Sample simulation. Basic Picture Complexity 1: Geometry

• Sample simulation

Basic Picture

Complexity 1: Geometry

Complexity 2: more than 1 scatterers

Complexity 3: more than 1 scattering mechanisms

• How to handle a scatterer with competing scattering mechanisms

Complexity 4: sample forms

• Single crystal

• Polycrystal

• Amorphous

Sample simulation framework

• Sample assembly– Scatterers

• Scattering kernels– (Phonon dispersion…)

Sample simulation framework - motivation

An extensible sample simulation framework has been constructed. It is designed with the following issues in mind:

• separation of physics and geometry. A clean separation of geometrical and physics properties will increase flexibility and extensibility.

• composite sample assembly. A sample is not alone. Usually it is inside some kinds of container. A sample simulation needs to take into account a collection of scatterers including sample and other objects.

• composite scatterer. Currently available sample simulation usually focus on one kind of scattering mechanism. A full simulation should take into account all possible scattering mechanisms with similar scattering strength.

Sample simulation UML

Sample simulation - algorithm

• The ScattererContainer is a container of scatterers. When a neutron comes in, it gathers the information of the position, orientation, and shape of all scatterers and passes the information to a PathFinder. A PathFinder will figure out the path of a particle through those shapes given the position and moving direction of the particle. With those information at hand, ScattererContainer will randomly choose a scatterer, and ask the scatterer to respond to the neutron event.

• Now the ball is on the scatterer's court. He is a container of scattering kernels. One of those kernels will be randomly picked and asked to respond to the neutron event. A scattering kernel has all information about the physics, and will figure out which direction the neutron should go and report back to the hosting scatterer. And then the scatterer will report back what he knows to ScattererContainer, where the fate of the neutron will be finally decided.

Coherent inelastic phonon scattering kernel

Fcc Ni Sample

Shape

Sample Assembly

Collection of scatterers

Aluminum Can

Collection of scattering kernels

Incoherent inelastic phonon scattering kernel

Collection of scattering kernels

Shape

A test case: simulation of an inelastic scattering experiment with bcc Tungsten

Instrument setup: general

Neutron Source

Sample Detector

Instrument setup 1: all ideal

Neutron Source

Sample Detector

Monochromatic (all neutrons are in

the same state)New

Bcc Tungsten polycrystal sample with only cohernt inelastic phonon scattering

New

Ideal detector that records neutron intensities as a function of Q, the momentum transfre, and E, the energy transfer

McStas

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Q (Angstrom^-1)

E(m

eV)

• 1st Brillouine Zone: optical branch is partially missing• Higher Brillouine Zone: sharp dispersions

Instrument setup 2: ARCS source

Neutron Source

Sample Detector

Simulated neutrons at sample position of

ARCS instrumentNew

Bcc Tungsten polycrystal sample with only cohernt inelastic phonon scattering

New

Ideal detector that records neutron intensities as a function of Q, the momentum transfre, and E, the energy transfer

McStas

ARCS neutrons at sample

Moderator (McStas)

Guides, Choppers(McStas)

Neutron recorder(new)

Q (Angstrom^-1)

E(m

eV)

• dispersions not as sharp• large smearing due to long tail of energy distribution of incident neutrons

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Energy resolution of Fermi chopper

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

ARCS neutrons at sample

(New)

I(E) monitor(McStas)

Instrument setup 3: ARCS source and detector

Neutron Source

Sample Detector

Bcc Tungsten polycrystal sample with only cohernt inelastic phonon scattering

New

ARCS detector. Reduction is done to reduced the detector

data to I(Q,E)New

Simulated neutrons at sample position of ARCS instrument

New

Q (Angstrom^-1)

E(m

eV)

• more smearing due to sample size, detector size

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

A test case: simulation of an inelastic scattering experiment with fcc Ni

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Simulation result. Monochromatic source. Ideal detector

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Simulation result.broadening due to Fermi chopper included

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Simulation result.broadening due to Fermi chopper, sample, and detector included