lecture41 - 1 Structure Nuclear physics At the center of an...

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Nuclear physics 1. Structure At the center of an atom there is a tiny but massive nucleus Nuclei are made up of protons and neutrons Proton has a positive charge with the same magnitude as electron (+e=1.6x10 -19 C) and a mass m p = 1.67262x10 -27 kg Neutron is electrically neutral (q=0) and has a mass m n = 1.67493x10 -27 kg that is very slightly lager the mass of the proton Nucleons: protons and neutrons Different nuclei are referred as nuclides 2. Terminology Z - the atomic number or proton number (the number of protons in the nucleus) N - the neutron number (the number of neutrons in the nucleus) A - the atomic mass number (the number of nucleons in the nucleus) A = Z + N X - chemical symbol for the element; it contains the same information as Z but in a more easily recognizable form Standard notation: Isotopes nuclei that contain the same number of protons but different number of neutrons X A Z
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3. Radius Because of wave-particle duality, the size of the nucleus is somewhat fuzzy. ( 29 3 1 15 3 3 4 10 2 . 1 A m r A r V - = π Example: Estimate the diameter of the following nuclei: . , 235 92 1 1 U H ( 29 ( 29 ( 29 ( 29 17 . 6 10 15 235 10 2 . 1 2 2 10 4 . 2 1 10 2 . 1 2 2 15 15 15 15 3 1 3 1 = = = = - - - - H U U U H H d d m m r d m m r d Example: Compare the density of nuclear matter to the density of normal solids 15 3 15 10 3 3 10 10 10 / / = = - - nucl atom atom atom nucl nucl atom nucl r r V m V m ρ
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4. Atomic mass units Masses of atoms are measured with reference to the carbon-12 atom A neutral atom is given the precise value 12.000000 u (unified atomic mass units) 1 u = 1.6605x10 -27 kg = 931.5 MeV/c 2 Proton: m p = 1.67262x10 -27 kg = 1.007276 u = 938.27 MeV/c 2 Neutron: m n = 1.67493x10 -27 kg = 1.008665 u = 939.57 MeV/c 2 1 H atom: m H = 1.67353x10 -27 kg = 1.007825 u = 938.78 MeV/c 2 (proton +electron) Electron: m e = 9.1094x10 -31 kg = 0.00054858 u =0.51100 MeV/c 2 C 12 6 What it is MeV/c 2 ? 2 27 2 35 2 16 19 16 16 2 8 2 2 / 10 94 . 560 / 10 6094 . 5 / 10 98740 . 8 1 / 10 6022 . 1 10 98740 . 8 10 98740 . 8 ) / 10 9979 . 2 ( 1 1 c MeV c eV c J kg eV J J J s m kg c kg mc E = = = = = = = -
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Binding Energy The total mass of a stable nucleus is always less than the sum of the masses of its separate protons and neutrons. Where has the mass gone?
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This note was uploaded on 02/11/2012 for the course PHYSICS 222 taught by Professor Ogilvie during the Fall '05 term at Iowa State.

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lecture41 - 1 Structure Nuclear physics At the center of an...

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