A screening facility for next generation lowbackground experiments

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A screening facility for next generation
low-background experiments

• Tom Shutt
• Laura Cadonati
• Princeton University

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Why?

• Next generation experiments require large
  advances in lower backgrounds.
• Dark Matter. 4 orders of magnitude needed.
• Solar Neutrinos lt 1 MeV.
• Double Beta decay
• Major effort on sophisticated detectors.
• Why not a major effort on backgrounds?

We need much better diagnostics!
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Current state of the art

• Ge detectors, Cu and Pb shielding
• Gran Sasso, Oroville
• Best case U, Th 50 ppt K 50 ppb
• Chemical assays, NAA for U, Th, K
• With major effort,
• U, Th at ppt K ppb
• Specialized concentration has done better
  (Munich)
• Problems
• Reliability
• Tiny sample size (g)
• Insensitivity to dust

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A new facility
Sample 20 cm Ø, 40 cm long Plastic - 13 Kg
plastic Cu - 110 Kg

• Mini-me version of Borexino
• Whole-body counting of sample
• 14C sets threshold near 250 KeV

scintillator
SS Sphere 6-8 m Ø
PMTs 100
Water shield
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Purification of scintillator

• Non-polar solvent
• Extremely low solubility for ionic impurities
• Purification methods developed
• Distillation
• Water extraction
• N2 stripping
• Solid-column adsorption
• Expect at least
• 10-16 g/g U,Th
• 10-14 g/g K.

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Sensitive to

• Photons emerging
• Betas, alphas on surface
• If sample is attacked by scintillator
• Seal in 50 µm film of nylon
• Not sensitive to alphas
• Alphas distinguished by pulse-shape
• Betas and photons distinguished by event shape

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Backgrounds

• Estimates based on Borexino work
• PMTs - dominant
• Nylon vessel ( ppt U, Th 20 ppb K)
• Nylon plumbing ( 50 ppb K)
• Scintillator (Borexino goal 10-16 g/g U,Th)
• Dominant radioactivity is external, so use
  position reconstruction.

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Fiducial Volume
?x 10 cm at 1 MeV
PMT background
Fiducial cut
Signal
Vessel radius
Radius (cm)
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Background

• At 30 days counting, have 3 counts.

• Same as 95 CL with no counts.

• Background free detector

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Photons detected outside sample
Outside sample
Energy Absorbed in sample
scintillator
sample
Inside sample
Detected energy

• This simulation
• Ge sphere
• Ø 20 cm
• M 22 Kg

Threshold
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Detection efficiency vs. Energy

• Reasonably good for E gt 500 KeV

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Consider equilibrium U chain
Rate outside 22 Kg Ge sphere with 10-14 g/g U

• Total Counts/day
• 0.15 total
• 0.10 fiducial

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U, as detected
?E 8 at 1 MeV
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U and background
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Sensitivity

• Total background 0.1 counts/day, E gt 250 keV
• U,Th, K Contamination limits, g/g
• Continuum background of Compton photons
• Surface a, b emitters, E gt 250 keV 0.8
  cnts/day/m2
• (not sensitive to a s if need to seal sample in
  film)

1 day counting 30 days counting U 3 E-13
1 E-14 Th 8 E-13 4 E-14 K
2 E-9 8 E-11
1 day counting 2 E-4 counts/Kg/keV/day 30
days counting 6 E-6 counts/Kg/keV/day
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Photon sensitivity
10-14 g/g U

• At MeV, good sensitivity to all photons.
• Below 500 keV, reduced sensitivity.
• Emergent continuum rate internal continuum rate

Outside sample
10-5
Inside sample
10-5 (cnts/kg/keV/day)
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What this wont do

• Internal beta, alpha contamination
• High resolution measurement of lines
• Modest ability to distinguish contamination,
  especially if several contaminants
• Low energy photons
• Reduced efficiency lt 500 keV
• Zero efficiency lt 250 keV

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Conclusion

• Can be built with existing technology
• 5000 - fold increase in sensitivity
• Old U,Th 50 ppt
• New U, Th 0.01 ppt
• Essential for next generation low E solar n DM,
  bb experiments.
• Unique opportunity with new National Underground
  Lab.