EsotericBackgroundCorrec.onforSANS

Measurements

ByJohnBarker8/13/2008

TalkOverviewBackgroundsignal:Anysignalcollectedbydetectorthatisnotproduced

BySmall‐anglescaJering(SAS)from‘desired’samplestructure.

1)BackgroundSourcesReview

•Internaltosample

•Externaltosample

2)AnnieBrulet’sPaperReview:largescaJeringangle(θ)correc.ons

•SampleTransmission

•Cell/WindowTransmission

•DetectorEfficiency

•DetectorGondolaEffect

3)SASTestswithVanadiumSingleCrystal

SourcesofBackgroundInternaltoSample

1)  LiquidSamples

a)  Quasi‐elas.cincoherentbackground[?nearly?Isotropic/flatscaJering,stronginhydrogenousmaterials.]

b)  Inelas.cIncoherentorCoherentBackground[Nottrulyisotropic,butmayappeartobeatsmallθ …]

c)  Elas.cCoherentWide‐AngleScaJering

[MostimportantforD2Oandamorphoussilica/quartz,

•DiffusescaJeringpeakproducedbynearestatomcorrela.ons.

•CompressibilitycreatesI(0)>0whichmayappearnearly

isotropic/flatatsmallangles.]

d)EnhancedIncoherentBgdfromD/Hisotopicmixtures

e)Mul.pleScaJeringDistor.ons

[whereTscat<0.9,Shape+amplitudealteredsamplegeometry]

Simula.onofMul;pleSca<eringFromIsotropicScaJeringDisks

ScaJeringispushedawayfrom90˚

ScaJeringisthusenhancedat0˚.

SourcesofBackgroundInternaltoSample

1)  CrystallineSamplesa)  Quasi‐elas.cincoherentbackground[?nearly?Isotropic/flatscaJering,stronginhydrogenousmaterials.]b)  Inelas.cIncoherentorCoherentBackground[Nottrulyisotropic,butmayappeartobeatsmallθ …]c)  DoubleBraggScaJering

•Polycrystal:IDBS~q‐m2<m<3Taglauer,E.(1968)•SingleCrystal:canproducestreaksorspots.

[eliminatebyusingλ > λcutoff,reducebyimprovingcrystallineperfec.on.d)GrainBoundaries,twinsordisloca.ons

•Veryweaksignalfromlowerdensitydisloca.oncore.•Strongsignalfromchemicalsegrega.on•Strongsignalinferromagnetsviademagne.za.on.

[Purematerialsannealedtoincreasegrainsizeorlowerdisloca.ondensity.]e)SurfaceScratches:I~q‐m2<m<3Roth,M.(1977).[Veryweakfortypicalpolishedsurface…]f)  Ferromagne.cDomainWalls‐VeryStrongmul.pleScaJering!!![Removewallsinsatura.ngmagne.cfield…]

SourcesofBackgroundExternaltoSample

1)  WindowsandSampleCells

•IncludesAllinternalsourcetypes

•SASfrominternalstructure(…precipitates…)

[Caremustbetakeninevalua;ngpossibleshadowingfromshielding.

…posi;oninmul;plesamplechanger…]

2)Airsca<ering

•Appearsnearlyisotropicatsmallangles.

•Severelyaffectedbyshadowingfromshielding.

•Largegeometricsolidanglegainproducedneardetector…

[VacuumP<0.1Torr,alsosensi;vetochangesinP]

3)  Collima;onSca<ering(Parasi;cHalo)

a)Apertureedgesca<ering(SAS+Refrac;on)I~q‐3

[UseGdfoilandpolishedge]

b)ApertureDiffrac;onI~q‐3

SourcesofBackgroundExternaltoSample

4)  FastNeutronBackground•weaksource(<0.2s‐1)frominstrumentguideextremelystable

•Externalsourcesmaychange!{NG‐6Physic’sshu<er…~10s‐1}

5)  ExternalThermalneutrons

[ProperCdshieldingofdetectorchambereliminatessource.]

6)  2DDetectorDomeSca<ering:I~q‐2

[0.5%ofneutronssca<er~isotropicallyviaphonons]

7)  Gamma‐Rays[Currentdetectorshaveverylowsensi;vity…]

8)  Reactor‐offBackground:(Stable~1s‐1)a)CosmicRays

b)Ac;vityinaluminumofdetector

c)Detectorelectronics“false”counts

AnnieBrulet’sPaperReview:largescaJeringangle(θ)correc.onsAnnieBruletetal,J.Appl.Cryst.(2007).40,165‐177.

Importantlargeanglecorrec;onsinBrulet’spaper:

1) Flatdetectorsolidanglecorrec.on~cos3(θ) [C.Glinka,1980’s]

2) Sampletransmissioncorrec.on[SteveHenderson,1990’s]

3) DetectorEfficiencyCorrec.on[Lindner,2000Kline,2008]

4) Cell+Windowtransmissioncorrec.on

[Brulet,2007…notpresentlyincorporatedatNIST….

5) “GondolaDetectorCorrec.on[Deformedcounterwindowcausesnonuniformityof3Hegasdepth]

•Produces–dependentdetectorsen.vity

•Sensi.vityvariesradiallyfromdetectorcenter

CurrentOrdela2660Ndetectorsseemtobeokay….constantdepth

€

εD(λ,θ ) = 1− exp −µ(λ)tcos(θ )

DetectorEfficiencyCorrec.on:

€

T (θ ) = T 1− T a(θ )

−a(θ )ln(T )

a(θ ) =1

cos(θ )−1

Angle‐dependentTransmissionCorrec.onBrulet’seq.9

Cell+Windowtransmissioncorrec.on(Brulet)

Brulet’sDataCorrec;ons:

•Detectorefficiency

•Sample+CellTransmission

VanadiumSingleCrystalasaDetectorSensi.vityStandard

VanadiumversusHydrogenousMaterial(Plexiglasorwater)Pro:Verysmallamountofinelas.cscaJeringPro:op.callythinsamplelimitsmul.plescaJering.Con:Smallandexpensive

Sample:99.99%pure,3.5mmthick,10mmdiameter(atλ=6Å,24%isotropicincoherentscaJering,76%absorp.on)

Vanadiumdatacollec.ononNG3usingSDD=1.3mInabsoluteunitsandcorrectedforbackground.(vacuuminsamplechamber…)