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)

Farhi – ILL/DS/CS – slide 2 Introducing McStas and (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,...

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 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

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 exp vs sim Powder rings

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)

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

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

Farhi – ILL/DS/CS – slide 8 Liquid rubidium, 350 K ● Agrees with experiments from Copley, Phys. Rev. A, 9 (1974) ● 2-body potential by Kahl, Phys. Rev. A 46 (1992) ● 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)

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

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

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 % Multiple scatt. ph.

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

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 !

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

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

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

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

Farhi – ILL/DS/CS – slide 18 : 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

Farhi – ILL/DS/CS – slide 19 : sample contribution Sample

Farhi – ILL/DS/CS – slide 20 : background estimate Cryostat+container contribution

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

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