A%20large%20water%20shield%20for%20dark%20matter,%20double%20beta%20decay%20and%20low%20background%20screening. - PowerPoint PPT Presentation

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A%20large%20water%20shield%20for%20dark%20matter,%20double%20beta%20decay%20and%20low%20background%20screening.

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Shielding for dark matter and bb decay. Pb shield for gammas ancient Pb/Cu inner liner. ... Tom Bowles proposal at first Lead meeting, 2001. ... – PowerPoint PPT presentation

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Title: A%20large%20water%20shield%20for%20dark%20matter,%20double%20beta%20decay%20and%20low%20background%20screening.


1
A large water shield for dark matter, double beta
decay and low background screening.
  • T. Shutt - Case
  • R. Gaitskell - Brown

2
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.
  • Borexinos CTF surrounds 2m Ø liquid
    scintillator
  • Boulby - UKDM

3
(No Transcript)
4
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.

5
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

6
Gamma shielding
  • 2 m 105 expected from 20 cm Pb shield.
  • 4 m affords extraordinarily low background.
  • Final rate will depend on water purity.

7
Neutrons from Rock
  • Neutrons from radioactivity lt 10 MeV.

8
High 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.

9
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
  • Can we live at shallow depth?

4850 mwe depth
10
Water 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.

11
Muon veto
  • Based on CTF3, 20 PMTs should give 99.9 or
    better efficiency.

12
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.

1.75 m
16 m
10 m
13
Mechanics
  • 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
14
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.

15
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.

16
How big?
  • Calculations in minimal supersymmetry framework
    (MSSM).

Current limits
10 ton experiment
Ellis, Olive, Santoso,Spanos, hep-ph/ 030875
17
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, dont need lowest depth
  • Is there the flexibility for DUSEL at Henderson
    to enter into this in a timely way?

18
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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