Ultra-pure Copper Electroplating for Background Suppression in - PowerPoint PPT Presentation

news-g.org Ultra-pure Copper Electroplating for Background Suppression in NEWS-G Patrick Knights University of Birmingham P Knights, DM UK Meeting, King's College London 11/04/2019 1

o NEWS-G@SNO o Copper radiopurity o Indirect Pb-210 measurements of copper used for NEWS-G@SNO o Development of electroplating of detector hemispheres P Knights, DMUK 2019 - London 11/04/2019 2

Spherical Proportional Counter o Grounded metallic shell ( ⌀ ~30-100 cm) ! " o Central sphere at high voltage as anode ( ⌀ ~1-6 mm) o Electrons drift to anode and amplified near surface P Knights, DMUK 2019 - London 11/04/2019 3

NEWS-G@SNO o NEWS-G dark matter search next generation detector – NEWS-G@SNO o 140 cm copper sphere to be installed in SNOLab o 4N Aurubis copper (99.99% pure) – Spun into two hemispheres P Knights, DMUK 2019 - London 11/04/2019 4

Background Contributions in Copper o Copper has no long-lived radio- 222 isotopes Rn 86 3.8 d o 63 Cu(n, ⍺ ) 60 Co by fast neutrons from cosmic muon spallation 218 214 210 Po Po Po o 238 U and 232 Th decay chain – 84 84 84 3.1 m 214 160 us 210 138 d naturally found and deposited by Bi Bi 83 83 222 Rn 214 20 m 210 5 d 206 Pb Pb Pb – Commercial copper ~1 μBq/kg 82 82 82 27 m 22 y Stable – Electroformed copper ( PNNL*) https://www.nndc.bnl.gov/ ~100 nBq/kg *Pacific Northwest National Laboratory, USA E. W. Hoppe et al. J. Radioanal. Nucl. Chem. v.277 p.103 P Knights, DMUK 2019 - London 11/04/2019 5

Best Estimate of 210 Po & 210 Pb from two measurements of 210 Po 210 Pb in Copper Preliminary o 2 measurements of 210 Po in our copper. Implied: – 210 Po ~ 80 mBq/kg – 210 Pb ~ 60 mBq/kg o Geant4 simulation indicates this 10 7 decays of 210 Pb and 210 Bi in Cu Walls in 2 bar Ne+10% CH 4 gives 4.58 dru < 1 keV 4.58 dru<1 keV – Reduced to 1.96 dru if 500 μm 1.96 dru<1 keV 1.59 dru<1 keV pure copper plated onto Preliminary surface XIA UltraLo-1800 See: XMASS collaboration arXiv:1707.06413 https://www.xia.com/ultralo-theory.html P Knights, DMUK 2019 - London 11/04/2019 6

Electrolytic Cell and Electroplating o Reactions governed by oxidation and reduction reactions o Ions reduced at cathode building up material – Current supplied to drive reactions – Mass deposited proportional to current supplied: # ! ∫ %(') )' ! = " – m ass *+ # ! – m olar m ass $(&) – current as function of tim e ( – num ber of electrons transferred in reduction reaction ) – Faraday Constant ( = + , - ) P Knights, DMUK 2019 - London 11/04/2019 7

Electroplating Ultra-Pure Copper o Some ions reduce more readily than others – reduction potentials o Copper benefits from ‘ electrowinning ’ – high reduction potential +0.34 V o Reduction potential of: – Uranium: -1.80 V – Thorium: -1.90 V – Lead: -0.13 V – All lower than copper → refined during electroplating o Using established techniques, similar to Majorana Experiment 1 1 arXiv:1308.1633 P Knights, DMUK 2019 - London 11/04/2019 8

o Cleaned with detergent Preparation Procedure o Sanded o Cleaned again o Surface chemically etched with 3% H 2 0 2 , 1% H 2 SO 4 in deionised (DI) water – Shown to be effective etchant while less aggressive than some alternatives o Electrolyte of H 2 SO 4 , H 2 O and CuSO 4 – Pump and filter to move electrolyte and remove particulates E. W. Hoppe et al. Nucl. Instrum. Methods Phys. Res v.579 p.486 P Knights, DMUK 2019 - London 11/04/2019 9

Electropolishing o Electropolishing serves several purposes – Removes layer from hemisphere without chemical or mechanical attack – Preferentially removes high spots from surface – Increases concentration of CuSO 4 in electrolyte − o First (second) hemisphere 21.2 � 0.1 μm + (28.2 � 0.1 μm) polished Cu Movement in Electropolishing P Knights, DMUK 2019 - London 11/04/2019 10

Electroplating o Electroplating performed with pulsed current o 0.27 to 0.3 V amplitude in forward and Preliminary reverse directions of plating – Established value for ultra-pure copper electroplating o Plating continued for ~ 15 days o In total first (second) hemisphere plated 502.1 � 0.2 μm (539.5 � 0.2 μm) ~0.036 mm/day o Passivation with Citric Acid ~1.3 cm/year + − Cu Movement in Electroplating P Knights, DMUK 2019 - London 11/04/2019 11

Result o Layer of Cu deposited on surface – Awaiting results of analysis of copper and electrolyte to verify purity o Hemispheres to be welded together o Final chemical etch to be performed on intact sphere o Detector to be installed in LSM for commissioning this year P Knights, DMUK 2019 - London 11/04/2019 12

Future NEWS-G Detectors o Next generation of detector will require even lower background material o Two options: – 6N copper sphere – Electroformed intact sphere o 6N more currently favoured; less pure than electroformed but commercially available and potentially lower cost – Electroformed copper could be the Electroformed copper (PNNL) ~<100 nBq/kg 238 U & 232 Th generation after – demonstrated growth ~ 1.3 cm/year – 10 bar, ⌀ 60 cm sphere requires 4 mm walls - ~ 4 months o Currently planned to be ⌀ 60 cm, installed in NEWS-G@LSM shielding P Knights, DMUK 2019 - London 11/04/2019 13

Summary o Recent indirect measurements of 210 Pb in copper show unacceptable contamination – additional measurements with our copper are ongoing o Electroplated ~0.5 mm pure Cu onto internal surface of NEWS-G@SNO detector to reduce experimental backgrounds – Hemispheres now being welded together o Commissioning in LSM later this year P Knights, DMUK 2019 - London 11/04/2019 14

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Additional Material P Knights, DMUK 2019 - London 11/04/2019 16

From: https://www.xia.com/ultralo-theory.html XIA Counter UltraLo-1800 XMASS collaboration arXiv:1707.06413 P Knights, DMUK 2019 - London 11/04/2019 17

Simulation of Effect of Electroplating Simulation of 10 7 decays of 210 Pb and 210 Bi within the 10.5 mm of thickness of the copper sphere with an inner radius of 67.5 cm. Data processed assuming 4000 V applied on a 3 mm radius sensor and a CREMAT pre-amplifier with ! =46 µs. Gas condition: 2 bars of Ne + 10 % CH4 for a mass of 2.03 kg 4.58 dru<1 keV 1.96 dru<1 keV 1.59 dru<1 keV Assuming 40 mBq/kg: Events with rise time lower than 500 µs and energy lower than 1 keV: No plating : 4.58 dru 500 µm : 1.96 dru (-58%) 1 mm : 1.59 dru (-65%) P Knights, DMUK 2019 - London 11/04/2019 18

Electroplating Copper o Some ions reduce more readily than others – reduction potentials o Voltage between anode and cathode limits electroplating of some species o Copper benefits from ‘ electrowinning ’ – high reduction potential +0.34 V o Reduction potential of: – Uranium: -1.80 V – Thorium: -1.90 V – Lead: -0.44 V – All lower than copper; refined during electroplating P Knights, DMUK 2019 - London 11/04/2019 19

Why don’t impurities plate too? ! - standard cell potential ' " # $ $ ! - standard cathode reduction potential ' % ! – standard anode reduction potential ' & o Which reaction proceeds determined by standard cell potential : ! − & & ! ! & "#$$ = & % o Related to Gibbs Free Energy: !" ! = −) * & "#$$ ! – If !" ! < $ then reaction is spontaneous – If !" ! > $ then need extra energy input P Knights, DMUK 2019 - London 11/04/2019 20

An Example $ - standard cell potential ' ! " # # $ - standard cathode reduction potential ' % $ – standard anode reduction potential ' & $ − ! & $ $ ! !"## = ! % o Example of solution containing U 3+ and Cu 2+ , with a Cu anode: – U 3+ to U o ! !"## = -2.138 V → Requires energy – Cu 2+ to Cu o ! !"## = 0 V → In equilibrium o Cu 2+ reduction will occur at lower potential o Still require a potential difference between electrodes to overcome energy losses P Knights, DMUK 2019 - London 11/04/2019 21

' – Electrode potential ! – Standard electrode potential Not Quite That Simple… ' ( – M olar gas constant ) – Tem perature * " – Chem ical activity of species + , " –  Stoichiom etric coefficient of species + o Assumes standard conditions o Electrode potentials modified outside of these conditions: # $ ∏ " + " $% ! = ! ! − &' ln # & ∏ % + % – In theory can have some plating of other species – heavily suppressed o Other effects dominant in contaminant plating P Knights, DMUK 2019 - London 11/04/2019 22

Why Pulse-Reverse Plating? o Waveform for plating with relaxation period and/or reverse bias (effectively polishing) o Benefits compared to DC plating: – High points are preferentially removed in reverse bias section – Greater uniformity of plate – relaxation period allows diffusion of ions – Higher density copper plate M . Chandrasekar and M . Pushpavanam , “Pulse and pulse reverse plating - conceptual, advantages and applications,” Electrochim ica Acta, vol. 53, no. 8, pp. 3313 – 3322, 2008. P Knights, DMUK 2019 - London 11/04/2019 23