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Measurements of electron emission reduction from grid electrodes in the R&D test platform for the LZ experiment Rachel Mannino University of Wisconsin - Madison CPAD 2019 On Behalf of the LZ Experiment LZ detector Search for WIMP dark

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SLIDE 1

Measurements of electron emission reduction from grid electrodes in the R&D test platform for the LZ experiment

Rachel Mannino University of Wisconsin - Madison CPAD 2019 On Behalf of the LZ Experiment

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SLIDE 2
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

LZ detector

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Instrumented Xe skin Gadolinium loaded liquid scintillator Water tank

Cathode HV feedthrough

TPC with 7 tonne LXe active volume

DD (NR) calibration conduit

  • Search for WIMP dark

matter candidate

  • 4850-ft underground at

Sanford Lab

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SLIDE 3
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

LZ TPC

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Electron extraction region

{

Drift region

TPC = Time Projection Chamber

LZ projected sensitivity

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SLIDE 4
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

LZ grids

Electric fields established by 4 woven SS mesh grids

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⌀ 1.456 m

Cathode Bottom grid

Wire pitch (mm) Wire diameter (μm) Trans- parency (%) Voltage (kV)

Anode 2.5 100 92 5.75 Gate 5 75 97

  • 5.75

Cathode

5 100 96

  • 50 /
  • 100

Bottom 5 75 97

  • 1.5

Anode Gate

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SLIDE 5
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Grid production: weave

  • Commercially available wire mesh does not come in the LZ grid

diameter

  • Challenges: Maintain wire spacing & tension
  • Video of weaving process

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Loom Installing warp wire through the heddles Weights set wire tensions

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SLIDE 6
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Grid production: glue

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Apply epoxy along engraved ring Install top ring to secure wire mesh Glue robot deposits epoxy on x-y stage

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SLIDE 7
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Electron emission

  • Electron emission from wires is problematic:
  • Impacts low energy dark matter search → Accidental

coincidence can mimic low energy events & limit S2-only search

  • Affects detector operability → high DAQ rate from electron

trains can increase dead time

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LZ projected sensitivity Events with an “S2” from electron emission can mimic NR event (red).

LZ simulated data set for a background-only 1000~live day run and a 5.6 tonne fiducial mass. ER and NR bands are indicated in blue and red, respectively (solid: mean; dashed: 10% and 90%). The 1σ and 2σ contours for the low-energy 8B and hep NR backgrounds, and a 40 GeV/c2 WIMP are shown as shaded regions.

S2 S2 Field emission Drifted electrons

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SLIDE 8
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Electron emission mitigation

  • 1. Dust removal: Construct grids in a

cleanroom & remove dust

  • 2. Passivation: Changes chemical

composition of the oxide layer & increases the Cr:Fe ratio.

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Tomás, A., et al. "Study and mitigation of spurious electron emission from cathodic wires in noble liquid time projection chambers." Astroparticle Physics 103 (2018): 49-61.

Collaborators at ICL measured reduction of electron emission from passivation Acid-cleaned Electropolished Untreated

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SLIDE 9
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

System test platform at SLAC

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Large (1.5-m ⌀ grids) Small (14-cm ⌀ grids) TPC Gas only

  • M. Kapust

2 PMTs 32-PMT array

LZ

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SLIDE 10
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Small 2-PMT gas-only detector

  • Scaled-down extraction region
  • Quick turnaround
  • Xenon gas, 3.3 bar

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Anode Gate PMT PMT 14 cm

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SLIDE 11
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Gas test nitric passivation

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35% Nitric acid at room temperature for 30 min

  • W. Ji PhD, Stanford, 2019.

LZ requirement 0.019 kV/cm LZ requirement: ΔVA-G = 6.8 kV

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SLIDE 12
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Gas test citric passivation

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LZ requirement: 0.019 kV/cm

  • W. Ji PhD, Stanford, 2019.

3-5% Citric acid at 175℉ for 2 hr

LZ requirement: ΔVA-G = 6.8 kV

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SLIDE 13
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Small 32-PMT detector

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Designed as TPC to test cryogenics, circulation, HV Test extraction region in gas-only mode by removing field cage PMTs Gate grid installed

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SLIDE 14
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

32-PMT gas detector: citric passivation results

Plots at ΔVA-G =16 kV

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✴ Before citric passivation, 2 hot spots ✴ After 130℉ and 140℉ citric passivation, same hot spots remain ✴ After passivation and 48 hr oxidation, hot spots gone

LZ equivalent ΔVA-G

Systematic errors not shown

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SLIDE 15
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

System test: Large gas-only detector

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LZ-scale grids Reflective MgF2 coated Al detector surfaces 32-PMT array

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SLIDE 16
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Emission from dust

Results from passivation of a prototype grid are being analyzed.

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SLIDE 17
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

LZ passivation & grid cleaning

  • Gate grid passivated in 3-5%

citric acid.

  • Cathodic and in the electron

extraction region

  • Each grid was spray washed

with DI water and UV-inspected for dust before assembly.

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SLIDE 18
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

HV in future experiments

  • HV issues affect many noble liquid detectors.
  • Fermilab’s 2013 HV in Noble Liquids workshop
  • Future larger-scale detectors affected by HV issues.
  • Scaling up can increase likelihood of dust or surface defects on

electrodes.

  • Techniques to mitigate electron emission may become

increasingly important.

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SLIDE 19
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Conclusions

  • SLAC R&D System Test studied passivation as a treatment for

electron emission reduction.

  • Promising results observed in many prototype grids
  • Paper in preparation now.

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SLIDE 20
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Thank you

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  • 1. Center for Underground Physics (South Korea) 14. Brandeis University (US)
  • 27. University of Alabama (US)
  • 2. LIP Coimbra (Portugal)
  • 15. Brookhaven National Lab (US)
  • 28. University of California, Berkeley (US)
  • 3. MEPhI (Russia)
  • 16. Brown University (US)
  • 29. University of California, Davis (US)
  • 4. Imperial College London (UK)
  • 17. Fermi National Accelerator Lab (US)
  • 30. University of California, Santa Barbara (US)
  • 5. Royal Holloway University of London (UK)
  • 18. Lawrence Berkeley National Lab (US)
  • 31. University of Maryland (US)
  • 6. STFC Rutherford Appleton Lab (UK)
  • 19. Lawrence Livermore National Lab (US)
  • 32. University of Massachusetts (US)
  • 7. University College London (UK)
  • 20. Northwestern University (US)
  • 33. University of Michigan (US)
  • 8. University of Bristol (UK)
  • 21. Pennsylvania State University (US)
  • 34. University of Rochester (US)
  • 9. University of Edinburgh (UK)
  • 22. SLAC National Accelerator Lab (US)
  • 35. University of South Dakota (US)
  • 10. University of Liverpool (UK)
  • 23. South Dakota School of Mines and Technology (US)
  • 36. University of Wisconsin — Madison (US)
  • 11. University of Oxford (UK)
  • 24. South Dakota Science and Technology Authority (US) 37. Yale University (US)
  • 12. University of Sheffield (UK)
  • 25. Texas A&M University (US)
  • 13. Black Hill State University (US)
  • 26. University at Albany (US)
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SLIDE 21
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Extra slides

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SLIDE 22
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

32-PMT TPC nitric & citric passivation

Plots at dV = 12.5 kV

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✴

Nitric, dirty, before spark

✴

Ntric, dirty, after spark

✴

Citric, clean

LZ equivalent field at 11.5 kV

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SLIDE 23
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Drift and reverse field region

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Grid

Voltage (kV)

Surface field (kV/ cm)

Cathode

  • 50
  • 100
  • 30.1
  • 61.4

Bottom

  • 1.5
  • 33.8
  • 68.6
  • 0.30 kV/cm
  • 0.65 kV/cm

3.5 kV/cm 7.1 kV/cm

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SLIDE 24
  • R. Mannino |

University of Wisconsin — Madison | CPAD 2019

Electron extraction region

  • Liquid-Anode gap = 8 mm
  • Gate-Liquid gap = 5 mm

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Grid Voltage (kV) Surface field (kV/cm) Anode 5.75 46.2 Gate

  • 5.75
  • 51.8
  • 48.4

z (cm) Electric field (kV/cm) r (cm)

Anode Gate

  • 10.2 kV/cm
  • 5.2 kV/cm

1.44 kV/cm

Cathode @ -50 kV Cathode @ -100 kV