PHYS 3446 – Lecture #9AnnouncementsDiscovery of Alpha and Beta RadiationsSlide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21AssignmentsMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu1PHYS 3446 – Lecture #9Monday, Oct. 9, 2006Dr. Jae Yu1. Nuclear Radiation•Beta Decay & Weak Interactions•Gamma Decay2. Energy Deposition in Media•Charged Particle Detection•Ionization ProcessMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu2Announcements•Term exam–First term exam results•Class average: 80.1•Max score: 107/100–Each term exam constitutes 15% Total 30%–Homework: 15%–Lab: 15%–Class projects: 20% (final paper) + 10% (Oral presentation)–Pop quizzes: 10%–Extra credit: 10%•Suggest going into today’s colloquiumMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu3Discovery of Alpha and Beta Radiations•After Bacquerel’s discovery of uranium effect on photo-films in 1896•The Curies began study of radio activity in 1898•Rutherford also studied using a more systematic experimental equipments in 1898–Measured the currents created by the radiations•While Bacquerel concluded that rays are observed in different levels•Rutherford made the observation using electrometer and determined that there are at least two detectable rays–Named and raysMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu4•Three kinds of -decays–Electron emission•Nucleus with large Nn•Atomic number increases by one•Nucleon number stays the same– Positron emission•Nucleus with many protons•Atomic number decreases by one•Nucleon number stays the same–You can treat nucleus reaction equations algebraically•The reaction is valid in the opposite direction as well•Any particle moved over the arrow becomes its anti particle Nuclear Radiation: -DecaysA ZX �A ZX �1A ZY+e-+1A ZY-e++A ZX �1A ZY-e++A ZX1A ZY-e-+ �Monday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu5–Electron capture•Nucleus with many protons•Absorbs a K-shell atomic electron•Proton number decreases by one•Causes cascade X-ray emission from the transition of remaining atomic electrons•For -decay: A=0 and |Z|=1Nuclear Radiation: -DecaysA ZX�1A ZY-e-+Monday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu6•Initially assumed to be 2-body decay•From energy conservation•Since lighter electron carries most the KENuclear Radiation: -DecaysXE =( )2e X Y eT E E m c= - -( )2X Y e Ym m m c T= - - -YQ T= -Q�•Results in a unique Q value as in -decay.•In reality, electrons emitted with continuous E spectrum with an end-point given by the formula above•Energy conservation is violated!!!!End-pointYeE E-+ =2Y e eE T m c+ +A ZX �1A ZY+e-+Monday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu7•Angular momentum is also in trouble•In -decays total number of nucleons is conserved –Recall |A|=0 and |Z|=1 in -decays?•Electrons are fermions with spin •Independent of any changes of an integer orbital angular momentum, the total angular momentum cannot be conserved–How much does it always differ by?•Angular momentum conservation is violated!!!Nuclear Radiation: -Decays2h2hMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu8•In 1931 Pauli postulated an additional particle emitted in -decays–No one observed this particle in experiments•Difficult to detect•First observation of e in 1956, in 1962 and in 1977 (direct 2000)–Charge is conserved in -decays •Electrically neutral–Maximum energy of electrons is the same as the Q value•Massless–Must conserve the angular momentum•Must be a fermion with spin•This particle is called neutrino (by Feynman) and expressed as Nuclear Radiation: -Decays2hMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu9•Have anti-neutrinos , just like other particles•Neutrinos and anti-neutrinos are distinguished through the spin projection on momentum–Helicity is used to distinguish them•Left-handed (spin and momentum opposite direction) anti-electron-neutrinos are produced in -decays •Right-handed electron-neutrinos are produced in positron emission–e- is a particle and e+ is the anti-particle to e-– is a particle and is the anti-particle to Nuclear Radiation: NeutrinosH p s� �r rvenenenMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu10•Electron emission•Positron emission•Electron capture–Decay Reaction Equations with NeutrinosA ZX �A ZX �A ZX e-+ �enenen1A ZY e+ -+ +1A ZY e- ++ +1A ZY-+Monday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu11•If the parent nucleus decays from rest, from the conservation of energy•Thus the Q-value of a -decay can be written•Electron emission can only occur if Q>0•Neglecting all small atomic BE, e emission can occur if-Decays with neutrinos2pM c =eDeT T Tn-+ + =( ) ( )( )2, , 1eeQ M A Z M A Z m m cn= - + - -2D DT M c+ +( )2 2ep D eM M m m c Mcn- - - =D =Q( ) ( )( )2 , , 1 0M A Z M A Z c� - + �2eeT m c-+ +2e eT m cn n+Monday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu12•Since the daughter nucleus is much heavier than e or , the small recoil energy of daughter can be ignored –Thus we can obtain•This means that the energy of the electron is not unique and can be any value in the range–The maximum electron kinetic energy can be Q–This is the reason why the electron energy spectrum is continuous and has an end point (=Q)•The same can apply to the other two -decays-Decays with neutrinoseeT T Qn-+ �0eT Q-� �Monday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu13•Baryon numbers: A quantum number assigned to baryons (particles consist of quarks)–Mostly conserved in many interactions –Baryons: +1–Anti-baryons: -1–Protons and neutrons are baryons with baryon number +1 each•Lepton numbers: A quantum number assigned to leptons (electrons, muons, taus and their corresponding neutrinos)–Leptons: +1–Anti-leptons: -1–Must be conserved at all times under SM in each species Particle NumbersMonday, Oct. 9, 2006 PHYS 3446, Fall 2006Jae Yu141A Z A ZX Ymn t+ -+ � +1A Z A ZX Y emn+ -+ � +•Three charged leptons exist in nature with their own associated neutrinos•These three types of neutrinos are distinct from each other–muon neutrinos never produce other leptons than muons or anti-muonsLepton Numberseen-� �� �� �� �1A Z A ZX Ymn m+ -+ � +mmn-� �� �� ��
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