Past Experience of reactor neutrino experiments

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Past Experience of reactor neutrino experiments

• Yifang Wang
• Institute of High Energy Physics, Beijing
• Nov. 28, 2003

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Contents

• Reactor neutrino sources
• Reactor neutrino detection
• Past experiments
• Summary

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Daya Bay
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Reactor Source of neutrinos
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How Neutrinos are produced in reactors ?

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Systematic Error Power
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Reactor thermal power
Known to lt1
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Fission rate in the Reactor
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Prediction of reactor neutrino spectrum

• Three ways to obtain reactor neutrino spectrum
• Direct measurement
• First principle calculation
• Sum up neutrino spectra from 235U, 239Pu, 241Pu
  and 238U
• 235U, 239Pu, 241Pu from their measured b
  spectra
• 238U(10) from calculation (10)
• They all agree well within 3

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Total error on neutrino spectrum
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Reactor neutrino detection
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Observed neutrino spectrum
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Background - Correlated
Background - Uncorrelated environmental

radioactivity
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Precautions for a reactor n experiment

• Cosmic-ray induced correlated background
• Enough overburden and shielding
• Active shielding, small enough and well known
  ineff.
• Environmental radiation(uncorrelated background)
• Clean scintillator
• PMT with Low radioactivity glass
• Clean surrounding materials
• Rn free environment
• Enough shielding
• Gd-loaded scintillator, good for bk. But aging
• Calibration
• Many sources at different positions
• Birks law, (Cerenkov) light transport/re-emission
  ,

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CHOOZ
5t 0.1 Gd-loaded scintillators Shielding 300
MWE 2 m scintillator 0.14m Fe 1km
baseline Signal 30/day Eff. 70 BK
corr. 1/day uncorr. 0.5/day
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Attenuation Length
l vs time
Acceleration of l aging
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Neutron energy spectrum
Gd capture
Proton capture
Edge effect
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Energy cut
Position cut
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Systematics
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Closer look -- Detection efficiency
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Experience gained

• Not stable Gd-loaded scintillator (l 5-2m)
• PMT directly in contact with scintillator ? too
  high uncorr. Background ? too high Eth(1.32 MeV)
• Good shielding ? low background
• Homogeneous detector ? Gd peak at 8 MeV
• 2m scintillator shielding gives a neutron
  reduction of 0.8106.

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Bad performance of reactor is a good news for
neutrino physics
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R1.01? 2.8
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Palo Verde
12t 0.1 Gd-loaded scintillators Shielding 32
MWE /1m water 0.9 km baseline Signal
20/day Eff. 10 BK corr.
15/day uncorr. 7/day
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Very stable Gd-loaded liquid scintillator
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Two trigger thresholds
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Two method used in Palo Verde

• Power method
• Neutrino signal follows the variation of reactor
  power

• Prompt (e) and delayed (n) are asymmetric
• But background (g-g, n-n) are symmetric
• N1NggNnnNnpNn
• N2NggNnn(1-e1)Nnp(1-e2)Nn

Y.F. Wang et al., PRD 62(2000)013012
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Systematics

• Error on n selection cuts obtained from
  multi-variable analysis

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Experience gained

• Good Gd-loaded scintillator(l 11m)
• Not enough shielding ? too high corr./uncorr.
  Background
• Segmentation makes Gd capture peak lt6MeV ? too
  high uncorr. Background
• Rn may enter the detector, problem ?
• Veto eff. is not high enough(97.5)
• Swap method to measure/cancel backgrounds ? key
  to success
• 1m water shielding gives a neutron reduction of
  106 (lower energy, complicated event pattern).

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Summary

• Reactor neutrino experiment is not trivial
• Chooz and Palo
• Verde give a limit
• of sin22q13lt0.1