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 SampleDetector

Instrument setup 1: all ideal Neutron Source SampleDetector 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

Q (Angstrom^-1) E(meV) 1st Brillouine Zone: optical branch is partially missing Higher Brillouine Zone: sharp dispersions

Instrument setup 2: ARCS source Neutron Source SampleDetector Simulated neutrons at sample position of ARCS instrument 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

ARCS neutrons at sample Moderator ( McStas ) Guides, Choppers ( McStas ) Neutron recorder ( new )

Q (Angstrom^-1) E(meV) dispersions not as sharp large smearing due to long tail of energy distribution of incident neutrons

Energy resolution of Fermi chopper

ARCS neutrons at sample ( New ) I(E) monitor ( McStas )

Instrument setup 3: ARCS source and detector Neutron Source SampleDetector 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(meV) more smearing due to sample size, detector size

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

Simulation result. Monochromatic source. Ideal detector

Simulation result.broadening due to Fermi chopper included

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