1. Farhi – ILL/DS/CS – slide 1 Separating coherent, incoherent and multiple scattering in virtual (and real) experiments Analysing experimental results using McStas 1.9 Optimizing sample environment and instrument setup Emmanuel Farhi, ILL/DS/CS Klaus Lieutenant Peter Willendrup (Risoe) Virginie Hugouvieux (CNRS)

2. Farhi – ILL/DS/CS – slide 2 Introducing McStas http://www.mcstas.org/http://www.mcstas.org/ and http://www.ill.fr/tas/mcstas mailto:neutron-mc@risoe.dkmailto:neutron-mc@risoe.dk (mailing list) McStas is developped by Risoe and ILL within the NMI3/MCNSI network Runs with all systems, and we also provide a LiveCD McStas is a neutron scattering simulation software. An Instrument description, what is it ? uses components from the existing library, or you own components text file compiled into a executable program behaves just as a real instrument Library contains sources, optics, detectors, samples,...

3. Farhi – ILL/DS/CS – slide 3 Recent news about McStas: version 1.9 released Main changes: component validation (choppers,... by Klaus) major manual updates gravitation support fixed parallel processing (MPI) more Warning messages and tips bug tracking system http://bugzilla.mcstas.org new powder sample PowderN new inelastic 'liquid' sample Isotropic_Sqw update of colloidal sample Sans_spheres single crystal diffraction Single_crystal to be used as sample or monochromator Manual

4. Farhi – ILL/DS/CS – slide 4 McStas 1.9 : the PowderN sample Handles single scattering diffraction Geometry is a filled cylinder or a box Future: more geometries, concentricity (sample env.) more input data formats (Lazy, Fullprof,...) Work from P. Willendrup DMC@PSI: exp vs sim Powder rings

5. Farhi – ILL/DS/CS – slide 5 McStas 1.9 : the Isotropic_Sqw sample Isotropic_Sqw sample component for McStas 1.9 coherent/incoherent scattering, elastic and inelastic scattering, absorption (with secondary extinction) multiple scattering may use Fullprof, Crystallographica, etc files for powders (but not as accurate as PowderN ) requires an Sqw table as input for inelastic scattering geometry is a box, cylinder, sphere – filled or hollow can be arranged in concentric geometry (sample env.) and more... l-Ge coh (log)

6. Farhi – ILL/DS/CS – slide 6 McStas 1.9 : the Isotropic_Sqw theory Holy Book (Squires) structure factor g(r → 0) gives |S| = f(  ) g()g()g q (q|  )probability functions See Egelstaff or H. Fischer, Rev. Prog. Phys. 69 (2005) 233

7. Farhi – ILL/DS/CS – slide 7 D2D2 last scattering ? D1D1 scattering point scattering direction k s next component monochromator sample  choose the scattering point D i along trajectory : with  and 2 nd extinction choose if coherent or incoherent scatt. and weight absorption choose  randomly in the DOS choose either  or -  (detailed balance) choose q randomly in a probability table P(q |  ) check selection rules and get |k f | : solve scattering direction : Q = k i – k f weight event with compute the distance d towards sample exit scatter again depending on a random choice on else: exit the sample Scattering events loop :  QQ McStas 1.9 : the Isotropic_Sqw : propagation From V. Hugouvieux S(Q,  g()g() g q (q|  ) See Hugouvieux et al. Physica B, 350 (2004) 151

8. Farhi – ILL/DS/CS – slide 8 Liquid rubidium, 350 K ● Agrees with experiments from Copley, Phys. Rev. A, 9 (1974) 1656. ● 2-body potential by Kahl, Phys. Rev. A 46 (1992) 3255. ● S(q,  ) computed by V. Hugouvieux, PhD (2004) ● classical MD with 520 atoms. Virtual experiment : the sample model for l-Rb Dynamic structure factor : Q < 1Å Dynamic structure factor : Q < 1Åphonon Interatomic potential r (Å)  (r) (K) From V. Hugouvieux  abs = 0.38 b  coh = 6.32 b  inc = 0.5 b l-Rb coh (log) l-Rb inc (log)

9. Farhi – ILL/DS/CS – slide 9 We start with a virtual experiment containing just: a source a sample l-Rb (cylinder  =2 cm) surrounding radial collimator monitors and beam stop Single sample : the instrument and monitors Computation time: about 10 3 events/s at detector. theta [deg] TOF S(q,  ) limited to q=0.2-3 Å -1 =3.4 A-1

10. Farhi – ILL/DS/CS – slide 10 Whole set of monitors attached to parts of the instrument Single sample : example of raw results AllCohIncMulti Inc Multi Coh Cryo In Cryo Out Cell ToF q w S(q,w) input

11. Farhi – ILL/DS/CS – slide 11 Single sample : coherent/incoherent signal single coh/all single inc/all Total signal (log) inc: About 10 % except at low q,  coh: About 85-95 % Multiple scatt. ph.

12. Farhi – ILL/DS/CS – slide 12 Single sample : multiple scattering Extract multiple scattering events: About 5-10 %. Up to 50 % at low q. Lower than 3 % on max(S(q)) 100 % where S(q,  ) is restricted

13. Farhi – ILL/DS/CS – slide 13 Single sample : sample geometry Determine best sample geometry ? Plate orientation Sample dimension Conclusion: Multiple scattering is usually over-estimated Better get bigger sample !

14. Farhi – ILL/DS/CS – slide 14 Sample Environment : the model We now add a cryo-furnace environment around sample 4 Al shields of 0.5-1 mm Sample Container Nb Computation time: about 600 events/s at detector (cryo-furnace takes 30 %). Cryo-furnace

15. Farhi – ILL/DS/CS – slide 15 Sample Environment : background estimate Scattering from sample environment

16. Farhi – ILL/DS/CS – slide 16 Sample Environment : sample scattering Cryo-furnace cumulated scattering Container coh and inc no cryo =

17. Farhi – ILL/DS/CS – slide 17 Sample Environment : error in standard analysis Usually perform experiment then empty cell measurement and substract

18. Farhi – ILL/DS/CS – slide 18 Focus@PSI : the model Computation time: 70 events/s at final detector The instrument model is now the ToF instrument Focus at PSI Source curved Guide curved monochromator Fermi chopper furnace sample l-Rb detectors

19. Farhi – ILL/DS/CS – slide 19 Focus@PSI : sample contribution Sample

20. Farhi – ILL/DS/CS – slide 20 Focus@PSI : background estimate Cryostat+container contribution

21. Farhi – ILL/DS/CS – slide 21 Focus@PSI : Effect of instrument Just sample part... Cryostat effect bigger with full Focus because beam is larger

22. Farhi – ILL/DS/CS – slide 22 You can do it ! To perform similar studies you need: to describe your instrument setup to describe the sample environment to know the sample S(q,  ) If you start such virtual experiments, send us your S(q,w) !!! currently: l-4He, l-Ge, l-Rb, l-para H2 with coh+inc parts S(q,  ) may come from: Molecular Dynamics/ab initio simulations previous experiments, with accurate data analysis