21 Nuclear and Particle physics

21 Nuclear and Particle physics - Plan for Today 1 Nuclear...

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Physics 13 Lecture 21 Prof. H. Gallagher 1 Plan for Today 1. Nuclear physics α , β , γ decays nuclear shell model nuclear physics in astronomy / astrophysics 2. Particle physics The Standard Model
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Physics 13 Lecture 21 Prof. H. Gallagher 2 Conservation laws In nuclear reactions the following are conserved: 1. Relativistic total energy and momentum 2. Angular momentum 3. Electric charge 4. mass number A (baryon number conservation) 5. Lepton number
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Physics 13 Lecture 21 Prof. H. Gallagher 3 Alpha Decay Alpha decay: an unstable nucleus (X) disintegrates into a lighter nucleus (X’) and an alpha particle (a nucleus of 4 He). All nuclei with Z>83 are theoretically unstable to α decay. Example of barrier penetration Reduces A by 4 units and Z by 2 units. Q = ((m(X) - m (X’) - m( 4 He))c 2 Kinetic energy shared by alpha particle and nucleus with K α ~ (A-4)*Q/A Produced by the strong force Four possible decay chains or sequences A=4n, (4n+1),(4n+2),(4n+3) All occur naturally except the (4n+1) chain t 1/2 ( 237 Np ) = 2 x 10 6 yrs Image from http://www.radongas.com/images/alpha_penetration.gif
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Physics 13 Lecture 21 Prof. H. Gallagher 4 Beta and gamma decay Beta Decay: neutron in the nucleus decays to produce a proton, electron, and anti-electron neutrino. n --> p + e - + ν e Produced by the weak force Changes Z and N by one unit but leaves A unchanged Kinetic energy shared by proton, electron, and anti-neutrino. for electron emission Q = (m( A X) - m( A X’)) c 2 Other examples are positron emission and electron capture (different Q values though)! Gamma Decay: nucleus in an excited state emits one or more photons and is left in the ground state. Excited states of the nucleus just like excited states of the atom! Often occurs following an alpha or beta decay which leaves the final nucleus in an excited state. Other processes also contribute (though more rarely): spontaneous fission, double beta decay
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Physics 13 Lecture 21 Prof. H. Gallagher 5 Neutrinos and the mystery of β decay Dear Radioactive Ladies and Gentlemen: I beg you to receive graciously the bearer of this letter who will report to you in detail how I have hit on a desperate was to escape from the problems of the … continuous beta spectrum … and the law of conservation of energy. I admit that my way out may look rather improbable at first since if the neutron (neutrino) existed it would have been seen long ago. But nothing ventured, nothing gained. … Well, dear radioactive friends, weigh it and pass sentence! Unfortunately, I cannot appear personally in Tubingen, for I cannot get away from Zurich on account of a ball which is
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21 Nuclear and Particle physics - Plan for Today 1 Nuclear...

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