The Background Model and Radio-assays for the LZ Experiment

Dr. Chamkaur Ghag

University College London

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IOP APP HEPP Annual Conference

10-12 April 2017

Dark Matter

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LUX-ZEPLIN (LZ)

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Xe TPC

Water tank

Gd loaded LS

LXe Heat Exchanger

Neutron Calib. Tube + external source tubes

494 TPC-PMTs (253 top, 241 bottom) + 131 skin-PMTs

120 Outer Detector PMTs

Instrumentation conduits

HV Feedthrough

Backgrounds

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Xenon contaminants

Environment, cosmogenic, surface contamination

Physics backgrounds

Detector components

Radio-assays

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Mass Spectrometry

Gamma-ray Spectroscopy

Radon Emanation

Optical Microscopy

Alpha detectors

Gamma Spectroscopy

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• Boulby Underground Germanium Suite (‘BUGS’)
• Sensitivity to ~100 ppt (g/g) ²³⁸U and ²³²Th chains
• Broad Energy Germainium (BEGe) for low energies
• Re-furbished P-type detector for high energies
• ‘Well-type’ BEGe/P-type hybrid for small samples

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• 150 assays with BUGS since UK project start (4/15)
• Custom simulation and analysis software
• Sample management (handling, tracking, ...)
• LZ also uses multiple high sensitivity systems at SURF

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• 3 new ‘S-ULB’ detectors to be added to BUGS
• Novel N-type BEGe, 160% P-type, 100% P-type
• Construction material assays with manufacturer (Canberra) using BUGS and UCL ICP-MS system
• Expect x10 improvement in sensitivity

Mass Spectrometry

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• Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
• Direct assays of ²³⁸U, ²³⁵U and ²³²Th
• Small samples digested for rapid analysis
• Careful and clean sample preparation is key
• Mitigates gamma assay throughput limitations

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• UCL system set up for LZ in dedicated ISO Class 6 facility
• Agilent 7900 mainframe with HF upgrades
• 10 ppt (g/g) D.L. in 1-2 days for ²³⁸U ²³²Th (50 ppt routine)
• Validated with IAEA sources
• ²³³U & ²³⁰Th spikes (~100% recovery)
• Closed vessel microwave digestion system
• Ashing microwave for pre-digestions
• Acid distillation to UpA grade

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• Systems also available to LZ at Alabama and BHUC in the U.S., and CUPS in S. Korea

Radon Emanation

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• Assaying to ~0.1 mBq of radon emanation from construction materials
• 4 room temperature systems in use, incl. UCL’s (SuperNEMO) at MSSL
• All wetted materials in LZ to be screened (~⅓ completed already)
• Estimate <8.5 mBq from materials incl. gas phase scrubbing (remainder from dust)
• Low temperature suppression for many components (difficult to quantify; no data!)

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• New UK radon system under development (UCL/RAL)
• Low temperature capability with large emanation chamber
• Key R&D for low-background material characterisation (future Dark Matter, 0𝜈𝛽𝛽, …)
• Similar systems in development by LZ colleagues in the U.S. (SDSM&T, Penn State)

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Plate-out and Fall-out

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• Radon emanation from dust estimated at 10 mBq in LZ
• Maximum of 1g of dust in LZ
• Assume 25% ²²⁶Ra activity of emanated as ²²²Rn (conservative!)
• Must maintain surface cleanliness to <500 ng/cm²

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• Microscopy at SDSM&T achieves 10-100 ng/cm²
• Aiming for 5 ng/cm²
• X-ray flourescence, tape-lifts, micro-balances

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• Rn daughters ‘plate-out’ onto material surfaces
• Produces neutrons (PTFE walls), and electron recoils (²¹⁰Bi)
• Maximum ²¹⁰Pb activity of 0.5 mBq/m² in LZ
• XIA sensitivity <0.5 mBq/m² (Brown University)
• AlphaBACH ~0.06 mBq/m² (SDSM&T, projected)

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• Controls on component construction and detector assembly

Summary

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