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Polarimetric Neutron Spin Echo

Polarimetric Neutron Spin Echo

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Polarimetric Neutron Spin Echo

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  1. Polarimetric Neutron Spin Echo • P M Bentley & C Pappas (HMI, Berlin) • E Lelièvre-Berna (ILL, Grenoble)

  2. Overview • Brief intro: `Normal’ Neutron Spin Echo • The extension to polarisation analysis and NSE • Wide Angle NSE: SPAN (HMI) and WASP (IN11/ILL) • Developing Polarimetric NSE using Artificial Intelligence • Current Status of Project

  3. Neutron Spin Echo • Traditional high-resolution instrumentation design sacrifices incident flux (monochromatic / sharp pulse) • In NSE, resolution is independent of incident monochromaticity • Measure S(Q, t) directly from the final polarisation • Can measure very slow dynamics and low-energy excitations • Note: neutron precession has `depolarised’ the beam by the time it reaches the sample

  4. Polarimetry & NSE • If polarisation analysis on the scattering is required, one needs more polarisers • Adding extra polariserscosts 50% signal • Encodes neutron velocity into intensity: “INTENSITY-MODULATED SPIN ECHO” • Can perform spin echo on depolarising samples F. Mezei et al./Physica B 297 (2001) 9

  5. Polarimetry & NSE • If polarisation analysis on the scattering is required, one needs more polarisers • Adding extra polariserscosts 50% signal • Encodes neutron velocity into intensity: “INTENSITY-MODULATED SPIN ECHO” • Can perform spin echo on depolarising samples F. Mezei et al./Physica B 297 (2001) 9

  6. Polarimetry & NSE • If polarisation analysis on the scattering is required, one needs more polarisers • Adding extra polariserscosts 50% signal • Encodes neutron velocity into intensity: “INTENSITY-MODULATED SPIN ECHO” • Can perform spin echo on depolarising samples Magnons in Fe measured with spin echo! F. Mezei et al./Physica B 297 (2001) 9

  7. Full (3D) Polarimetry • Extends the polarisation analysis NSE technique • Will measure quasi-elastic processes in the off-diagonal elements of the neutron scattering tensor. • eg. Chirality => Symmetry breaking processes and fundamental physics of phase transitions • Use CRYOPAD & Compact Polarisers

  8. Wide-Angle Spin Echo • Magnetic field is radially symmetric • Field integral is independent of scattering angle • This is the `next-generation spin echo instrument’ (WASP at ILL is based on this design, gain 30x count rate over existing IN11!!!) Side View Above View

  9. Re-Design of Magnetic Field CRYOPAD Walls • CRYOPAD can tolerateonlysmallfields (<8G) • High NSE resolution requireslargefields • What is the best possible resolution (largest field integral) that does not saturate the shielding?

  10. A good solution is a Genetic Algorithm • Life evolves to meet demands of the environment • Genetic algorithm allows the instrument design to evolve • Overall, GA is a very efficient search of parameter space • Order of magnitude slower than gradient search • Avoids local minima -> end result near global optimum • Very easy to use: no partial derivatives of figure of merit function required http://www2.lucidcafe.com/lucidcafe/library/96feb/darwin.html http://www.rit.edu/~rhrsbi/GalapagosPages/DarwinFinch.html

  11. Darwin’s Finches • Note that the beaks are all shaped differently, depending on the kinds of foods available on each island.

  12. A good solution is a Genetic Algorithm • Life evolves to meet demands of the environment • Genetic algorithm allows the instrument design to evolve • Overall, GA is a very efficient search of parameter space • Order of magnitude slower than gradient search • Avoids local minima -> end result near global optimum • Very easy to use: no partial derivatives of figure of merit function required • We model the instruments as a virtual creatures, and impose a virtual environment upon them http://www2.lucidcafe.com/lucidcafe/library/96feb/darwin.html http://www.rit.edu/~rhrsbi/GalapagosPages/DarwinFinch.html

  13. One GA Iteration • Start with a figure of merit function, the “fitness”: • aB and aC are weighting factors. • Parameter values in binary are `chromosomes’ • Rank instruments in order of fitness, and chose random parents weighted towards the top of the list • Child genes are a mix of parent genes, also with low-probability mutations • First generation is completely random • Future generations tend to be superior via `survival of the fittest’

  14. Note in margin: Artificial Intelligence layer in design / configuration of instruments is extremely useful =>GA now being developed to configure triple axis instrument FLEX (HMI) -Enter desired energy resolution and obtain maximum possible flux (c.f. desired field limit on CRYOPAD and obtain maximum possible field integral) =>GA also being used to design WASP, the next-generation spin echo instrument at the ILL. =>AI layer means that a user can safely concentrate on science rather than the technical details Results • Currents initially decrease to prevent saturation of CRYOPAD’s shields • Currents then increase to achieve large field-integrals • An increase of 1 m coil separation by 4 cm is also being implemented. • We should be able to access t = 1.34 ns, which is 1/6 of the usual SPAN resolution.

  15. Polarimetric NSE Project Status 1 • Component tests/simulations checked • Optimum resolution (1/6th normal SPAN) • Evolved coil design is built (adjustable) • New SPAN Sample position is defined

  16. Polarimetric NSE Project Status 2 • Write instrument driver software • CORBA, C / C++ • XCode => Igor Pro XOP • Same software interface for ILL & HMI, and feed back into ILL for WASP • Simulations of instrument performance (Vitess) • Switching to Monte Carlo simulations directly within Mathematica & Radia (more accurate line integral calculations) • Test & develop software • Science tests beginning 2006

  17. Acknowledgements • Catherine Pappas (HMI) • Eddy Lelièvre-Berna (ILL) • Robbie Kishnik (HMI) • Klaus Habicht (HMI) • Thomas Krist (HMI) • Bela Farago (ILL) • Peter Fouquet (ILL) • Judith Peters (HMI) • Serguei Manochine (HMI→DUBNA) • Feri Mezei (HMI)

  18. PNCMI 2006 Berlin!