Double Chooz: Outer Veto

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Double Chooz Outer Veto

• Sophie Berkman
• Nevis Labs, Columbia University

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Outline

• Neutrino Oscillations
• Double Chooz
• Outer Veto
• Some Studies
• PMT Characterization
• Scintillator Tests
• Efficiency
• Cross-Talk
• Pulse Height vs. Distance

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Neutrino Oscillations

• In the standard model neutrinos are massless
  leptons - cannot mix.
• BUT - neutrinos oscillate so by the current
  interpretation
• Neutrinos have mass
• Lepton family number is not conserved

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What it means that neutrinos oscillate

• In a 2-neutrino simplification
• Mass states ?1, ?2
• Flavor (weak) states ?? , ?e

Probability of oscillation P(?? -gt
?e)sin2(2?)sin2(1.27?m2L/E) Tmixing angle
?m2difference in squares of neutrino masses
Ldistance of oscillation Eenergy of
neutrinos
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Neutrino Mixing with 3 flavors
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Double Chooz

• Measure ?13
• Reactor experiment
• Look at ?e from reactors
• Disappearance experiment - reactors only produce
  ?e
• Two Detectors - identical, cancel uncertainties
  in neutrino flux and cross-section
• Near - unoscillated neutrino flux
• Far - after oscillation

-
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Muon Background

• Double Chooz looks for inverse beta decay
• ?e p n e
• Double coincidence of neutron capture and
  positron signal (within 100?s)
• Cosmic muon background
• Muon interacts to form neutrons
• Neutrons knock protons out of scintillator
• Protons emit light as they move through
  scintillator and neutron captured by gadolinium
• Looks like inverse-beta decay signal

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Double Chooz Detectors
7m
7m

• Target liquid scintillator, doped with
  Gadolinium - n capture
• Gamma catcher measure gammas from n capture
• Buffer holds PMTs, shields detector from PMT
  radiation
• Inner veto reject fast neutron/muon background
• Outer Veto atmospheric muons

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Outer Veto

• Reject atmospheric muon background
• Stacked scintillator strips
• Wavelength shifting fibers
• Light transmitted to PMT and DAQ
• Nevis developing electronics/software
• All tests done in light tight boxes

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PMT Characterization

• Why Characterize?
• Want all pixels to respond in the same way to
  light
• Pulse height of 350 ADC counts
• 350ADC counts 10pe 35 ADC/pe

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Characterization Process

• Take Baseline with laser off
• Turn laser on and allow it to stabilize for 30
  min
• Adjust HV to get an average pulse height for all
  pixels to be 350 ADC counts
• Adjust gain across preamplifiers to get a mean
  pulse height of 350 ADC counts across each
  individual pixel
• Turn off the laser and allow it to stabilize for
  30 minutes
• Take noise data for different DAC thresholds

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Before and After Characterization
Spread18
Spread2.9
Conclusion characterization process narrows the
spread of the pulse height distributions. Use to
determine if bad PMTs.
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Gain Constant Distribution

• Gain Constant measure of gain adjustment
• Gain constant of 16 means adjust by a factor of 1

Conclusion Centered around 16 (ie. Adjustment by
factor of 1)
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Scintillator Setup

• Four stacked strips 1.5m long
• Four sets of trigger counters
• Wavelength Shifting fibers
• Fiber Holder

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Some Standard Modifications

• Spacers to protect the face of the PMT
• Large spacer space of 1.27mm
• Small spacer space of 0.48mm
• No spacer space of 0.000mm
• Optical Grease

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Efficiency Test
Trigger on trigger counters and one strip
Entries326
91
Efficiency

Entries359
Trigger on trigger counters

• Events over 1pe for triggered strip/trigger
  counter
• Repeat with more coincidences

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Efficiency Results

• Repeated for more coincidences
• Large spacer 4.3pe
• Small spacer 5.2pe

Large Spacer
Small Spacer
Conclusion more efficient with more
coincidences, and with smaller spacer.
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Cross Talk

• Optical Cross talk the amount surrounding pixels
  receive light from the illuminated pixel
• pe smaller than expected
• Add pulse heights in surrounding pixels to the
  signal pixel
• Can find maximum pe without crosstalk
• Note different numbers of surrounding pixels for
  different pixels

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PH distribution before and after addition - no
spacer, strip 2
Conclusioncross-talk is on average 10 and pe
increases to 5-8pe in the nearest position
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Pulse Height vs Distance Setup

• Noticed dependence on distance from previous
  studies
• All strips at all positions
• Use optical grease without spacer
• Require 5-fold coincidence
• 1 photoelectron cut on non signal strip/trigger

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Strips at Position 3
PH281.7pe8.049
PH206.4pe5.897
PH 305.3 Pe8.723
PH 221.8 pe6.337
Conclusion Four strips have different pulse
heights because of polishing of fibers or
scintillator
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Strip 4 at four different positions
Mean 246.8 Pe7.051
Mean269.9 Pe7.711
Mean355.1 Pe10.14
Mean308.3 Pe8.809
Conclusion Pulse Height increases as move closer
to the PMT because more light will reach the PMT
from closer positions. (Higher PH than previous
because of Trigger 2)
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Trigger Counters at Position 3
Mean106.4 Pe3.040
Mean 76.88 Pe2.197
Conclusion Trigger counters have lower PH than
strips because light will be lost from muons that
hit them at the edge
Mean305.3 Pe8.723
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Attenuation Length

• Find using PH vs. distance data
• Find by fitting plot of PH vs distance to
  exponential

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Conclusion and Thanks

• Process for characterizing PMTs works well and
  will be possible to implement for all outer veto
  PMTs
• Still generally not as many photoelectrons as
  expected, but we can use optical grease/other
  trigger modes to increase the number

Thanks to everyone I worked with this summer for
teaching me so much about physics and for this
extraordinary opportunity to work on Double Chooz.
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Bibliography/Picture Permissions

• Camilleri, Leslie. Slides.
• Shaevitz, Mike. Reactor Neutrino Experiment and
  the Hunt for the Little Mixing Angle. 30 Nov
  2007.
• Sutton, Christine. Spaceship Neutrino.

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Efficiency Test

• Find the mean of the pulse height distribution in
  strip 1 when both trigger counters have at least
  1pe
• Find the mean pulse height distribution in strip
  1 when both trigger counters and strip 2 have at
  least 1pe.
• Efficiency Second Mean/First mean
• Require more strips to have 1pe
• Look at efficiencies with different requirements
  for events
• Repeat with large and small spacer

Conclusion More efficient with more
requirements. -Large Spacer went from
83-90 -Small Spacer went from 91-96