CREXTarget

History (as I know it)• Isotopicallyenriched(~90.04%),createdbyOakRidge• purchasedbyJLab~700k$.• acEvated(eitherCaorcontaminant,230nC)bybeamexposure• StoredinargonatmospheretopreventsurfacereacEon• OxidaEonevent

- about48houratmosphericexposureduringgamma-specmeasurementbyRadCon

- “white”layer.MaybeCaO,orCaC03ifhumiditypresent.Unknownthickness(guessedtobe“thin”)

ConcernsforCREX• oxygen,carboncontaminaEon(isoscaler0+…notworsethan40Ca).• knowledgeonisotopicraEo• OthercontaminaEon(N,ArabsorbEon,nitride,hydroxide?)• Reportedtobetoothin(~0.4g/cm2,1g/cm2proposed)

Not Too Thin (?)

Actual:~20mmdiameter,2.4g->0.76g/cm2(~4mmthick)

Reported:~25mmdiameter,2g->0.4g/cm2(~2.2mmthick)

Proposed:1g/cm2,5.3mmthick

FromdiscussionwithDaveMeekins,andreportedin2011HallAtargetconfiguraEonnoteondocushare.Thisnotequotes0.779+/-0.005g/cm2(or,0.819+/-0.01g/cm2)

Salesquote:providesbatchnumber,assayvalue(90.04%),mass(2.434g),size(D=20mm,t=5mm)+/-10%ondimensions,andthicknessvariaEons

Discrepanciesinnumbersarenotresolved

Can beam current make up the difference (190uA)? 1/5 the dE/dX, 6x the thermal conductance, and a melting point ~2x farther from LN2 temperature, the calcium target is about 60x easier than lead. Calculations say lead runs without diamond at 50uA.

Oxidation rates Moist argon atmosphere Material being added to surface. Timescales of hours 1 mg/ cm2/hr which slows as time goes on

The Low-Temperature Water Vapor Oxidation of Calcium. D. A. Nissen. Oxidation of Metals, Vol. 11, No. 5, 1977

5

Assuming 1 mg/cm2/hr and a 48 hr exposure time, that means ~185 um on

each side, or 15% of the calcium is oxidized

SummaryfromJulieie

FromDr.LommelatGSI:“ifthematerialwaskeptunderhumidatmosphere,calciumhasturnedintocalciumcarbonate”

•ReacEonpathandratesensiEvetohumidity,temperature,andsurface•EnEresamplecanreact-noneutralizedlayer•ReacEonisnotguaranteedtobeuniform(grainboundaries)

• Samplewaskeptforonly50hours,unknownhumidity.• Sampledoesn’tlook“alltheway”white

Surface studies of Calcium

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(slidefromJulieie)

Microfissures

Oxidation can penetrate because of grain boundaries or micro-fissures if not a single crystal Vapor deposited 0.013 cm Ca on 0.013 cm iron substrate (thicker Ca layer for higher oxidation rate studies)

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(slidefromJulieie)

Characterization

EDS(OrEDX)energydispersivex-rayspectroscopy(orotherxrayspectroscopytechniques,likePIXE).ContaminantraEoanddepth(nearsurface)

RutherfordBackscaiering(RBS)withprotonsorHe.ResonancepeaksforN,C,Ocontaminants.SomesurfaceraEo,somedepth.

Massspectrometry:checkisotoperaEo

AugerelectronSpectroscopy.SurfacecharacterizaEon.Canalsospuierholetomeasuredepthprofile

Plan: Calcium studies at ManitobaNatural Calcium

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Calcium came oxidized Need to purify it before performing tests

pressed

Scattered Electron Images Unprepared Calcium

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Scattered Electron Images Cut Calcium

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Scattered Electron Images Cut Calcium

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zoomed

Manitoba

• EDS,Auger,electronmicroscope• TestoxidaEonrates• TestoxidaEonlayercharacterizaEon• Testcleaning,reforming?

Onepossibleoutcome:canweboundthesurfacecontamina<onifitis“shiny”?

• RBS,PIXE• TestsofthesecharacterizaEontechniques?

Ohio

Preliminary Results

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After etching on Tuesday night

Wednesday morning after being stored in vacuum (10-10 Torr)

RBS at Ohio University● Ohio University has a

4.5 MV tandem Pelletron accelerator– alpha beams of up to

~12 MeV can be delivered on target

● A significant portion of the research program involves accelerator-based material characterization, including Rutherford Back Scattering (RBS)

● The Rutherford cross section is proportional to the square of the nuclear charge

● The energy of the scattered alpha depends on the mass of the target nucleus

● RBS can be used to study composition of thin films on thick substrates

RBS Models of contaminated 48Ca● Calculated energy

spectrum for 168 with 2.2 MeV beam on Ca with uniform 1:1 C, N, O, and Ar contamination– Ar and Ca have

similar cross-section but different energies

– C, N, O are barely visible at thsi energy

RBS Models of contaminated 48Ca● At beam energy of

3.035 MeV, there is a resonance for a-O scattering

● A 1:1 ratio of Ca to O should be readily observable

RBS Models of contaminated 48Ca● At beam energy of 3.035

MeV, there is a resonance for a-O scattering

● A 10:1 ratio of Ca to O should still be observable

● Note that this probes a specific thickness of material, as the alphas will fall out of resonance as they penetrate further in the material.

Measuring depth of contaminantsThe thickness of the contamination layer could be determined by increasing the beam energy above the resonance energy.

For insance, going from 3.05 MeV to 3.35 MeV will penetrate a 1 µm layer, but will still be sampling a 10 µm layer

3.35 MeV on 10 µm

3.05 MeV on 1 µm

3.35 MeV on 1 µm

3.05 MeV on 10 µm

No O peak!

O peak