Lecture21_NP2

Lecture21_NP2 - Nuclear Nuclear Physics 2 1 Topics Topics...

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Nuclear Physics 2 1
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Topics z Liquid drop model z Radioactivity z Nuclear Reactions z Nuclear Fusion z Summary 2
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Nuclear Models Nuclear Models – liquid drop model liquid drop model z liquid drop model (Bohr, Bethe, Weizsäcker): ucleus = drop of incompressible nuclear fluid z nucleus drop of incompressible nuclear fluid. z fluid made of nucleons, nucleons interact strongly (by nuclear force) with each other, just like molecules in a drop of liquid. z introduced to explain binding energy d mass of nuclei and mass of nuclei z predicts generally spherical shape of nuclei z good qualitative description of fission of large nuclei z provides good empirical description of binding energy 3 vs A
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Bethe Bethe – Weizsäcker formula for binding energy Weizsäcker formula for binding energy z Bethe - Weizsäcker formula: z an empirically refined form of the liquid drop model for the binding ergy of a nucleus of mass number A with Z protons and N neutrons energy of a nucleus of mass number A with Z protons and N neutrons z binding energy has five terms describing different aspects of the binding of all the nucleons: olume energy volume energy surface energy Coulomb energy (electrostatic repulsion of the protons,) asymmetry term (N s ) an asymmetry term (N vs Z) an exchange (pairing) term (even-even vs odd-even vs odd-odd number of nucleons) ( ) 3/4 - P 2 Sym /3 2 C 3 / 2 S V A a N Z a Z a A a A a ) Z , A ( B λ = 4 1/3 A A
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“liquid drop” terms in B “liquid drop” terms in B-W formula W formula 5
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Radioactivity z Discovery of Radioactivity z Antoine Becquerel (1896): serendipitous discovery of radioactivity: penetrating radiation emitted by substances containing uranium z A. Becquerel, Maria Curie, Pierre Curie(1896 – 1898): also other heavy elements (thorium, radium) show radioactivity three kinds of radiation, with different penetrating power (i.e. amount of material necessary to attenuate beam): “Alpha ( α ) rays” (least penetrating – stopped by paper) “Beta ( β ) rays” (penetrate 3mm of Al, need 2mm lead to absorb) amma ( rays” (need several cm of lead to be attenuated) Gamma ( γ ) rays (need several cm of lead to be attenuated) three kinds of rays have different electrical charge: α: +, β: −, γ: 0 entification of radiation: z Identification of radiation: z Ernest Rutherford (1899) Beta ( β ) rays have same q/m ratio as electrons lpha ( rays have same q/m ratio as He 6 Alpha ( α ) rays have same q/m ratio as He Alpha ( α ) rays captured in container show He-like emission spectrum
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Radioactivity z Alpha and beta rays are bent in opposite directions in magnetic field, while gamma rays are not bent at a magnetic field, while gamma rays are not bent at all. 7
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Radioactivity z A nucleus will be radioactive if its mass is greater than the sum of the masses of the decay products.
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This note was uploaded on 02/17/2010 for the course PHY PHY3101 taught by Professor Prosper during the Spring '10 term at FSU.

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Lecture21_NP2 - Nuclear Nuclear Physics 2 1 Topics Topics...

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