Ultra-pure Copper Electroplating for Background ... · Electroplating 11 o Electroplating performed...

Ultra-pure Copper Electroplating forBackground Suppression in NEWS-GPatrick Knights

University of Birmingham

11/04/2019 1P Knights, DM UK Meeting, King's College London

news-g.org

http://news-g.org/

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

11/04/2019 2P Knights, DMUK 2019 - London

Spherical Proportional Counter

3P Knights, DMUK 2019 - London

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

11/04/2019

!"

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

11/04/2019

Po218

84

Pb214

82

Rn222

86

Bi214

83

Po214

84

Pb210

82

Bi210

83

Po210

84

Pb206

82

3.8 d

3.1 m

27 m

20 m

160 us

22 y

5 d

138 d

Stable

Background Contributions in Copper

o Copper has no long-lived radio-isotopes

o 63Cu(n,⍺)

60Co by fast neutrons from

cosmic muon spallation

o 238U and

232Th decay chain –

naturally found and deposited by

222Rn

– Commercial copper ~1 μBq/kg

– Electroformed copper (PNNL*)

~100 nBq/kg

5P Knights, DMUK 2019 - London 11/04/2019

https://www.nndc.bnl.gov/

E. W. Hoppe et al. J. Radioanal. Nucl. Chem. v.277 p.103

*Pacific Northwest National Laboratory, USA

210Pb in Coppero 2 measurements of 210Po in our

copper. Implied:– 210Po ~ 80 mBq/kg– 210Pb ~ 60 mBq/kg

o Geant4 simulation indicates this gives 4.58 dru < 1 keV– Reduced to 1.96 dru if 500 μm

pure copper plated onto surface


XIA UltraLo-1800

https://www.xia.com/ultralo-theory.htmlSee: XMASS collaboration arXiv:1707.06413

4.58 dru<1 keV1.96 dru<1 keV1.59 dru<1 keV

107 decays of 210Pb and 210Bi in Cu Walls in 2 bar Ne+10% CH4

Best Estimate of 210Po & 210Pb from two measurements of 210Po

Preliminary

Preliminary

https://www.xia.com/ultralo-theory.html

https://arxiv.org/abs/1707.06413

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:

! =#! ∫ %(') )'

*+

7

" – m ass

# ! – m olar m ass$(&)– current as function of tim e(– num ber of e lectrons transferred in reduction reaction

) – Faraday C onstant (= + ,- )

Electrolytic Cell and Electroplating

P Knights, DMUK 2019 - London 11/04/2019

Electroplating Ultra-Pure Coppero 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 Experiment1


1arXiv:1308.1633

http://arxiv.org/abs/arXiv:1308.1633

Preparation Procedure o Cleaned with detergento Sandedo Cleaned againo Surface chemically etched with 3% H202,

1% H2SO4 in deionised (DI) water – Shown to be effective etchant while less

aggressive than some alternativeso Electrolyte of H2SO4, H2O and CuSO4

– Pump and filter to move electrolyte and remove particulates

9P Knights, DMUK 2019 - London 11/04/2019E. W. Hoppe et al. Nucl. Instrum. Methods Phys. Res v.579 p.486

Electropolishing

o Electropolishing serves several purposes– Removes layer from hemisphere

without chemical or mechanical attack– Preferentially removes high spots

from surface– Increases concentration of CuSO4 in

electrolyteo First (second) hemisphere 21.2�0.1 μm

(28.2�0.1 μm) polished


Cu Movement in Electropolishing

+−

Electroplating

11

o Electroplating performed with pulsed current

o 0.27 to 0.3 V amplitude in forward and reverse directions of plating– Established value for ultra-pure copper

electroplatingo Plating continued for ~15 days o In total first (second) hemisphere plated

502.1�0.2 μm (539.5�0.2 μm) o Passivation with Citric Acid


~0.036 mm/day~1.3 cm/year

Cu Movement in Electroplating

+−

Preliminary

Result

12

o Layer of Cu deposited on surface – Awaiting results of analysis of copper and electrolyte to verify purity

o Hemispheres to be welded togethero Final chemical etch to be performed on intact sphereo Detector to be installed in LSM for commissioning this year


Future NEWS-G Detectors

13

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 availableand potentially lower cost– Electroformed copper could be the

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


Electroformed copper (PNNL) ~<100 nBq/kg 238U & 232Th

Summary

14

o Recent indirect measurements of 210Pb 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


29/11/2018 15P Knights, DMUK 2019 - London

Additional Material


XIA Counter UltraLo-1800


From: https://www.xia.com/ultralo-theory.html

XMASS collaboration arXiv:1707.0641311/04/2019

https://www.xia.com/ultralo-theory.html

https://arxiv.org/abs/1707.06413

Simulation of Effect of ElectroplatingSimulation of 107 decays of 210Pb and 210Bi 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


Assuming 40 mBq/kg:Events with rise time lower than 500 µs and energy lower than 1 keV:No plating : 4.58 dru500 µm : 1.96 dru (-58%)1 mm : 1.59 dru (-65%)

11/04/2019

4.58 dru<1 keV1.96 dru<1 keV1.59 dru<1 keV

Electroplating Coppero Some ions reduce more readily than

others – reduction potentialso Voltage between anode and cathode

limits electroplating of some specieso Copper benefits from ‘electrowinning’ –

high reduction potential +0.34 Vo Reduction potential of:

– Uranium: -1.80 V– Thorium: -1.90 V– Lead: -0.44 V – All lower than copper; refined during

electroplating


Why don’t impurities plate too?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

20

&"#$$! = &%! − &&!

'" # $ $! - standard cell potentia l

'%! - standard cathode reduction potentia l

'&! – standard anode reduction potentia l

!"! = −) * &"#$$!


An Example

o Example of solution containing U3+ and Cu2+, with a Cu anode:– U3+ to U

o !!"## = -2.138 V → Requires energy– Cu2+ to Cu

o !!"## = 0 V → In equilibriumo Cu2+ reduction will occur at lower

potentialo Still require a potential difference between

electrodes to overcome energy losses

21

!!"##$ = !%$ − !&$

'! " # #$ - standard cell potentia l

'%$ - standard cathode reduction potentia l

'&$ – standard anode reduction potentia l


Not Quite That Simple…o Assumes standard conditionso Electrode potentials modified outside of

these conditions:

– In theory can have some plating of other species – heavily suppressed

o Other effects dominant in contaminant plating

22

! = !! − $%&' ln

∏" +"#$

∏% +%#&

'– E lectrode potentia l

'! – Standard electrode potentia l(– M olar gas constant

)– Tem perature* " – C hem ical activ ity of species +, " – Stoichiom etric coeffic ient of species +


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 . C handrasekar and M . Pushpavanam , “Pulse and pulse reverse p lating - conceptual,

advantages and applications,” E lectrochim ica A cta, vo l. 53, no . 8, pp . 3313 – 3322, 2008.

Ultra-pure Copper Electroplating for Background ... · Electroplating 11 o Electroplating performed...

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