MeV An electron can be annihilated (converted completely to energy). A 0.51 MeV photon is produced. Mass changes when energy is lost or gained Gasoline + O 2 -> CO 2 +water + kinetic energy (heat) The energy released is equal to the change in mass in the reaction. E= mc 2 CO 2 + water is lighter.
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