Monte Carlo Simulations of

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Monte Carlo Simulations of Neutron Scattering
Instrumentation
Dr Michael James mja_at_ansto.gov.au
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PLATYPUS Time-of-Flight Neutron Reflectometer
2D Helium-3 Detector
Sample Stages Collimation Slits
Slit PackagesBeam Attenuator
Disc Chopper
Neutron Guide
Detector Vacuum Chamber
Collimation Optics
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PLATYPUS June 2006
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Monte Carlo Optimisation of Neutron Optics

• Neutron source
• Beam transport (neutron guides)
• Beam shaping slits
• Collimation optics
• Sample configurations
• Disc chopper (neutron pulses)
• Detector design

From source to detector 20 optical
elements (Each element may have 5-6 adjustable
parameters)
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The Package McStas
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What is McStas?

• Versatile software for Monte Carlo neutron
  ray-tracing simulations
• Based on a meta-language
• Translated into simulations codes in ANSI-C
• Library of components for reactor and pulsed
  neutron sources
• Developed maintained by Risø and ILL.

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The McStas System

• Model the physical properties of neutrons
  interacting with optical components
• Model the instrument using components
• Accurate calculation of instrument properties
  using Monte Carlo techniques (ray tracing)
• Graphical presentation of results

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• Advantages
• Ability to place detectors and monitors at
  any point in the instrument
• Ability to graphically follow neutron
  trajectories through the instrument
• Supported by a variety of platforms PC, Linux
• Constraints
• Flux limitations for simulations 109 n ? 10
  hours
• (Platypus flux 109 n/s Flight time 30 ms)
• Timely integration of optical components

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2-D Position Sensitive Detector
Real He-3 neutron detectors are based on capture
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Vertical and Horizontal Divergence Map
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Wavelength Spectrum Monitor
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Flux Limitations 107 neutron trajectories
SIit dimensions 10 mmRun Time 2
minutes(Limited monitors)
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Flux Limitations 108 neutron trajectories
SIit dimensions 10 mmRun Time 21
minutes(Limited monitors)
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Flux Limitations 108 neutron trajectories
SIit dimensions 1 mmRun Time 21
minutes(Limited monitors)
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Summary

• Simulation of basic instrument 6 months
• Each simulation run gt 10 hrs
• Instrument design is a highly interactive process
• Desire to run simulations in minutes, not hours