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