R.Svoboda, U.C. Davis /LLNL

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Chemical Additives in Water Cerenkov Detectors

• R.Svoboda, U.C. Davis /LLNL

This work was performed under the auspices of
the U.S. Department of Energy by University of
California, Lawrence Livermore National
Laboratory under Contract W-7405-Eng-48.
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Improved neutron detection antineutrino
tagging (Super-K, SONGS) active neutron
shield (LUX) national security (LLNL Portal
Monitoring Project) Improved light collection
antineutrino spectral measurements reduce
required PMT coverage Unwanted additives what
makes good water go bad?
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Pros and Cons

• Viable
• GdCl3 relatively inexpensive
• Small concentrations of Gd in water improve
  neutron capture significantly
• Gd capture signature (8 MeV ?- cascade) easily
  detectable
• Workable?
• What is GdCl3 effect on transparency?
• What are the physical effects of GdCl3 caused by
  extended exposure to SK detector components?
• Optimal?

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Baffled joints
Nitrogen purge and relief valve
Injection and Measurement Optics
Light transmission arm
Alignment mirror
drain
mixing tank and pump
Nitrogen purge
Storage tank
PMT tank
Recirculation pump
Deionizing Filtering Sterilization
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LLNL Test Set-Up
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LLNL Test Set-Up
337 N2 Laser w/ dye attachement
9.54 meters
Not shown collamators, baffles, filters
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Typical Waveform for 337nm
ns
V
reflected
primary
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By putting filters of known transmittance (lt1
uncertainty) into the injected primary beam, the
system is seen to be linear to within 2 over a
40 variation in transmission
The system is also stable to variations in PMT
gain to better than 1
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Stability
pure water fall off in transparency over time
(337 nm)
Stopped recirc
Preliminary
/- 2
0.9 /day
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Test of GdCl3 Addition at 337 nm
Injected mixing tank Water and filtered
Injected mixing tank Water and filtered
Added 0.2 GdCl3
Removed GdCl3
13/day
Preliminary
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Test of GdCl3 Addition at 400 nm
Injected Pure Water _at_13 MOhm
Added 0.2 GdCl3
Injected Pure Water _at_ 8 MOhm
Preliminary
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Test of GdCl3 Addition at 420 nm
Injected Pure Water _at_ 13 MOhm
Added 0.2 GdCl3
Injected Pure Water _at_ 8 MOhm
8.5 /day
Preliminary
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Results

• Pure water in stainless steel slowly looses
  transparency at 337, 400 and 420 nm.
• For 337nm measurements, the water was
  deoxygenated via nitrogen bubbler to 0.9 ppm
  (typical air is 8-9 ppm) as measured by dissolved
  oxygen measurement.
• For 400nm 420nm measurements, initial dissolved
  oxygen was measured at .15 ppm.
• Addition of GdCl3 makes the water transparency
  drop much faster (factor of 15).
• Injection of water from polypro tank shows that
  water stored there suffered no/little degradation
  in transparency.
• Loss of transparency directly from GdCl3 very
  small (consistent with 0 at all three
  wavelengths).

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GdCl3 effect on transparency
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Conclusion GdCl3 is not a suitable additive
for detectors with steel walls. May be OK for
other materials.
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Current WorkWhat makes good water go bad?
Super-Kamiokande water must be continuously and
cleaned else transparency drops slowly similar
behavior seen in IMB (plastic walls) and SNO
(acrylic walls but much slower
degradation) REDUCING THE REQUIREMENT FOR RECIRC
WILLLOWER COST OF MEGATON SCALE DETETOR
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Test with FeCl3

• 10 ppm Fe3 ion makes water look like ice tea.
  Clearly very low levels can affect transparency
• next week we will test 0.1 ppm
• slowly raise concentration to measure molar
  attenuation coefficient
• test Ni, Cr metal ions for similar behavior

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Future

• Change steel pipe for acrylic one
• use polypro tank for materials testing of HDPE
  and other potential plastic liners for LUX and
  future detectors
• Investigate coatings for steel for cryostat
  treatment
• monitor Water SONGS for stability (acrylic sides)

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Backup slides
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Water SONGS
1 cm black Delryn lid
8 ea. 8 PMTs (1 cm spacing in both directions)
Upper tank (5 sides) 3/8 thick UVT acrylic 4
sides external Tyvek wrap Pure water fill to 10 cm

• lower tank (6 sides)
• - 3/8 thick UVT acrylic,
• Gd-water fill
• 5 side external Tyvek wrap

- total external dimensions 100 cm x 50 cm w x
80 cm ht
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Antineutrino Detection
_ ??e p n e

• The antineutrino interacts with a proton
  producing
• A 0-7 MeV positron ( annihilation gammas)
• A neutron which thermalizes, captures and
  creates a delayed 8 MeV gamma cascade
• mean time interval 30 µsec capture time of
  neutron
• Both energy depositions and the time interval
  are measured
• The time since the most recent muon is also
  measured

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Background

• Antineutrinos are not the only particles that
  produce our coincident signal
• Cosmic ray muons produce fast neutrons, which
  scatter off protons and can then be captured on
  Gd
• Important to tag muons entering the detector

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