Title: A%20large%20water%20shield%20for%20dark%20matter,%20double%20beta%20decay%20and%20low%20background%20screening.
1A large water shield for dark matter, double beta
decay and low background screening.
- T. Shutt - Case
- R. Gaitskell - Brown
2Shielding 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.
- Borexinos CTF surrounds 2m Ø liquid
scintillator - Boulby - UKDM
3(No Transcript)
4Multiple 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.
5Shielding 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
6Gamma shielding
- 2 m 105 expected from 20 cm Pb shield.
- 4 m affords extraordinarily low background.
- Final rate will depend on water purity.
7Neutrons from Rock
- Neutrons from radioactivity lt 10 MeV.
8High energy neutrons from muons
(Mei and Hime, astroph/0512125)
- Muons in rock, outside of veto
- Low rate, but important
- Cross section on hydrogen dropping
- Conversion in Pb multiplies them. N 20.
9High 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 - Can we live at shallow depth?
4850 mwe depth
10Water purity
- Assumption bulk contaminants will be very low
with moderate cost commercial purification - 18 M? deionization
- Radon is main question.
- From initial water let decay. (5.82 half-life).
- From Ra.
- Main concern of SNO
- Borexinos 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.
11Muon veto
- Based on CTF3, 20 PMTs should give 99.9 or
better efficiency.
12LXe (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.
1.75 m
16 m
10 m
13Mechanics
- Detector grid hangs from ceiling, supports
modules. - Detector modules either water-tight, or sealed in
plastic - Feedthrough plate handles sealing of each module.
14 m
14Sealing 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.
15Water 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.
16How big?
- Calculations in minimal supersymmetry framework
(MSSM).
Current limits
10 ton experiment
Ellis, Olive, Santoso,Spanos, hep-ph/ 030875
17Water 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, dont need lowest depth
- Is there the flexibility for DUSEL at Henderson
to enter into this in a timely way?
18Summary
- 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