This research has been funded by the European Unions Horizon 2020 - - PowerPoint PPT Presentation

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This research has been funded by the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No 841261 (DarkSphere)

NEWS-G Searching for low mass Dark Matter

Using an innovative gaseous detector the Spherical Proportional Counter

NEWS-G collaboration Canada, France, Greece, UK, USA

6th collaboration meeting LPSC, Grenoble June 2019

I.Giomataris et al ,JINST,2008, P09007

Anode

• Metallic
• Semiconducting

Supporting tip

• Insulator

Wire

• Metallic core
• Insulating

surface Supporting Rod

• Metallic
• Resistive

coating

The sensor

• Simple design
• Single readout

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• ○

○

Polyethylene 30 cm Lead 15cm Copper 8 cm Sedine 60 cm Ø SPC

• NOSV Copper vessel (Ø 60 cm)
• Equipped with a 6.3 mm Ø sensor
• Chemically cleaned several times for Radon

deposit removal

NEWS-G (2017)

Crest-II (2015) DAMIC CDMS-lite

Gas Mixture: Ne+0.7%CH4 at 3.1 bar (280 g) Exposure: 9.6 kg*days (34.1 live-days x 0.28 kg)

NEWS-G collaboration, Astropart. Phys. 97, 54 (2018), doi: 10.1016/j.astropartphys.2017.10.009

• NEWS-G is preparing to install a new detector at SNOLAB
• H-rich mixtures
• Expected to be sensitive to WIMP masses ~100 MeV
• Detector already operating at LSM for a commissioning run

CH4 Ne+ 6%CH4

Snoglobe at LSM Preliminary NEWS-G Snolab

• NEWS-G is preparing to install a new detector at SNOLAB
• H-rich mixtures
• Expected to be sensitive to WIMP masses ~100 MeV
• Detector already operating at LSM for a commissioning run

Preliminary Snoglobe at LSM

Electric field strength in large volume SPCs

Low E-field region

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• Decoupling the E-field needed for gain vs the E-field to drift charge

Giganon, A. et al, 2017. “A Multiball Read-out for the Spherical Proportional Counter.”, JINST

Giganon, A. et al, 2017. “A Multiball Read-out for the Spherical Proportional Counter.”, JINST

S i n g l e A C H I N O S

Results with the prototypes

He:Ar:CH4(80:11:9) 640 mbar HV1 = 2015 V HV2 = -200 V 2 mm Ø anodes

Giganon, A. et al, 2017. “A Multiball Read-out for the Spherical Proportional Counter.”, JINST

He:Ar:CH4 (56:37:7) 455 mbar HV1 = 1100 V, HV2 = -100 V 2 mm Ø anodes

Measurement of the 5.9 keV 55Fe X-ray line

5.9 keV Escape peak

• Resistive layer materials tested:
• Araldite/Cu, Araldite/Graphite
• Commercial resistive paste
• DLC (Diamond Like Carbon)

3D design Implemented modules using 3D printing

600 mbar He+10% CH4 without contaminant filtering 600 mbar He+10% CH4 with contaminant filtering 9310 ADU 8.9% (σ)

1.49 keV Al Fluorescence 5.9 keV

55Fe

5.9 keV

55Fe

1.49 keV Al Fluorescence

Using Oxisorb

Contaminants: Oxygen, Water, electronegative gases... Purifiers:

• Getter
• Oxysorb

4620 ADU 22.4% (σ)

• SAES MicroTorr Purifier (MC700 902-F) then used
• Improved filtering efficiency in large sphere – attachment problem ‘solved’
• Incorporated into recirculation system with RGA

Charge Loss and Oxygen Concentration

• ver Time while gas

passes circulated through MicroTorr Purifier

Tunable transmission to control the mean number of electrons Common DAQ for timing analysis between two channels Parallel photo-detector to tag laser events A powerful UV laser capable of extracting 100s

• f electrons
• 213 nm laser used to extract primary electrons from wall of SPC
• Photo detector in parallel tags events and monitors laser power
• Laser intensity can be tuned to extract 1 to 100 photo electrons
• Q. Arnaud et al. (NEWS-G Collaboration), Phys. Rev. D 99, 102003 (2019)

• N photo-electrons are

extracted from the surface of the sphere: Poisson

• Each photo-electron creates

S avalanche pairs: Nth convolution of Polya

• Sum the contributions of all

N photo-electrons

• The overall response is

convolved with a Gaussian to model baseline noise

Laser in pulsed mode fixed to a low intensity

• Q. Arnaud et al. (NEWS-G Collaboration), Phys. Rev. D 99, 102003 (2019)

Fit results θ = 0.09 ±0.02 <G> = 30.26 ± 0.21 ADU χ2/ndf = 0.97

• Ar37 produced by

irradiating Ca power with a high flux of fast neutrons

• Together with laser

calibrations, can find W (mean Ionization energy) with 1% precision for target gas, and set upper limits on F (Fano factor) L-Shell: 270 eV W = 27.6 eV/pair F = 0.26 K-Shell: 2.82 keV W = 27.6 eV/pair* F = 0.19 Fit of 270 eV and 2.82 lines with flat background Detector response modeled:

• Primary ionisation

(COM-Poisson)

• Avalanche (Polya)

*The W-value at 2.82 keV was calculated directly from <G> and fixed for this fit

• Q. Arnaud et al. (NEWS-G Collaboration), Phys. Rev. D 99, 102003 (2019)
• D. Durnford et al, Phys. Rev. D 98, 103013 (2018)

37Ar 2.82 keV corrected

The laser can be used to monitor the detector response during physics runs Long-term fluctuations in gain can be caused by temperature changes, O2 contamination, sensor damage... Laser monitoring data could even be used to correct for long-term fluctuations

37Ar 2.82 keV peak

Laser events

• Q. Arnaud et al. (NEWS-G Collaboration), Phys. Rev. D 99, 102003 (2019)

5.5 MeV 6.0 MeV 7.7 MeV 5.3 MeV

• 4N Aurubis copper (99.99% pure)

○ Spun into two hemispheres

• Copper has no long-lived isotopes
• 63Cu(n,⍺)60Co from fast neutrons – mostly cosmic

muon spallation

• Contaminants : U and Th decay chain traces

○ Measured for NEWS-G ~10 μBq/kg ○

210Pb out of equilibrium - 28.5 mBq/kg

• Using PNNL expertise in

electroforming Cu

• The inner surface of the detector was

electroplated to stop Bremsstrahlung X-rays from 210Pb and 210Bi β-decays in copper

• 0.5 mm pure copper plated on inner

surface at LSM: expected background from 210Pb and 210Bi under 1 keV reduced from 4.58 dru<1 keV to 1.96 dru The setup during electroplating at LSM

• Good surface quality achieved
• Hemispheres electron-beam welded together
• Detector already operating at LSM
• Copper was deposited at a rate of ~36 μm/day

○ Result is promising for possibly a whole detector electroformed underground

~0.036 mm/day ~1.3 cm/year Plated surface Hemisphere after plating

Preliminary

• Known that these filters emanate radon:

○ Observed ~600 mBq alpha particles, up from ~1 mBq without filter ○ ~40 days before 222Rn acceptably low, but depositing daughters (e.g. 210Pb)

• Alpha particles not directly a great problem

Daughter nuclei are a problem

• Tests with an Carboxen 100 active carbon filter to

capture Rn ○ Side effect, removal of some CH4 from mixtures ○ Looking into optimum temperature of filter

• Recirculation system in development

Comparison between a CH4 and Ne/CH4 run

He/CH4 (90/10) 600 mbar HV1=1820 V HV2=+225 V Ball Φ2 mm No OXISORB He/CH4 (90/10) 600 mbar HV1=1840 V HV2=+300 V Ball Φ2 mm OXISORB

5.9 keV (55Fe) 1.49 keV (27Al)

Conversion e-

Improvements

Vacuum conditions

1.E-4 mbar→1.E-5 mbar→1.E-6 mbar Leak Rate ≈ 1.4E-6 mbar*L/s

⇒ Not a dramatic effect

Gas quality Contaminants ~ppm ↓ Oxisorb ~100 ppb ⇒ Big improvement Increased drift velocity less attachment

Rise time vs Amplitude 2D Histo Amplitude 1D Histo Attachment

e

• e
• Wall event

Volume event

Gas quality (2)

Gas filtering

2 measurements of 210Po in our copper. Implied:

• 210Po ~ 80 mBq/kg
• 210Pb ~ 60 mBq/kg

Simulation shows 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.html See: XMASS collaboration arXiv:1707.06413 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

Capacities for a 1 m3 detector in different geometries

rA

≈ 3500 pF ≈ 115 pF

≈ 1.5 pF

Parallel Plate Cylindrical

Spherical

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Advantage of spherical geometry - A

C~Electronic noise ↑Electronic noise ↑Threshold

• Copper benefits from ‘electrowinning’

– has high reduction potential

• Reduces on surface more readily than

most contaminants - 238U, 232Th ect

• Refined ultra-pure copper surface builds

up

Preparation Procedure

• Cleaned with detergent
• Sanded
• Cleaned again
• Surface chemically etched with 3% H202,

1% H2SO4 in deionised (DI) water ○ Shown to be effective etchant while less aggressive than some alternatives

• Electrolyte of H2SO4, H2O and CuSO4

○ Pump and filter to move electrolyte and remove particulates

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Electroplating Copper

• Some ions reduce more readily than
• thers – reduction potentials
• Copper benefits from ‘electrowinning’ –

high reduction potential +0.34 V

• Reduction potential of:

○ Uranium: -1.80 V ○ Thorium: -1.90 V ○ Lead: -0.44 V ○ All lower than copper → refined during electroplating

• Using PNNL expertise, already

electroformed Cu for Majorana Experiment

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Electropolishing

• Electropolishing serves several purposes

○ Removes layer from hemisphere without chemical or mechanical attack ○ Preferentially removes high spots from surface ○ Increases concentration of CuSO4 in electrolyte

• First (second) hemisphere 21.2±0.1 μm

(28.2±0.1 μm) polished

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Cu Movement in Electropolishing

+

−

Electropolishing

• Electropolishing serves several purposes

○ Removes layer from hemisphere without chemical or mechanical attack ○ Preferentially removes high spots from surface ○ Increases concentration of CuSO4 in electrolyte

• First (second) hemisphere 21.2±0.1 μm

(28.2±0.1 μm) polished

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Cu Movement in Electropolishing

+

−

Electroplating

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□ Electroplating performed with pulsed

current

□ 0.27 to 0.3 V amplitude in forward and

reverse directions of plating – Established value for copper plating

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

502.1±0.2 μm (539.5±0.2 μm)

□ Passivation with Citric Acid

~0.036 mm/day ~1.3 cm/year

Cu Movement in Electroplating

+

−

Future NEWS-G Detectors

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□ Next generation of detector will require

even lower background material

□ Two options:

– 6N copper sphere – Electroformed intact sphere

□ 6N more currently favoured; less pure

than electroformed but commercially available and 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

□ Currently planned to be ⌀ 60 cm, installed

in NEWS-G@LSM shielding

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

Electroplating Copper

• Some ions reduce more readily than
• thers – reduction potentials
• Copper benefits from ‘electrowinning’ –

high reduction potential +0.34 V

• Reduction potential of:

○ Uranium: -1.80 V ○ Thorium: -1.90 V ○ Lead: -0.44 V ○ All lower than copper → refined during electroplating

• Using PNNL expertise, already

electroformed Cu for Majorana Experiment

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North South

Resistive central electrode Spherical metallic anodes Insulated wires Supporting metallic rod