MCB124lecture7

MCB124lecture7 - MCB 124 Critical Dimensions for Life...

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Unformatted text preview: MCB 124 Critical Dimensions for Life Sciences light microscopes electron microscopes scanning probe microscopes X-ray diffraction NMR MCB 124 Transmission Electron Microscopy MCB 124-100’000 Volt ! Sample Viewing Screen Vacuum ! The TEM MCB 124 Thermionic Gun φ e- kT e AT J φ- = 2 Tungsten, LaB 6 MCB 124 Thermionic Gun (2)…Optimizing Brightness MCB 124 Field Emission Gun φ e- Apply field φ e- r V E = φ e- MCB 124 Brightness current density per unit solid angle ( 29 2 4 α π β d i e = d = source diameter i e = emission current α divergence semi-angle Highest Brightness is the Cold Field Emission Source MCB 124 Temporal Coherency - Energy Spread E vh c ∆ = λ Defines the Coherence of the Beam- usually not an issue for most applications- ∆ E is in the range 0.2-3eV MCB 124 Spatial Coherency related to the size of the source α λ 2 〈〈 c d d c = effective source size λ = wavelength α = angle subtended to the specimen More spatial coherence increases resolution MCB 124 Electron Lenses Polepiece Bore and Gap are Important Parameters MCB 124 Types of Lenses MCB 124 Apertures Used to limit angular range and stop stray signals MCB 124 Aberrations 3 β s sph C r = β E E C r c chr ∆ = MCB 124 Theoretical Resolution β λ 61 . = th r Why not increase b? MCB 124 The Effect of C s ( 29 2 1 2 2 s r r r th + = Add in quadrature: ( 29 ( 29 2 1 2 3 2 61 . + = β β λ β s C r Minimum r when: ( 29 ( 29 5 2 3 2 6 61 . 2 β β λ β β s C d dr +- = = 4 1 4 1 77 ....
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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.

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MCB124lecture7 - MCB 124 Critical Dimensions for Life...

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