Why Diffraction, Why Neutrons?J. A. Dura

dura@nist.gov

Neutron Small Angle Scattering and Reflectometry

NCNR Summer School on

June 26, 2006

Why Diffraction?3 choices for microscopic structural information

Examples Advantages Disadvantages

Microscopy

Optical

TEM

Field Ion

Direct Local information

Scanning Probes

AFM

STM

SEM

Direct Local 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

Neutron Energy, Momentum, and Wavelength

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 )

Destructive Interference

S( )=0

S( )>0

Scattering Function S( ,)•Im=I0 * 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

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

Elastic vs. Inelastic Scattering

Probes dynamics: energy transferred from excitations in the sample

Probes structures by interference of neutrons scattered from them

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

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