# Presentation: 179643

## 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

**Scattering Events and Reciprocal Space**

## Destructive Interference

- 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

## Slide 8

## 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

## Slide 11