Using Reactor Neutrinos to Study Neutrino Oscillations

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Using Reactor Neutrinos to Study Neutrino
Oscillations Jonathan Link Columbia
University Heavy Quarks and
Leptons 2004 June 1, 2004
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Doing Physics With Reactors Neutrinos The
original Neutrino discovery experiment, by Reines
and Cowan, used reactor neutrinos
Reines and Cowan at the Savannah River Reactor
The first successful neutrino detector
actually anti-neutrinos. The ?e interacts with
a free proton via inverse ß-decay
Later the neutron captures giving a coincidence
signal. Reines and Cowan used cadmium to capture
the neutrons.
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Nuclear Reactors as a Neutrino Source

• Nuclear reactors are a very intense sources of
  ?e deriving from the b-decay of the neutron-rich
  fission fragments.
• A typical commercial reactor, with 3 GW thermal
  power, produces 61020 ne/s

From Bemporad, Gratta and Vogel
Arbitrary
Observable n Spectrum

• The observable ne spectrum is the product of the
  flux and the cross section.
• The spectrum peaks around 3.6 MeV.
• Visible positron energy implies ? energy

Cross Section
Flux
E? Ee 0.8 MeV ( mn-mpme-1.022)

• Minimum energy for the primary signal is 1.022
  MeV from ee- annihilation at process threshold.
• Two part coincidence signal is crucial for
  background reduction.

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Uses of Reactor Neutrinos

• Measure cross sections or observe new processes
  (e.g. neutral current nuclear coherent
  scattering)
• Search for anomalous neutrino electric dipole
  moment
• Measure the weak mixing angle, sin2?W
• Monitor reactor core (non-proliferation
  application)
• Measure neutrino oscillation parameters (?m2s
  and mixing angles)

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Observations of Neutrino Oscillations
Reactor neutrinos can probe oscillations in all
three observed ?m2 regions.
Oscillations are observed as a deficit of ?e with
respect to expectation.
Short Baseline 10 to 100 meters Bugey, Gosgen
Krasnoyarsk Medium Baseline 1 to 2 km Chooz,
Palo Verde future Long Baseline 100 km
KamLAND
?m23 to 310-2 eV2 ???
?m22.510-3 eV2 ?23 ?13
?m27.510-5 eV2 ?12
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Short Baseline Oscillation Searches
Experiments like Bugey rule out the low ?m2,
large mixing angle region of the LSND signal.
The Bugey Detector
Neutrons capture on Lithium
Reactor Excluded
Bugey looked for evidence of oscillations between
15 45 meters. Gosgen was closer and
Krasnoyarsk farther away.
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Observations of Neutrino Oscillations
Reactor neutrinos can probe oscillations in all
three observed ?m2 regions.
Oscillations are observed as a deficit of ?e with
respect to expectation.
Short Baseline 10 to 100 meters Bugey, Gosgen
Krasnoyarsk Medium Baseline 1 to 2 km Chooz,
Palo Verde future Long Baseline 100 km
KamLAND
?m23 to 310-2 eV2 ???
?m22.510-3 eV2 ?23 ?13
?m27.510-5 eV2 ?12
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Medium Baseline Oscillation Searches
Experiments like Chooz looked for oscillations in
the atmospheric ?m2. At the time they ran the
atmospheric parameters were determined by
Kamiokande, not Super-K (larger ?m2).
1050 m baseline
Chooz Nuclear Reactors, France
Gadolinium loaded liquid scintillating target.
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Medium Baseline Oscillation Searches

• No evidence found for ne oscillation.
• This null result eliminated nm?ne as the primary
  mechanism for the Super-K atmospheric deficit.

• sin22q13lt 0.18 at 90 CL (at Dm22.010-3)
• Future experiments should try to improve on
  these limits by at least an order of magnitude.
• Down to sin22q13 0.01
• In other words, a measurement to better than 1
  is needed!

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Observations of Neutrino Oscillations
Reactor neutrinos can probe oscillations in all
three observed ?m2 regions.
Oscillations are observed as a deficit of ?e with
respect to expectation.
Short Baseline 10 to 100 meters Bugey, Gosgen
Krasnoyarsk Medium Baseline 1 to 2 km Chooz,
Palo Verde future Long Baseline 100 km
KamLAND
?m23 to 310-2 eV2 ???
?m22.510-3 eV2 ?23 ?13
?m27.510-5 eV2 ?12
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Long Baseline Oscillations
The KamLAND experiment uses neutrinos from 69
reactors to measure the solar mixing angle (?12)
at an average baseline of 180 km.
Scatter plot of energies for the prompt and
delayed signals
Neutron Capture on Hydrogen results in a 2.2 MeV
gamma
In 145 days of running they saw 54 events where
86.85.6 events where expected. The fit energy
confirms the oscillation hypothesis.
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Long Baseline Oscillations
KamLAND Results
Eliminates all but the large mixing angle (LMA)
solution. The best fit sin22?12 0.91
The best fit ?m2 6.910-5 eV2
The ?m2 sensitivity comes primarily from the
solar measurements
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Future Experiments to Search for a Non-zero Value
of sin22?13 Subject of a lot of interest because
of it relevance to lepton CP violation and
neutrino mass hierarchy. See Whitepaper
hep-ex/0402041
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Sin22?13 Reactor Experiment Basics
Well understood, isotropic source of electron
anti-neutrinos
Oscillations observed as a deficit of ?e
E? 8 MeV
1.0
Unoscillated flux observed here
Probability ?e
Survival Probability
Distance
1200 to 1800 meters
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Proposed Sites Around the World
Status
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What is the Right Way to Design the
Experiment? Start with the dominate systematic
errors from previous experiments and work
backwards
CHOOZ Systematic Errors, Normalization
Near Detector
Identical Near and Far Detectors
The combination of these two plus a complex
analysis gives you the anti-neutrino flux
Movable Detectors, Source Calibrations, etc.
CHOOZ Background Error BG rate
0.9
Muon Veto and Neutron Shield (MVNS)
Statistics may also be a limiting factor in the
sensitivity.
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Backgrounds

• There are two types of background
• Uncorrelated - Two random events that occur
  close together in space and time and mimic the
  parts of the coincidence.
• This BG rate can be estimated by measuring
  the singles rates, or by switching the order of
  the coincidence events.
• Correlated - One event that mimics both parts of
  the coincidence signal.
• These may be caused fast neutrons (from
  cosmic ms) that strike a proton in the
  scintillator. The recoiling proton mimics the e
  and the neutron captures.
• Or they may be cause by muon produced isotopes
  like 9Li and 8He which sometimes decay to ßn.
• Estimating the correlated rate is much more
  difficult!

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Reducing Background

• Go as deep at you can (300 mwe ? 0.2 BG/ton/day
  at CHOOZ)

• Veto ms and shield neutrons (Big effective
  depth)
• Measure the recoil proton energy and extrapolate
  into the signal region. (Understand the BG that
  gets through and subtract it)

Shielding
6 meters
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Isotope Production by Muons
A ½ second veto after every muon that deposits
more that 2 GeV in the detector should eliminate
70 to 80 of all correlated decays. The vetoed
sample can be used to make a background
subtraction of in a fit to the energy spectrum.
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Movable Detector Scenario
The far detector spends about 10 of the run at
the near site where the relative normalization of
the two detectors is measured head-to-head.
Build in all the calibration tools needed for a
fixed detector system and verify them against the
head-to-head calibration.
1500 to 1800 meters
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Reactor Sensitivity

• Sensitivity to sin22?13 0.01 at 90 CL is
  achievable.
• Combining with off-axis some of the CP phase, d,
  range can be ruled out.
• Unexpected results are possible might break
  the standard model.

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Reactor Sensitivity
Is the mixing angle ?23 is not exactly 45º then
sin2?23 has a two-fold degeneracy. Combining
reactor results with off-axis breaks this
degeneracy. With the 0.03 precision of the Double
Chooz experiment the degeneracy is not broken.
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Aggressive Experiment Timeline
2003
Years
2004
2005
2006
2007
2008
2009
2010
2011
Run
Construction
Site Selection
Proposal
1 year 2 years 2 years
3 years (initially)
Site Selection Currently underway. The early
work on a proposal is currently underway. With
movable detectors, the detectors are constructed
in parallel with the civil construction Run
Phase Initially planned as a three year run.
Results or events may motivate a longer run.
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Conclusions and Prospects

• Reactor neutrinos are relevant to oscillations
  in all observed ?m2 regions.
• The KamLAND experiment has been crucial to
  resolving the oscillation parameters in the solar
  ?m2 region.
• There are many ideas for reactor ?13 experiments
  around the world and it is likely that more than
  one will go forward.
• Controlling the systematic errors is the key to
  making this measurement.
• With a 3 year run, the sensitivity in sin22q13
  should reach 0.01 (90 CL) at Dm2 2.010-3.
• Reactor sensitivities are similar off-axis and
  the two methods are complementary.
• The physics of reactor neutrinos is interesting
  and important.

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Question Slides
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Why Use Gadolinium?
Gd has a huge neutron capture cross section. So
you get faster capture times and smaller spatial
separation. (Helps to reduce random coincidence
backgrounds)
30 µs
With Gd Without Gd
With Gd Without Gd
200 µs
Also the 8 MeV capture energy (compared to 2.2
MeV on H) is distinct from primary interaction
energy.
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Characterizing BG with Vetoed Events
Matching distributions from vetoed events outside
the signal region to the non-veto events will
provide an estimate of correlated backgrounds
that evade the veto.

• Other Useful Distributions
• Spatial separation prompt and delayed events
• Faster neutrons go farther
• Radial distribution of events
• BGs accumulate on the outside of the detector.

n interactions
Proton recoils
?
From CHOOZ
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Medium Baseline Oscillation Searches

• Homogeneous detector
• 5 ton, Gd loaded, scintillating target
• 300 meters water equiv. shielding
• 2 reactors 8.9 GWthermal
• Baselines 1115 m and 998 m
• Used new reactors ? reactor off data for
  background measurement

Chooz Nuclear Reactors, France
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Palo Verde
Palo Verde Generating Station, AZ

• 32 mwe shielding (Shallow!)
• Segmented detector
• Better at handling the cosmic rate of a shallow
  site
• 12 ton, Gd loaded, scintillating target
• 3 reactors 11.6 GWthermal
• Baselines 890 m and 750 m
• No full reactor off running

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Exelon has agreed to work with us to determine
the feasibility of using their reactors to
perform the experiment. We are excited about the
possibility of participating in a scientific
endeavor of this nature At this time we see no
insurmountable problems that would preclude going
forward with this project. They have given us
reams of geological data which we are currently
digesting.
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Quantitative Analysis of Movable vs. Fixed
Detectors
Both the Kashiwazaki and Krasnoyarsk proposals
assume that they can get the relative
normalization systematic down to 0.8 with fixed
detectors.
Double Chooz believes that 0.6 is achievable.
Even if you halve the relative normalization,
fixed detector are not as sensitive for a two
year (or longer) run. All fixed detector scenario
quickly become systematics limited.
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Optimal Far Baseline
One must consider both the location of the
oscillation maximum (2200 m at ?m2210-3) and
statistics loss due to 1/r2 flux.
Kinimatic Phase 1.27?m2L/E for E3.6 MeV
At the preferred Dm2 the optimal region is quite
wide. In a configuration with a tunnel
connecting the two detector sites, one should
choose a far baseline that gives the shortest
tunnel (1200 to 1400 meters).
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Sensitivity Scaling with Systematic Error For a
rate only analysis
The optimal baseline is very sensitive to the
level of systematic error. The standard
assumption of 0.8 relative efficiency error for
fixed detectors is 250 of the statistical error
after 3 years at Braidwood.
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Comparison of Shape Rate
Systematics Limited
Statistics Limited
Systematic Error 0
Systematic Error 200
Systematic Error 600
The optimal Baseline for the systematics limited
shape analysis is 40º. The optimal baseline for
the systematics limited counting experiment is at
the least optimal spot for a shape analysis! You
better know what regime your working in.
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Sensitivity wrt Near Baseline
Ultimately the location of the near detector will
be determined by the reactor owners. The main
question here is what can we live with?
There is a 1/r2 dependence in statistics (a small
effect) and increasing oscillation probability
with distance.
Sensitivity degrades with increasing near
baseline. When LnearLfar the sensitivity is
about the same as CHOOZ.