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Physics 30: Nuclear Physics and Radioactivity

Physics 30: Nuclear Physics and Radioactivity. Christopher Chui. Structure and Properties of the Nucleus. A proton is the nucleus of the simplest atom, H 2 The neutron, discovered by Chadwick in 1932, is electrically neutral Both protons and neutrons are nucleons, ~10 -27 kg

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Physics 30: Nuclear Physics and Radioactivity

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  1. Physics 30: Nuclear Physics and Radioactivity Christopher Chui Nuclear Physics and Radioactivity

  2. Structure and Properties of the Nucleus • A proton is the nucleus of the simplest atom, H2 • The neutron, discovered by Chadwick in 1932, is electrically neutral • Both protons and neutrons are nucleons, ~10-27 kg • The number of protons in the nucleus is called the atomic number, Z • The total number of nucleons is called the atomic mass number, A • The neutron number, N = A – Z • For a nuclide X, the symbol is AZ X • Isotopes are nuclei that contain the same number of protons but different numbers of neutrons Nuclear Physics and Radioactivity

  3. Binding Energy and Nuclear Forces • Atomic masses are for neutral atoms • Atomic mass unit, 1 u = 1.6605 x 10-27 kg = 931.5 MeV/c2 • In solving problems, keep track of electron masses • Binding energy refers to the amount of energy to break the nucleus • Average binding energy per nucleon is the total binding energy of a nucleus divided by A • Strong nuclear force makes stable nuclei, which is short range • Electric and gravitational forces are long range • Radioactivity is called weak nuclear force Nuclear Physics and Radioactivity

  4. Radioactivity-1 • Henri Becquerel discovered radioactivity in 1896 • Marie and Pierre Currie discovered radium in 1906 • Rutherford in 1898 discovered a, b, and g rays • Alpha decay refers to giving off helium nuclei • Change from one kind of element into another is called transmutation, from parent to daughter • Disintegration energy, Q = (Mp - Md - ma) c2 • Beta decay is the emission of an electron from the nucleus (n  p + e- ) • Neutrino has 0 charge and 0 rest mass • A positron has the same mass as an electron but + charge • Electron capture occurs when a nucleus absorbs one e- Nuclear Physics and Radioactivity

  5. Radioactivity-2 • K-capture refers to capturing an electron in the innermost (K) shell. The electron disappears and a proton becomes a neutron, a neutrino is emitted • Gamma decay refers to emission of photons by excited atoms, which are results from collision with another particle • If atoms remain excited for some time, they are in a metastable state and are called isomers • An excited nucleus returns to the ground state by a process called internal conversion with no g emitted • X-rays are produced by an electron-atom interaction, and g rays are produced in a nuclear process Nuclear Physics and Radioactivity

  6. Half-Life and Rate of Decay • Radioactive decay law: N = N0 e-lt • Number of decays/sec, DN/Dt, is the activity of sample • Half-life is the time it takes for half of the original amount to decay • There are several natural radioactive decay series:U238Pb206, Th232Pb207 • Many objects can be dated by C14 Nuclear Physics and Radioactivity

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