20. Nuclear Physics

Nuclear Physics Rutherford Alpha (He2+) Scattering Experiment Key features of the Setup:  An Alpha source  Thin Metal Foil  Gold foil  Fluorescence detector  ZnS Screen  Vacuum Chamber o Prevent collision with air Results:  Most He2+ have little or no deviation o Volume of the atom is made up mainly of empty spaces  A small fraction of He2+ were deflected by more than 90° and a very small proportion were deflected by 180° o Nucleus consists of a positive charge concentrated in a very small space Nuclide – a particular species of a nucleus that is specified by its proton and neutron number Nuclide notation: A Z X Where X is the symbol of the element, Z is the proton number, A is the nucleon number (sum of the number of protons and neutrons) Subatomic Particles Sub-atomic particle Rest mass Charge Proton ( H1 1 or p1 1 ) 1.6726 x 10-27 kg +1.6 x 10-19 C Neutron ( n1 0 ) Approximately mass of proton 1.6749 x 10-27 kg 0 Electron ( e0 1 ) 9.11 x 10-31 kg ( ≈ 1/1800 x mass of protons) -1.6 x 10-19 C Isotopes – are nuclides having the same atomic number (same element) but different mass number Atomic mass unit (u) – a unit of mass. 1u is defined as 1/12 the mass of a carbon-12 atom, 1u = 1.661 x 10-27 kg Electronvolt (eV) – is a non-SI unit for energy equal to the energy gained by an electron when it is accelerated through a potential difference of one volt. (1eV = 1.60 x 10-19 J) Gold Foil

Mass Defect  Mass of an atom is less than the sum of masses of their separate constituents – protons, neutrons and electrons  The difference between the mass of a nucleus and the mass of its constituent particles taken separately is known as mass defect o Mass defect = Mass of protons and neutrons – Nuclear mass o Z Mp + (A – Z)Mn – M  where Z: proton number; A: mass number; Mp: mass of a proton; Mn mass of a neutron; M: mass of the nucleus Nuclear Binding Energy  Binding energy is the work that would have to be done (energy absorbed) to separate a nucleus into its constituent protons and neutrons. OR  Binding energy is the energy released if a nucleus is formed from its constituent separate protons and neutrons Mass-Energy Equivalence  Einstein proposed that mass and energy are equivalent, that is, it is possible to interchange mass and energy, 2E mc .  Binding energy is the energy equivalent of the mass defect of the nucleus, 2E mc , m is the mass defect Nuclear Processes  A nuclear reaction (or process) can be represented as, o 13 7 12 6 NHC  1 1  In a nuclear process, the following are conserved o Nucleon number o Proton number o Mass-Energy o Momentum  Note: Nuclear reaction/process outlined is only one of the many possible reactions. It is unlike chemical reactions that are definite without other product. In a nuclear reactions, many different products are possible even if the initial products are the same  Calculate energy released in a