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 environmentand instrument setup

Emmanuel Farhi, ILL/DS/CSKlaus Lieutenant

Peter Willendrup (Risoe)Virginie Hugouvieux (CNRS)

Farhi – ILL/DS/CS – slide 2

Introducing McStas

http://www.mcstas.org/ and http://www.ill.fr/tas/mcstasmailto:neutron-mc@risoe.dk (mailing list)

McStas is developped by Risoe and ILL within the NMI3/MCNSI networkRuns 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 componentstext file compiled into a executable programbehaves just as a real instrument

Library contains sources, optics, detectors, samples, ...

0

2

0

2

4

-0.5

0

0.5

x/[m]

LightIN6Instr

z/[m]

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

Farhi – ILL/DS/CS – slide 4

McStas 1.9 : the PowderN sample

Handles single scattering diffractionGeometry 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

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 scatteringgeometry 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() gq(q|)probability functions

See Egelstaff or H. Fischer, Rev. Prog. Phys. 69 (2005) 233

Farhi – ILL/DS/CS – slide 7

D2

last scattering ?last scattering ?

D1

scattering pointscattering point

scattering direction scattering direction kkss

next component

monochromator

sample

choose the scattering point Di along trajectory :

with and 2nd 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 |kf | :

solve scattering direction : Q = ki – kf

weight event with

compute the distance d towards sample exit

scatter again depending on a random choice on

else: exit the sample

ℏ=k i2−k f

2

1− e− s d

Scattering events loop :Scattering events loop :

1− e− x

Q

McStas 1.9 : the Isotropic_Sqw : propagation

From V. Hugouvieux

S(Q,

g()

gq(q|)∣S∣

k f

k i

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) 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 : Dynamic structure factor : Q < 1Q < 1ÅÅ

phononphonon

Interatomic potentialInteratomic 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 sourcea sample l-Rb (cylinder =2 cm)surrounding radial collimatormonitors and beam stop

Single sample : the instrument and monitors

Computation time: about 103 events/s at detector.

thet

a [d

eg]

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

All Coh Inc MultiInc

MultiCoh

CryoIn

CryoOut

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 85-95 %

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 orientationSample dimension

Conclusion:Multiple scattering is usually over-estimatedBetter get bigger sample !

Farhi – ILL/DS/CS – slide 14

Sample Environment : the model

We now add a cryo-furnace environment around sample4 Al shields of 0.5-1 mmSample 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

Containercoh and inc

no cryo =

Farhi – ILL/DS/CS – slide 17

Sample Environment : error in standard analysis

Usually perform experimentthen empty cell measurementand substract

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

Sourcecurved Guidecurved monochromatorFermi chopperfurnacesample l-Rbdetectors

Farhi – ILL/DS/CS – slide 19

Focus@PSI : sample contribution

Sample

Farhi – ILL/DS/CS – slide 20

Focus@PSI : background estimate

Cryostat+container contribution

Farhi – ILL/DS/CS – slide 21

Focus@PSI : Effect of instrument

Just sample part...

Cryostat effect bigger with full Focusbecause beam is larger

Farhi – ILL/DS/CS – slide 22

You can do it !

To perform similar studies you need:to describe your instrument setupto describe the sample environmentto 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 simulationsprevious experiments, with accurate data analysis