US- -Japan Seminar on Japan Seminar on US Double- -Beta Decay Beta Decay Double 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 Pacific Northwest National Laboratory U.S. Department of Energy
Outline Motivation Electroformed Copper ICP-MS Copper Assay Lead Surface Preparation Summary 2 2005/9/26
Materials are Critical Depth is only part of the equation Must also have � Pure materials � Environmental gamma shielding � Environmental neutron shielding � Residual muon shielding Muon-induced secondary neutrons can dominate under good conditions 3 2005/9/26
See talk by T. Hossbach 4 2005/9/26
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. 5 2005/9/26
Ultra- -Low Low- -Background Background Ultra Ultra-Low-Background Electroformed Copper Electroformed Copper Electroformed Copper Strength equal to OFHC Technology has small physical footprint for production Can be easily formed into thin, low-mass parts Purity established with IGEX* experience, development continues Electroformed cups shown have wall *(International Germanium EXperiment) thickness of only 250 µ m! 6 2005/9/26
Low- -Background Electroformed Copper Background Electroformed Copper Low Low-Background Electroformed Copper Key Elements Key Elements Key Elements Semiconductor-grade acids Glassware-free handling Copper sulfate purified by recrystallization Baths circulated with continuous microfiltration to remove oxides and precipitates Continuous barium scavenge removes radium Cover gas in plating tanks reduces oxide formation Periodic surface machining during production minimizes dendritic Low-background detector and growth electroformed cryostat during assembly 7 2005/9/26
Electroforming Overview Electroforming Overview Electroforming Overview N 2 cover gas 230 Th tracer 230 230 Th tracer Th tracer study shows study shows study shows >8,000 rejection >8,000 rejection >8,000 rejection F C I O Chiller/ + - L I Heater T L E H 2 SO 4 R BaSO 4 CuSO 4 PUMP Secondary Tank 8 2005/9/26
Plating Bath Process Parameters Plating Bath Process Parameters Plating Bath Process Parameters Constituent Concentration Plating is done onto polished, cleaned, stainless steel CuSO 4 188 g/l mandrels in the shape of the desired parts H 2 SO 4 75 g/l Current density is ~40 mA/cm 2 Plating rate is ~0.05 mm/h HCl 30 mg/l BaSO 4 collects in the micro- filtration stage and acts as radium scavenge Thiourea 3 mg/l CoSO 4 was added as a holdback carrier for the cosmogenic CoSO 4 1 mg/l 56,57,58,60 Co present in the starting copper BaSo 4 ~1 mg/l HCl and Thiourea affect copper crystal nucleation and grain size 9 2005/9/26
Chemistry & Cosmogenics Chemistry & Cosmogenics Chemistry & Cosmogenics Further improvements were made in the chemistry for electroformed Cu production U, Th progeny reduced substantially (100x, 10x) over early work [Bro95] Underground production would totally eliminate cosmogenic 60 Co, other less- important cosmogenics Current chemistry development continues (tracer studies, mass balance, etc.) Both ~1995 to pr e se nt ~1995 to pr e se nt E ar ly data showing c osmoge nic s E le c tr ofor me d c oppe r E le c tr ofor me d c oppe r NIM A292 (1990) 337-342. r adioc he mistr y gains: r adioc he mistr y gains: Cu • H 2 SO 4 Pur ity • H 2 SO 4 Pur ity Ge • R e c rystalize d CuSO 4 • R e c rystalize d CuSO 4 • Bar ium sc ave nge • Bar ium sc ave nge R e sults: R e sults: <25 µ Bq/ kg <25 µ Bq/ kg 226 R a 226 R a 9 µ Bq/ kg 9 µ Bq/ kg 228 T h 228 T h (Br odzinski e t al, Jour nal of R adioanalytic al and (Br odzinski e t al, Jour nal of R adioanalytic al and Nuc le ar Che mistry, 193 (1) 1995 pp. 61-70) Nuc le ar Che mistry, 193 (1) 1995 pp. 61-70) L NGS NOSV High-Pur ity Cu: L NGS NOSV High-Pur ity Cu: <18 µ Bq/ kg <18 µ Bq/ kg 226 R a 226 R a Re-analysis (in <12 µ Bq/ kg <12 µ Bq/ kg 228 T h 228 T h progress) suggests (M. L aube nste in e t al, Applie d R adiation and (M. L aube nste in e t al, Applie d R adiation and greater purity… Isotope s, 61 (2004) 167- 172) Isotope s, 61 (2004) 167- 172) 10 2005/9/26
Electroformed Copper Electroformed Copper Electroformed Copper Surface Cleaning & Passivation Surface Cleaning & Passivation Surface Cleaning & Passivation Goal was to find copper cleaning process to replace destructive nitric acid etch Surface passivation was also desired Experiments inspired by CUORE conversations Tested several oxide removal methods Tested ~30 passivation chemistries H 2 O 2 -based cleaning & citric acid passivation were final result 11 2005/9/26
Examples Examples Examples from from from MEGA MEGA MEGA Detector Detector Detector 12 2005/9/26
Examples Examples Examples from from from MEGA MEGA MEGA Detector Detector Detector 13 2005/9/26
Electroforming R&D is Ongoing Electroforming R&D is Ongoing Electroforming R&D is Ongoing 14 2005/9/26
LANL- -PNNL PNNL LANL LANL-PNNL Underground Cu Experiment Underground Cu Experiment Underground Cu Experiment Equipment underground at WIPP LANL, Majorana team will operate Will demonstrate cosmogenic suppression 15 2005/9/26
ICPMS Copper Purity Assay ICPMS Copper Purity Assay ICPMS Copper Purity Assay
Motivation Motivation Motivation 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 parts Inductively-Coupled Plasma – Mass Spectrometry (ICP-MS) has good potential for reaching radiopurity goals 17 2005/9/26
Basic ICP/MS PLASMA mass spectrometer Quadrupole ion detection sample & aerosols: EM liquid FC gases Daly solids ASAT 18 2005/9/26
PNNL ICP/MS Equipment PNNL ICP/MS Equipment PNNL ICP/MS Equipment 19 2005/9/26
Sample Introduction Methodologies Sample Introduction Methodologies Sample Introduction Methodologies 20 2005/9/26
DETECTION IMPROVEMENT IN ICP/MS DETECTION IMPROVEMENT IN ICP/MS 1E10 4 fg/mL 1E9 45 fg/mL 1E8 450 fg/mL ACPS 1E7 4500 fg/mL 17000 fg/mL 1E6 1E5 1986 1990 1993 1996 1996+USN YEAR 21 2005/9/26
ICP- -MS DETECTION RANGES MS DETECTION RANGES ICP ICP-MS DETECTION RANGES Aqueous Standards Aqueous Standards Aqueous Standards WEIGHT PREFIX 238 U ATOMS/ml 10 -3 (ppt) 2.53x10 18 Milli 10 -6 (ppm) 2.53x10 15 Micro NORMAL ICP-MS 10 -9 (ppb) 2.53x10 12 Nano RANGE 10 -12 (ppt) 2.53x10 9 Pico 10 -15 (ppq) 2.53x10 6 Femto 10 -18 (pp?) Atto 2530 THIS WORK 10 -21 (pp??) Zepto 2.53 10 -24 (pp???) Guaca 0.00253 22 2005/9/26
Direct Atto Atto- -gram/mL gram/mL Detection Detection Direct Direct Atto-gram/mL Detection 1E4 250 ag/mL Np-237 1.8E+05 1E3 25000 MHz/ppm 1.2E+04 Response (cps) 1E2 1.2E+03 1E1 2.0E+02 2.5E+02 1.0E+02 7.2E+01 4.0E+01 4.6E+01 5.6E+01 2.2E+01 1E0 2.9E+01 1.9E+01 2.7E+01 1E-1 1E-2 230 232 234 236 238 240 242 244 amu 23 2005/9/26
Copper Sample Preparation Copper Sample Preparation Copper Sample Preparation Nominal 1g copper sample is placed in 75ml clear Teflon bottle 20ml 7.5M HNO 3 (<0.05pg/ml) is added Tracer ( 229 Th or 230 Th) is added at about 10% of expected 232 Th value Gentle heat is applied until dissolution is complete Copper goes to +2 state 24 2005/9/26
Thorium Separation Thorium Separation Thorium Separation Column is 200-400 mesh anion resin Column is first washed with H 2 O Column is conditioned with 7.5M HNO 3 Sample is loaded (20 ml 7.5M HNO 3 ) Wash copper from column void volume (7.5M HNO 3 ) Elute (strip) thorium with 0.5M chemist explains to me HNO 3 how this works on my whiteboard… � Elute in 3 ml directly to MS 25 2005/9/26
ICP- -MS Instrument MS Instrument ICP-MS Instrument ICP Condition instrument with 0.5M HNO 3 until stable background is achieved Switch in eluent (also 0.5M HNO 3 ) and wait for signal to stabilize Measure (integrate) mass response during eluent ionization � Typically ~6 integration periods of 30 seconds each � Provides 10 seconds on each of three mass peaks (230.0, 230.5, 232.0) for each integration period 26 2005/9/26
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