A large water shield for dark matter, double beta decay and low

background screening.T. Shutt - Case

R. Gaitskell - Brown

Water shields for dark matter or decay.

• Conventional Pb + Poly shield for DM, decay expensive, inflexible at large size.– Ancient Pb (or Cu) to avoid 210Pb - $$.– Thick polyethylene - $$.– Higher intrinsic gamma background than water shield.

• Existing water shields– SNO light water.– Borexino’s CTF: surrounds 2m Ø liquid scintillator– Boulby - UKDM

• Liquid noble detectors: At a 1st order phase transition.– Hundred-kg LXe, LAr, bubble chamber modules not expensive.– Rapid evolution and scale-up to ton scale could happen very

rapidly...

…. if shielding weren’t prohibitive.

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Multiple User Facility• Tom Bowles proposal at first Lead meeting, 2001.• 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 summary

Gammas:• 2 m ~ 105 expected from 20 cm Pb shield.• 4 m affords extraordinarily low background.• Final rate will depend on water purity.

L. DeViveiros, R. Gaitskell, Brown

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/0512125)

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 10-46 cm2 (~1 ton) sensitivity at 4850 mwe

• Can we live at shallow depth?

4850 mwe depth

10-46 cm2

WIMP rates

L. DeViveiros, R. Gaitskell, Brown

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. (3.82 half-life).– From Ra.

• Main concern of SNO• Borexino’s CTF: ~ 1 mBq/m3 with commerical system.

– Make-up water. Membrane stripping/degassing.

• Stable water– SNO, Kamland: should get stagnant water -> Rn decays.– Chiller with recirculation to enforce gradient.

• Dark matter with discrimination may not drive high requirement.– Screening, other experiments may drive this.

16 m

10 m

1.75 m

From a proposal for Homestake DUSEL

(R. Gaitskell, Brown / XENON)

• 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.

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

Sealing against Rn• Cavern lined same as SNO cavern. 107

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.

Where?

• Possible “Early Implementation” at DUSEL.– Strong endorsement by both Homestake and Henderson

DUSEL sites.

• Implementation soon would provide very powerful boost to promising next-generation, very large scale detectors.

• Tremendous opportunity for collaborative effort for liquid-noble gas DM detectors

• SNOLAB?

Noble Liquid Dark Matter ConsortiumAdam Bernstein54

David Cline54 Rick Gaitskell54

Yongsheng Gao54

Andrew Hime10,18

Ed Kearns10,18

Dan McKinsey10,18,54

Tom Shutt54

Hanguo Wang54

James White10,54

Frank Wolfs54

So far:

US based effort from CLEAN+DEAP, XENON, ZEPLIN.

Open to further participation.

Consortium• Follows informal discussions over last ~1.5 years.

– Previous DMSAG meeting catalyzed letter to committee.

• First step: letter to DMSAG (6/26/06):– “We believe it would be beneficial to operate a US

consortium, which could exploit common infrastructure and specific shared R&D projects. A prime example is a large multi-module water shield that could be used by a number of experiments (and also for ultra-sensitive low background screening).”

– “In addition to benefiting the next phase of technical development, this consortium will also help lay the groundwork for what we anticipate to be a very large-scale experiment based on the noble liquid technology (or technologies) that prove most sensitive for detecting WIMP dark matter.”

PMTs • PMT radioactivity, and n, is dominant issue in all

experiments.• Idea: unified R&D effort with manufacturer(s)• Development efforts to date:

– Hamamatsu: XMASS

– ETL: ZEPLIN, DAMA, WARP

• Larger effort may gain critical mass• Overlap of goals:

– Radioactivity: common goal

– Size: large, apart from top dual-phase array

– Temperature: need extra metal coating for Ar/Ne.

Liquid-phase purification• Liquid phase purification needed at large mass

– Heat load from gas phase: XENON10 rate -> 0.5 kW @ 100 kg

• Common purifier technology:– “Spark-gap” or cold-getter– Ne can also use charcoal.

• Key technical challenge: Clean fluid pumping

Internal neutron backgrounds• From PMTs, will become and issue below WIMP

sensitivity of ~10-45 cm2 (nominal 100 kg active mass).

• Common approach to mitigating makes sense, especially in context of water shield.– Outer liquid (or solid?) scintillator

– Gd doping in water?

– LAr/Ne shield?

• Measurements to calibration of Monte Carlos?

Other possible joint R&D activities

• Waveshifter.– Essential for Ar + Ne, may be good for Xe.

• Cryogenics

• DAQ

• Rn screening

• Monte Carlo

• Nuclear recoil calibration techniques