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Lecture physics a2 intro to nuclear physics huynh quang linh

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1 Intro to Nuclear Physics 2 Nuclear Physics Topics  Composition of Nucleus  features of nuclei  Nuclear Models  nuclear energy  Fission  Fusion  Summary 3 About Units  Energy electron volt [.]

Intro to Nuclear Physics Nuclear Physics Topics Composition of Nucleus features of nuclei Nuclear Models nuclear energy  Fission  Fusion  Summary     About Units  Energy - electron-volt  electron-volt = kinetic energy of an electron when moving through potential difference of Volt; o eV = 1.6 × 10-19 Joules o kWãhr = 3.6 ì 106 Joules = 2.25 × 1025 eV o MeV = 106 eV, GeV= 109 eV, TeV = 1012 eV  mass - eV/c2 o o o o eV/c2 = 1.78 × 10-36 kg electron mass = 0.511 MeV/c2 proton mass = 938 MeV/c2 = 0.938 GeV/ c2 neutron mass = 939.6 MeV/c2  momentum - eV/c: o eV/c = 5.3 × 10-28 kg m/s o momentum of baseball at 80 mi/hr  5.29 kgm/s  9.9 × 1027 eV/c  Distance o femtometer (“Fermi”) = 10-15 m Radioactivity  Discovery of Radioactivity  Antoine Becquerel (1896): serendipitous discovery of radioactivity: penetrating radiation emitted by substances containing uranium  A Becquerel, Maria Curie, Pierre Curie(1896 – 1898): o also other heavy elements (thorium, radium) show radioactivity o three kinds of radiation, with different penetrating power (i.e amount of material necessary to attenuate beam):  “Alpha (a) rays” (least penetrating – stopped by paper)  “Beta (b) rays” (need 2mm lead to absorb)  “Gamma (g) rays” (need several cm of lead to be attenuated) o three kinds of rays have different electrical charge: a: +, b: -, g:  Identification of radiation:  Ernest Rutherford (1899) o Beta (b) rays have same q/m ratio as electrons o Alpha (a) rays have same q/m ratio as He o Alpha (a) rays captured in container show He-like emission spectrum Proton  “Canal rays”  1898: Wilhelm Wien: opposite of “cathode rays”  Positive charge in nucleus (1900 – 1920)  Atoms are neutral o positive charge needed to cancel electron’s negative charge o Rutherford atom: positive charge in nucleus  periodic table  realized that the positive charge of any nucleus could be accounted for by an integer number of hydrogen nuclei protons Neutron  Walther Bothe 1930:  bombard light elements (e.g 49Be) with alpha particles  neutral radiation emitted  Irène and Frederic Joliot-Curie (1931)  pass radiation released from Be target through paraffin wax  protons with energies up to 5.7 MeV released  if neutral radiation = photons, their energy would have to be 50 MeV puzzle  puzzle solved by James Chadwick (1932):  “assume that radiation is not quantum radiation, but a neutral particle with mass approximately equal to that of the proton”  identified with the “neutron” suggested by Rutherford in 1920  observed reaction was: a (24He++) + 49Be  613C* 13  612C + n C* Beta decay neutrino  Beta decay puzzle : o decay changes a neutron into a proton o apparent “non-conservation” of energy o apparent non-conservation of angular momentum  Wolfgang Pauli predicted a light, neutral, feebly interacting particle (called it neutron, later called neutrino by Fermi) Positron  Positron (anti-electron)  Predicted by Dirac (1928) needed for relativistic quantum mechanics  existence of antiparticles doubled the number of known particles!!! Positron track going upward through lead plate  P.A.M Dirac  Nobel Prize (1933)  member of FSU faculty (1972-1984)  one of the greatest physicists of the 20th century Structure of nucleus  size (Rutherford 1910, Hofstadter 1950s):  R = r0 A1/3, r0 = 1.2 x 10-15 m = 1.2 fm;  i.e ≈ 0.15 nucleons / fm3  generally spherical shape, almost uniform density;  made up of protons and neutrons  protons and neutron “nucleons”; are fermions (spin ½), have magnetic moment  nucleons confined to small region (“potential well”)   occupy discrete energy levels  two distinct (but similar) sets of energy levels, one for protons, one for neutrons  proton energy levels slightly higher than those of neutrons (electrostatic repulsion)  spin ½  Pauli principle  only two identical nucleons per eng level Nuclear Sizes - examples r  ro (A ) ro = 1.2 x 10-15 m Find the ratio of the radii for the following nuclei: 1H, 12C, 56Fe, 208Pb, 238U 3 3 : 12 : 56 : 208 : 238 : 2.89 : 3.83 : 5.92 : 6.20 10 .. .Nuclear Physics Topics Composition of Nucleus features of nuclei Nuclear Models nuclear energy  Fission  Fusion  Summary     About... material necessary to attenuate beam):  “Alpha (a) rays” (least penetrating – stopped by paper)  “Beta (b) rays” (need 2mm lead to absorb)  “Gamma (g) rays” (need several cm of lead to be attenuated)... released from Be target through paraffin wax  protons with energies up to 5.7 MeV released  if neutral radiation = photons, their energy would have to be 50 MeV puzzle  puzzle solved by James

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