sample simulation. basic picture complexity 1: geometry
Post on 21-Dec-2015
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• Sample simulation
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Basic Picture
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Complexity 1: Geometry
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Complexity 2: more than 1 scatterers
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Complexity 3: more than 1 scattering mechanisms
• How to handle a scatterer with competing scattering mechanisms
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Complexity 4: sample forms
• Single crystal
• Polycrystal
• Amorphous
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Sample simulation framework
• Sample assembly– Scatterers
• Scattering kernels– (Phonon dispersion…)
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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.
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Sample simulation UML
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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.
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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
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A test case: simulation of an inelastic scattering experiment with bcc Tungsten
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Instrument setup: general
Neutron Source
Sample Detector
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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
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QuickTime™ and aTIFF (LZW) decompressor
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Q (Angstrom^-1)
E(m
eV)
• 1st Brillouine Zone: optical branch is partially missing• Higher Brillouine Zone: sharp dispersions
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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
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ARCS neutrons at sample
Moderator (McStas)
Guides, Choppers(McStas)
Neutron recorder(new)
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Q (Angstrom^-1)
E(m
eV)
• dispersions not as sharp• large smearing due to long tail of energy distribution of incident neutrons
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Energy resolution of Fermi chopper
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ARCS neutrons at sample
(New)
I(E) monitor(McStas)
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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
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Q (Angstrom^-1)
E(m
eV)
• more smearing due to sample size, detector size
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A test case: simulation of an inelastic scattering experiment with fcc Ni
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Simulation result. Monochromatic source. Ideal detector
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Simulation result.broadening due to Fermi chopper included
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QuickTime™ and aTIFF (LZW) decompressor
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Simulation result.broadening due to Fermi chopper, sample, and detector included