A large water shield for dark matter, double beta decay and low background screening. T. Shutt - Case R. Gaitskell - Brown

Shielding for dark matter and  decay. Pb shield for gammas + ancient Pb/Cu inner liner. Polyethylene neutron moderator for DM. –Becoming important for  decay. Local active shielding - e.g., ZEPLIN, WARP 100kg Existing water shields –SNO light water. –Borexino’s CTF: surrounds 2m Ø liquid scintillator –Boulby - UKDM

Multiple User Facility Tom Bowles proposal at first Lead meeting, Modular approach from 100 kg - ton scale for modular dark matter experiments. –Dual-phase detectors have some natural size limit (as opposed to XMASS/CLEAN/DEAP). Modular approach will accommodate other experiments –Experiments may not have the same internal backgrounds. Spacing, arrangement. Good platform for advanced screening –Ge counters –Beta cage, alpha screening. –Moderate-sized liquid scintillator.

Shielding goals Shield ambient gammas –Pb is fine to point, but then 210 Pb is problem –Cu is very good, but have cosmogenics –Liquid shields will have lowest ultimate backgrounds Shield neutrons from radioactivity Muon veto (especially at shallow depths) High energy neutrons from muons in rock –Very difficult to stop

Gamma shielding 2 m ~ 10 5 expected from 20 cm Pb shield. 4 m affords extraordinarily low background. Final rate will depend on water purity.

Neutrons from Rock Neutrons from radioactivity: < 10 MeV.

High energy neutrons from muons Muons in rock, outside of veto –Low rate, but important Cross section on hydrogen dropping Conversion in Pb multiplies them. N ~ 20. (Mei and Hime, astroph/ )

High energy neutrons in water Elastic scattering primarily on O. –But forward scattered Overcome by simple thickness 2m water better than feasible Pb/Poly shield 4m water sufficient for 1 ton Xe exp ( cm 2 ) sensitivity at 4850 mwe Can we live at shallow depth? 4850 mwe depth

Water purity Assumption: bulk contaminants will be very low with moderate cost commercial purification –18 MΩ deionization Radon is main question. –From initial water: let decay. (5.82 half-life). –From Ra. Main concern of SNO Borexino’s CTF: ~ 1 mBq/m 3 with commerical system. –Make-up water. Membrane stripping/degassing. Stable water –SNO, Kamland: should get stagnant water, Rn decays. –Chiller with recirculation to enforce. Dark matter with discrimination may not drive high requirement. –Screening, other experiments may drive this.

Muon veto Based on CTF3, ~ 20 PMTs should give 99.9% or better efficiency.

16 m 10 m 1.75 m LXe (XENON) proposal - Homestake 10 module system 4 m shielding –Could be reduced to 3 Cavern: 16m x 10m x 15 m. Davis cavern +3m depth.

14 m Mechanics Detector grid hangs from ceiling, supports modules. Detector modules either water-tight, or sealed in plastic Feedthrough plate handles sealing of each module.

Sealing against Rn Cavern lined same as SNO cavern reduction. Deck structure sealed to walls with flexible membrane. Each detector module contains all conduit seals. –Use same mechanism for sealing against water. N2 pure on blanket.

Water shield for dark matter Dark matter detection is possibly entering a 1st order phase transition. –Hundred-kg LXe, LAr, bubble chamber modules are not expensive. –Scale-up to ton scale may happen very rapidly. WIMP hypothesis will be tested at the ~ton scale.

How big? Current limits Ellis, Olive, Santoso,Spanos, hep-ph/ Calculations in minimal supersymmetry framework (MSSM). ~ 10 ton experiment

Water shield for dark matter Dark matter detection is possibly entering a 1st order phase transition. –Hundred-kg LXe, LAr, bubble chamber modules are not expensive. –Scale-up to ton scale may happen very rapidly. WIMP hypothesis will be tested at the ~ton scale. Water shield requires large space –Not obvious at SNOlab, Gran Sasso –With large space, don’t need lowest depth Is there the flexibility for DUSEL at Henderson to enter into this in a timely way?

Summary Unique opportunity for new national lab Strong physics potential for both dark matter and double beta decay experiments Powerful platform for low-background screening Opportunity for collaborative effort