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Unformatted text preview: MCB 124 What are Xrays? Visible XRay Gamma Electron Microscope λ 500 nm 0.03 nm 0.0003 nm 0.0006 nm ν 6*10 14 Hz 1*10 19 Hz 1*10 21 Hz 5*10 19 Hz E 3.6*1019 J = 2.2eV 6.4*1015 J = 40 keV 6.4*1013 J = 4 MeV 3.2*1014 J = 200 keV 10 2 1 101 104 107 108 1010 1012 1014 Wavelength in meters MCB 124 Condensed History 1912: crystals exhibit diffraction qualities first photographic diffraction patterns 1913: successful analysis of (PO 4 Cl and NaCl) 1960’s: Watson and Crick 1960’s: computer technology W. H. Bragg and W. L. Bragg, Nobel Prize in Physics in 1915 Crick, Watson and Wilkins Nobel Prize in Physiology or Medicine in 1962 MCB 124 Generation of Xrays λ hc mv eV = = 2 2 1 nm V 8 . 1239 min = λ MCB 124 The Xray Spectrum Continuum or Bremsstrahlung radiation plus characteristic peaks – caused by electronic transitions in the target MCB 124 Practical Xray Tubes MCB 124 Wavelength Selection x x e I I μ = x x e I I ρ ρ μ  = MCB 124 Absorption Edges and Xray Filters Use a Nickel window to filter the Cu X ray peaks MCB 124 Xray Detectors •Scintillator •Conical Fiberoptic Taper •4k x 4k CCD chip MCB 124 Determining Structures – Xray Diffraction ( 29 ( 29 ( 29 t kx A t x E ν π = 2 cos , ( 29 ( 29 ( 29 ct x k A t x E = π 2 cos , ( 29 t i ikx e Ae t x E πν π 2 2 , = K is a vector in reciprocal space MCB 124 Phase Relationship Between Waves λ π φ x ∆ = 2 ( 29 ( 29 ( 29 2 cos 2 cos 2 cos cos φ φ φ + = + + = x x x x S Can get constructive and destructive interference MCB 124 Young’s Twoslit Experiment Need to find “slits” small enough to work for Xrays MCB 124 Scattering of Xrays by Lattice Planes λ θ n d = sin 2 Bragg’s Law λ θ = sin 2 hkl d MCB 124 kvectors λ λ π 1 2 ≈ = = D I k k Elastic Scattering: MCB 124 …kvectors (2) k 2 sin Κ = θ λ θ sin 2 = Κ Bragg: λ θ B B sin 2 = Κ MCB 124 …kvectors (3) Now have 2 versions of Bragg’s Law B d n θ λ sin 2 = λ θ B B sin 2 = Κ Combining the equations gives g d B = = Κ 1 MCB 124 What is n in Bragg’s Law? P 1 & P 3 in phase if: λ θ n d = sin 2 2 g g 2 2 = So… λ θ = sin 2 n d MCB 124 What is n in Bragg’s Law?...
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This note was uploaded on 11/09/2010 for the course MCB 124 taught by Professor Baldwin during the Fall '09 term at UC Davis.
 Fall '09
 BALDWIN

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