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…)