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Why Diffraction, Why Neutrons? J. A. Dura Neutron Small Angle Scattering and Reflectometry NCNR Summer School on June 26, 2006.

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Presentation on theme: "Why Diffraction, Why Neutrons? J. A. Dura Neutron Small Angle Scattering and Reflectometry NCNR Summer School on June 26, 2006."— Presentation transcript:

1 Why Diffraction, Why Neutrons? J. A. Dura dura@nist.gov Neutron Small Angle Scattering and Reflectometry NCNR Summer School on June 26, 2006

2 Why Diffraction? 3 choices for microscopic structural information ExamplesAdvantagesDisadvantages Microscopy Optical TEM Field Ion DirectLocal information Scanning Probes AFM STM SEM DirectLocal information Surfaces only Diffraction Probes Electron (RHEED, LEED) X-ray Neutron Quantitative data on correlations and distribution of structural features Probes entire sample Requires Fitting

3 Neutron Energy, Momentum, and Wavelength

4 Scattering Events and Reciprocal Space A Neutron Scattering Instrument: Creates a beam with a well defined Measures the amount of scattered neutrons into a well defined Thereby probes various Q or “Scans” Reciprocal space to determine S( Q )

5 Destructive Interference S( )=0 S( )>0 Scattering Function S(,  ) I m =I 0 * S(,  ) Depends only on sample/independent of instrument Each point in reciprocal space derived from entire sample Representative sampling of whole Reciprocal space needed to fully recreate sample Constructive Interference

6 Why Neutrons? The properties of the neutron lead to unique experimental techniques with particular advantages Subatomic Particle Particle & Related Physics Nuclear Activation PGAA, NDP, Radiography Energies ~excitations in materials Inelastic Scattering Neutron Interactions and Dosimetry Group / Physics Laboratory Studies the weak interactions of the neutron Neutron decay lifetime, Decay angular correlations, Low energy neutron-nucleon interactions Consequences for Cosmology, Weak interactions physics Tests of the Standard Model of Particle physics Cold Neutron Trap PGAA NDP

7 Elastic vs. Inelastic Scattering Probes dynamics: energy transferred from excitations in the sample Probes structures by interference of neutrons scattered from them

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9 Why Neutrons? The properties of the neutron lead to unique experimental techniques with particular advantages Subatomic Particle Particle & Related Physics Nuclear Activation PGAA, NDP, Radiography Energies ~excitations in materials Inelastic Scattering Wavelength ~ atomic spacing lower limit on sizes Geometry of the motions Interferometry Elastic Scattering Techniques ( SANS, NR, Diffraction) Spin 1/2 Particle magnetic sensitivity Neutral Particle - Interacts with Nucleus via Strong Force light element sensitivity (independent of Z) isotope effect isotropic scattering (no form factor) Magnetic Nuclear

10 Why Neutrons? The properties of the neutron lead to unique experimental techniques with particular advantages Subatomic Particle Particle & Related Physics Nuclear Activation PGAA, NDP, Radiography Energies ~excitations in materials Inelastic Scattering Wavelength ~ atomic spacing lower limit on sizes Geometry of the motions Interferometry Elastic Scattering Techniques ( SANS, NR, Diffraction) Spin 1/2 Particle magnetic sensitivity Neutral Particle - Interacts with Nucleus via Strong Force light element sensitivity (independent of Z) isotope effect isotropic scattering (no form factor) Neutral Particle – Weakly interacting & penetrating Simplified scattering theory Non-destructive Simplified sample environments Penetrates the whole sample Imaging Residual Stress Analysis Probes the entire sample simultaneously Statistics on sample wide distributions of features

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