Document 7344939

Report
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 bb decay.
• Pb shield for gammas + ancient Pb/Cu inner
liner.
• Polyethylene neutron moderator for DM.
– Becoming important for bb 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
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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 goals
• Shield ambient gammas
– Pb is fine to point, but then 210Pb 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 ~ 105 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
(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
• 4m water sufficient for
1 ton Xe exp (10-46
cm2) sensitivity at
4850 mwe
4850 mwe depth
• Can we live at shallow
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/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.
• 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.
LXe (XENON)
proposal - Homestake
1.75 m
• 10 module system
• 4 m shielding
– Could be reduced to
3
16 m
• Cavern: 16m x 10m
x 15 m.
• Davis cavern +3m
depth.
10 m
Mechanics
14 m
• 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. 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.
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?
• Calculations in minimal supersymmetry framework (MSSM).
Current limits
~ 10 ton experiment
Ellis, Olive, Santoso,Spanos, hep-ph/ 030875
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

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