A large water shield at 4850 Homestake

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 bb decay.
• Conventional Pb + Poly shield for DM, bb 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
…. 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 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
L. DeViveiros, R. Gaitskell, Brown
• 2 m ~ 105 expected from 20 cm Pb shield.
• 4 m affords extraordinarily low background.
• Final rate will depend on water purity.
High energy neutrons from muons
• Muons in rock, outside
of veto
(Mei and Hime, astroph/0512125)
– Low rate, but important
• Cross section on
hydrogen dropping
• Conversion in Pb
multiplies them. N ~ 20.
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
10-46 cm2
• 4m water sufficient for
10-46 cm2 (~1 ton)
sensitivity at 4850 mwe
• Can we live at shallow
4850 mwe depth
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
– Screening, other experiments may drive this.
From a proposal for Homestake
(R. Gaitskell, Brown /
• 10 module system
1.75 m
• 4 m shielding
16 m
– Could be reduced to 3
• Cavern: 16m x 10m x
15 m.
10 m
• Davis cavern +3m
• Detector grid hangs from
ceiling, supports modules.
• Detector modules either
water-tight, or sealed in
14 m
• Feedthrough plate handles
sealing of each module.
• Muon veto: Based on
CTF3, ~ 20 PMTs should
give 99.9% or better
Sealing against Rn
• Cavern lined same as SNO cavern. 107
• Deck structure sealed to walls with flexible
• Each detector module contains all conduit
– Use same mechanism for sealing against water.
• N2 pure on blanket.
• 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
• Tremendous opportunity for collaborative effort for
liquid-noble gas DM detectors
Noble Liquid Dark Matter Consortium
Adam 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
Open to further participation.
• 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 largescale experiment based on the noble liquid technology
(or technologies) that prove most sensitive for detecting
WIMP dark matter.”
• PMT radioactivity, g and n, is dominant issue in all
• Idea: unified R&D effort with manufacturer(s)
• Development efforts to date:
– Hamamatsu: XMASS
• 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
• 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.
Rn screening
Monte Carlo
Nuclear recoil calibration techniques

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