Pacific Northwest National Laboratory U.S. Department of Energy US US - - Japan Seminar on Japan Seminar on Double Double - - Beta Decay Beta Decay and Neutrino Mass and Neutrino Mass Materials Purity: Ultra-Low-Background Copper, ICP-MS Assay, and Lead Surface Preparation for the Majorana Project Craig Aalseth Craig Aalseth Pacific Northwest National Laboratory Pacific Northwest National Laboratory Richland, WA, USA Richland, WA, USA September 19, 2005 September 19, 2005
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Pacific Northwest National LaboratoryU.S. Department of Energy
USUS--Japan Seminar on Japan Seminar on DoubleDouble--Beta Decay Beta Decay and Neutrino Massand Neutrino Mass
Materials Purity: Ultra-Low-Background Copper,
ICP-MS Assay, and Lead Surface Preparation
for the Majorana Project
Craig AalsethCraig AalsethPacific Northwest National LaboratoryPacific Northwest National Laboratory
Pure materialsEnvironmental gamma shieldingEnvironmental neutron shieldingResidual muon shielding
Muon-induced secondary neutrons can dominate under good conditions
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See talk byT. Hossbach
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Copper Motivation
Commercial high-purity copper is an attractive material for constructing ultra-low-background spectrometers.Thermal, mechanical, electrical, and vacuum properties enable vacuum cryostats, crystal mounts, heat conductors, electrical interconnects, etc.When even higher purity is required, additional electrolytic and chemical purification can be combined with the final fabrication step, resulting in “electroformed” copper parts of extreme purity. This process can be done underground, providing a potential way to eliminate cosmogenic activation products seen in copper with above-ground exposure. Additional purity improvements seem possible with modest additional chemistry.
Strength equal to OFHCTechnology has small physical footprint for productionCan be easily formed into thin, low-mass partsPurity established with IGEX* experience, development continues
*(International Germanium EXperiment)Electroformed cups shown have wall thickness of only 250 µm!
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Low-Background Electroformed CopperLowKey Elements
Low--Background Electroformed CopperBackground Electroformed CopperKey ElementsKey Elements
Semiconductor-grade acidsGlassware-free handlingCopper sulfate purified by recrystallizationBaths circulated with continuous microfiltration to remove oxides and precipitatesContinuous barium scavenge removes radiumCover gas in plating tanks reduces oxide formationPeriodic surface machining during production minimizes dendritic growthLow-background detector and
electroformed cryostat during assembly
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Electroforming OverviewElectroforming OverviewElectroforming OverviewN2 cover gas
H2SO4
CuSO4
+ -F
I
L
T
E
R
BaSO4 PUMP
Secondary Tank
CO
I
L
Chiller/
Heater
230Th tracer study shows
>8,000 rejection
230230Th tracer Th tracer study shows study shows
>8,000 rejection>8,000 rejection
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Plating Bath Process Parameters Plating Bath Process Parameters Plating Bath Process Parameters
3 mg/lThiourea
~1 mg/lBaSo4
1 mg/lCoSO4
30 mg/lHCl
75 g/lH2SO4
188 g/lCuSO4
ConcentrationConstituentPlating is done onto polished, cleaned, stainless steel mandrels in the shape of the desired partsCurrent density is ~40 mA/cm2
Plating rate is ~0.05 mm/hBaSO4 collects in the micro-filtration stage and acts as radium scavengeCoSO4 was added as a holdback carrier for the cosmogenic 56,57,58,60Co present in the starting copperHCl and Thiourea affect copper crystal nucleation and grain size
Further improvements were made in the chemistry for electroformed Cu productionU, Th progeny reduced substantially (100x, 10x) over early work [Bro95]Underground production would totally eliminate cosmogenic 60Co, other less-important cosmogenicsCurrent chemistry development continues (tracer studies, mass balance, etc.)
NIM A292 (1990) 337-342.Early data showing cosmogenics
GeCu
Both
Re-analysis (in progress) suggests
greater purity…
~1995 to presentElectroformed copperradiochemistry gains:
•H2SO4 Purity•Recrystalized CuSO4•Barium scavenge
Results:226Ra <25 µBq/kg228Th 9 µBq/kg(Brodzinski et al, Journal of Radioanalytical and Nuclear Chemistry, 193 (1) 1995 pp. 61-70)
~1995 to presentElectroformed copperradiochemistry gains:
•H2SO4 Purity•Recrystalized CuSO4•Barium scavenge
Results:226Ra <25 µBq/kg228Th 9 µBq/kg(Brodzinski et al, Journal of Radioanalytical and Nuclear Chemistry, 193 (1) 1995 pp. 61-70)
LNGS NOSV High-Purity Cu:226Ra <18 µBq/kg228Th <12 µBq/kg(M. Laubenstein et al, Applied Radiation and Isotopes, 61 (2004) 167-172)
LNGS NOSV High-Purity Cu:226Ra <18 µBq/kg228Th <12 µBq/kg(M. Laubenstein et al, Applied Radiation and Isotopes, 61 (2004) 167-172)
Goal was to find copper cleaning process to replace destructive nitric acid etchSurface passivation was also desiredExperiments inspired by CUORE conversationsTested several oxide removal methodsTested ~30 passivation chemistriesH2O2-based cleaning & citric acid passivation were final result
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Examplesfrom
MEGADetector
ExamplesExamplesfromfrom
MEGAMEGADetectorDetector
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Examplesfrom
MEGADetector
ExamplesExamplesfromfrom
MEGAMEGADetectorDetector
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Electroforming R&D is OngoingElectroforming R&D is OngoingElectroforming R&D is Ongoing
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LANL-PNNL LANLUnderground Cu Experiment
LANL--PNNL PNNL Underground Cu ExperimentUnderground Cu Experiment
Equipment underground at WIPPLANL, Majorana team will operateWill demonstrate cosmogenic suppression
Direct radiometric methods require large sample mass (~10 kg), long count time (~3 months), have reached limit Producing material for next-generation detector (Majorana) will require careful QA of even small partsInductively-Coupled Plasma – Mass Spectrometry (ICP-MS) has good potential for reaching radiopurity goals
Nominal 1g copper sample is placed in 75ml clear Teflon bottle20ml 7.5M HNO3 (<0.05pg/ml) is addedTracer (229Th or 230Th) is added at about 10% of expected 232Th valueGentle heat is applied until dissolution is completeCopper goes to +2 state
Condition instrument with 0.5M HNO3 until stable background is achievedSwitch in eluent (also 0.5M HNO3) and wait for signal to stabilizeMeasure (integrate) mass response during eluent ionization
Typically ~6 integration periods of 30 seconds eachProvides 10 seconds on each of three mass peaks (230.0, 230.5, 232.0) for each integration period
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Data ReductionData ReductionData Reduction
Subtract instrument background from eluent signalThis is from 0.5M HNO3 reagent and is small
Subtract process blank from eluent signalThis is from an eluent blank prepared without copper and is larger
Quote result (pg/g) based on tracerConvert to µBq/kg equivalent 232Th (4x multiplier)
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First Copper ResultFirst Copper ResultFirst Copper Result
Two 1-g samples of MEGA inner-can copper were analyzedSample #1 (0.882 g)
The process blank was 6.0±0.3 pg/gThe sample yielded a value of
7.3±0.7 pg/g (gross) or 1.2±0.8 pg/g (net)
This is a net 232Th activity of 4.9±2.9 µBq/kgSample #1 (0.936 g)
The process blank was 5.7±0.3 pg/gThe sample yielded a value of
7.0±0.6 pg/g (gross) or 1.3±0.7 pg/g (net)
This is a net 232Th activity of 5.2±2.8 µBq/kg
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Cu ICP-MS Next StepsCu ICPCu ICP--MS Next StepsMS Next Steps
SensitivityPreliminary result appears to be first positive indication of Th in CuAnion column cleanup of 7.5M HNO3 plannedSub-boiling distillation to further clean HNO3 if necessaryInstrument background now 6x lower
DocumentationProcess will eventually transition into “service center” activityWill benefit from QA, standardized reporting
PriorityRepeat process blanksRepeat and extend copper measurements (starting stock, other Cu)Test reagent cleanup chemistry
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Pb CleaningPb CleaningPb Cleaning
Ancient lead is desired to avoid 210Pb backgrounds, present in all modern lead
Materials are usually recovered needing extensive surface cleaning
Even new lead may need oxide removal for safety reasons