The Braidwood Neutrino Experiment

1
The Braidwood Neutrino Experiment
Ed Blucher, Chicago

• Outstanding questions in neutrino oscillation
  physics importance of ?13
• Experimental approaches to ?13 motivation for a
  precise reactor experiment
• The Braidwood Experiment

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Neutrino Oscillations

• During last few years, oscillations among
  different flavors of neutrinos have been
  established physics beyond the S.M.
• Mass eigenstates and flavor eigenstates are not
  the same

mass eigenstates
flavor eigenstates
MNSP matrix

• Raises many interesting questions including
  possibility of CP violation in neutrino
  oscillations.
• CP violation in neutrino sector could be
  responsible for the matter-antimatter asymmetry
  (leptogenesis)

The antilepton excess is converted to a baryon
excess through nonperturbative S.M. BL
violating, but B-L conserving processes.
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2-Flavor Neutrino Mixing
The time evolution of the flavor states is
For a beam that is pure ?? at t0,
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What do we know? Oscillations established with
two distinct mass differences
1. Atmospheric ?m22.5????? eV2 Experiments
using neutrinos produced by cosmic rays in
atmosphere (e.g., SuperK) verified with
long-baseline accelerator experiment (K2K).
K2K
Super Kamiokande
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2. Solar ?m25????5 eV2 Series of experiments
using neutrinos from the Sun (e.g., Ray Davis
37Cl experiment, SNO) and KAMLAND experiment
using reactors in Japan.
Ray Davis
SNO
KAMLAND
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What about LSND?
Unconfirmed observation of oscillations with
?m21 eV2 by LSND does not fit into 3 generation
model (with 2 independent mass
splittings). MiniBoone should have results early
next year.
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Neutrino mixing and masses
?12 30
?23 45
sin2 2?13 lt 0.15 at 90 CL
What is ?e component of ?3 mass eigenstate?
normal
inverted
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Key questions in neutrino mixing

• What is value of ?13?
• What is mass hierarchy?
• Do neutrino oscillations violate CP symmetry?

• Why are quark and neutrino mixing matrices so
  different?

Value of ??3 central to these questions it sets
the scale for experiments needed to resolve mass
hierarchy and search for CP violation.
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Methods to measure sin22?13

• Accelerators Appearance (????e) at ?m2?2.5?10-3
  eV2

T2K ltE?gt 0.7 GeV, L 295 km
NO?A ltE?gt 2.3 GeV, L 810 km

• Reactors Disappearance (?e??e) at ?m2?2.5?10-3
  eV2

Use reactors as a source of ?e (ltE?gt3.5 MeV)
with a detector 1-2 kms away and look for
non-1/r2 behavior of the ?e rate
Reactor experiments provide the only clean
measurement of sin22??? no matter effects, no
CP violation, almost no correlation with other
parameters.
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Recommendation 2 (of 3)

• We recommend, as a high priority, a comprehensive
  U.S. program to complete
• our understanding of neutrino mixing, to
  determine the character of the neutrino
• mass spectrum, and to search for CP violation
  among neutrinos. This program
• should have the following components
• An expeditiously deployed multi-detector reactor
  experiment with sensitivity to
• disappearance down to sin22??? 0.01, an order
  of magnitude below present
• limits.
• A timely accelerator experiment with comparable
  sin22??? 0.01 sensitivity and
• sensitivity to the mass hierarchy through matter
  effects.
• A proton driver in the megawatt class or above
  and neutrino superbeam with an
• appropriate very large detector capable of
  observing CP violation and measuring
• the neutrino mass-squared differences and mixing
  parameters with high precision.

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Both reactor and accelerator experiments have
sensitivity to sin22???, but accelerator
measurements have ambiguities
Example T2K. ?P(????e)0.0045 ? ?sin22?130.028
dcp

• normal
• inverted

(5 yr n)
/- 0.028
?m22.5?10-3 eV2
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Reactor and accelerator sensitivities to sin22???
90 CL exluded regions with no osc.signal
Braidwood
sin22?13 0.05, dCP0, ?m2 2.510-3 eV2 (3
yr reactor, 5 yr T2K)
dCP0, ?m2 2.510-3 eV2 (3 yr reactor, 5 yr
Nova)
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Resolving the ?23 Degeneracy
Green Nova OnlyBlue Braidwood Reactor plus
Nova Red Double-Chooz plus offaxis

• ?? disappearance experiments
• measure sin22?23, while
• P(????e)?sin2?23sin22?13.
• If ?23?45?, ?? disappearance
• experiments, leave a 2-fold
• degeneracy in ?23 it can be
• resolved by combination of a
• reactor and ????e appearance
• experiment.

Example sin22 ?23 0.95 ? 0.01 ?m2
2.510-3 eV2 sin22?13 0.05
?m2 2.510-3 eV2 sin22q13 0.05
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CP Violation and the Mass Hierarchy
T2K
Nova
P(????e)
sin22?130.1
?CP
?CP
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Example Reactor T2K ? running
T2K ? - 5 years
?sin22????0.01 from reactor
P(????e)
sin22?130.1
Neutrino, normal hierarchy
Neutrino, inverted hierarchy
?CP
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Nova and T2K Sensitivity to ?CP and Mass Hierarchy
If Braidwood does not see an oscillation signal,
it will be difficult for long-baseline
superbeam experiments to investigate mass
hierarchy and CP violation.
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Reactor Measurements of Neutrino Oscillations
Reactors are copious sources of
per second.
Flux
Cross section
Detection of antineutrino by
(100 events /GW/ yr / ton at L 1500 m)
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Reactor Measurements of
?13 Search for small oscillations at 1-2 km
distance (corresponding to
Past measurements
Pee
Our sensitivity goal sin22???0.01. Level at
which long-baseline accelerator experiments can
be used to measure mass hierarchy, CP violation.
Distance to reactor (m)
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Chooz Current Best ??? Experiment
P8.4 GWth
L1.05 km
D300mwe
m 5 tons, Gd-loaded liquid scintillator
sin22???lt 0.15 for ?m22.5?10?3 eV2
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• How to improve on previous reactor experiments?
• ?Add an identical near detector
• Eliminate dependence on reactor flux only
  relative
• acceptance of detectors needed
• ? Optimize baseline (1500 m)
• ? Larger detectors (5 ton ? 100
  tons) ? Reduce backgrounds
• (Go deeper 100m ? 150 to 300 m active
  veto systems)

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• Many sites have been discussed
• Kraznoyarsk (Russia)
• Chooz (France)
• Kashiwazaki (Japan)
• Diablo Canyon (California)
• Braidwood, Byron (Illinois)
• Wolf Creek (Kansas)
• Brazil
• Taiwan
• Daya Bay (China)

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Kr2Det Reactor ?13 Experiment at Krasnoyarsk
Features - underground reactor - existing
infrastructure
Detector locations constrained by existing
infrastructure
Reactor
Ref Marteyamov et al, hep-ex/0211070
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The Chooz site, Ardennes, France
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Daya Bay, China
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U.S. Nuclear Power Plants
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BraidwoodNeutrino Experiment

• Features of Braidwood Site
• 2?3.6 GW reactors 7.17 GW maximum power
• Flat flexibility, equal overburden at near and
  far sites, surface
• transportation of detectors
• Favorable geology (dolomitic limestone) good
  for excavation,
• low radioactivity (order of magnitude lower U,
  Th than granite)

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The Braidwood Collaboration
14 Institutions 70 Collaborators
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Braidwood Baseline Design

• Goals Flexibility, redundancy, cross checks
• 4 identical 65 ton fiducial mass detectors 2 at
  near site (L270m), 2 at far site (L1510m)
• Two zone detectors inner zone with Gd-loaded
  LS and r2.6 m outer zone with mineral oil and
  r3.5 m.
• Movable detectors with surface transport for
  cross-calibration vertical shaft access to
  detector halls
• Oscillation measurements using both rate and
  energy spectrum
• Full detector construction above ground
  detectors
• filled simultaneously with common scintillator.
• Near and far detectors at same depth of 183
  (464 mwe) gives equal spallation rates that can
  be exploited for detector and background checks

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Braidwood Site
Far Detector
Near Detector
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Bore Hole Project at the Exelon Site

• Bore hole project completed in January 2005
• Bore holes drilled to full depth (200m) at near
  and far shaft positions on Braidwood site.
• Provided detailed information on geology, ground
  water, radioactivity, etc.
• Confirmed feasibility of detectors down to
  depths of 460mwe.
• Reduces contingency required for underground
  construction
• Demonstrated willingness of Exelon to allow
  construction on their site.

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Braidwood Design Sensitivity

• GOALS
• Discovery potential (at 3?) for sin22?13 gt 0.01
• Sensitivity (90 CL) down to the sin22?13 0.005
  level With cross checks and redundancy to
  establish signal and check systematic errors
• See signal in both rate and energy spectrum
  measurements
• Cross calibrate detector pairs at high-rate near
  site
• Cross calibrate near/far detectors using
  spallation isotopes like 12B
• Multiple near and far detectors give direct cross
  checks on detector systematics at 0.05 for the
  near set and 0.3 for far
• Large detectors allow studies of the radial
  dependence of the IBD signal and backgrounds.

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Normalization and spectral information

• Counting analysis Compare number
• of events in near and far detector
• Systematic uncertainties
• relative normalization of near and
• far detectors
• relatively insensitive to energy
• calibration
• Energy spectrum analysis Compare
• energy distribution in near and far
• detectors
• Systematic uncertainties
• energy scale and linearity
• insensitive to relative efficiency of
• detectors

Predicted spectrum ?130 (from near detector)
Observed spectrum (far detector) sin22?130.04
E? (MeV)
E? (MeV)
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Detectors and analysis strategy designed to
minimize relative acceptance differences
Central zone with Gd-loaded scintillator
surrounded by buffer regions fiducial mass
determined by volume of Gd-loaded
scintillator Events selected based on
coincidence of e signal (Evisgt0.5 MeV) and ?s
released from nGd capture (Evisgt6 MeV). No
explicit requirement on reconstructed event
position little sensitivity to E
requirements.
Shielding
Neutrino detection by
n mGd ? m1Gd ?s (8 MeV) ?20?sec
6 meters
Gd-loaded liquid scintillator
To reduce backgrounds depth active and
passive shielding
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Conceptual Mechanical Design

• Outer steel buffer oil containment vessel (7m
  diameter)
• 1000 low activity glass 8 PMTs evenly
  distributed on inside surface (25 coverage)
• Inner acrylic Gd-loaded scinitillator containment
  vessel (5.2m diameter)
• Top access port can be used to insert
  calibration sources

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Detector With Moveable Veto System and Shielding
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Acceptance Issues
Must know (relative) number of protons in
fiducial region (relative) efficiency for
detecting IBD events
Known volume of stable, identical
Gd-loaded liquid scintillator in each
detector Well understood efficiency of positron
and neutron energy requirements
37
Monte Carlo Studies
Reconstructed e and n-capture energy
Studies based on hit-level simulation with
parameterizations of many detector effects.
Studies using full GEANT4 simulation are
underway.
n Capture on Gd

• Reconstructed Energy Cuts
• positron Evis gt 0.5 MeV
• n-Gd capture Evis gt 6 MeV

n Capture on H
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Energy Scale
Use neutron capture peaks from IBD events to
measure energy scale. In each far detector, E
scale can be measured to 0.3 every 5 days. (This
calibration averages over detector in exactly the
same way as signal events.) Acceptance
uncertainty from energy scale should be 0.1.
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3-zone versus 2-zone detectors
I. Gd-loaded liquid scintillator II. ? catcher
liquid scintillator (no Gd) III.
Non-scintillating buffer
(Braidwood 2-zone Design)
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Acceptance Sensitivity to Energy Scale
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Gd - Liquid Scintillator (Gd-LS)

• Detectors must be filled simultaneously common
  scintillator
• relative volume measurement with lt0.2
  uncertainty.
• We plan to use 0.2 Gd 20 PC 80 dodecane
  mixture
• developed by BNL Nuclear Chemistry group.
• (Dick Hahn, Minfeng Yeh, et al.)
• Long-term stability tests in progress
• So far, stable with attenuation length gt 18 m.

Stability of Gd-LS (Absorbance of 0.002
corresponds to attenuation Length of 20 m).
Chooz degradation was 0.4/day
x - Braidwood scintillator
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Movable Detectors

• Transport is necessary to move detectors from
  construction/filling area to below ground halls
• Movable detectors allow direct check of relative
  detector acceptances at
• near site
• Possible scenario
• Possible method Use climbing jack system with
  cable to lift and put detectors on multi-wheeled
  trailer (standard method used in industry).

A
B
A
B
C
D
A
C
B
D
Goldhofer Trailer Moving 400 tons
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Using Isotope Production to Measure Fiducial Mass

• Unique feature of the Braidwood site
• Near and far detectors have equal,
  well-understood, substantial overburden
• ? Can use produced 12B events to measure
• Near/far relative target mass from the total rate
• Near/far energy calibrations from the relative
  energy distribution
• 50,000 12B beta-decay events per year per
  detector can be tagged and isolated giving a
  statistical uncertainty of 0.45
• Systematic uncertainties related to the knowledge
  of relative near/far overburden must be known to
  few percent from
• Geological survey information (Bore hole data
  near/far agreement lt1)
• Cosmic muon rates in the near and far locations

44
Summary of Acceptance Uncertainties
45
Backgrounds

• Even though near and far shielding is the same,
  backgrounds do
• not cancel signal/background ratios in the near
  and far detectors are different.
• Uncorrelated backgrounds from random coincidences
  (not a problem)
• Reduced by limiting radioactive materials
• Limestone rock at Braidwood site has low
  radioactivity
• Directly measured from rates and random trigger
  setups
• Correlated backgrounds
• Neutrons that mimic the coincidence signal
• Cosmogenically produced isotopes that decay to a
  beta and neutron
• (9Li and 8He).

46
Cosmic Muon Rates at Braidwood Depths

• Calculation of muon rate at 464 mwe (600 ft)
• Used data from boreholes for density and material
• Average muon flux 0.213 /m2/sec
• Average muon energy 110.1 GeV

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Veto (Tagging) System
Goal lt 1 n background event/day/detector. Strateg
y tag muons that pass near the detector. Use
shielding to absorb neutrons produced by muons
that miss the veto system.

• Residual n background
• Veto inefficiency - 99 efficiency ?
  0.25/detector/day
• Fast neutron created outside the shielding -
  0.5/detector/day

Shielding
With µ rate in the veto system of 21 Hz and the
tag window of 100 µs ? 0.2 dead time
6 meters
Muon identification must allow in situ
determination of the residual background rate
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Background Simulations
Neutrons that reach the vessel wall

• For a veto system with 2 mwe of
• shielding, both a GEANT4 and a
• MARS calculation give
• 170 n/ton/day produced in the surrounding rock
• 4500 n/day emerging from the rock
• Background rate of 0.75 events/ dayafter the
  veto requirements

Fraction of Neutrons
Detector
Untaggedneutrons
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9Li and 8He
Isotopes like 9Li and 8He can be created in µ
spallation on 12C and can decay to ßn. Long
lifetimes make veto difficult 9Li178ms KAMLAND
found isotope production correlated with muons
that shower in the detector.
from the thesis of Kevin McKinny
Tagging showering muons and rejecting events in a
0.5 s window eliminates 72 of 9Li and results in
7 deadtime.
Expect 0.078 9Li/ton/day half decay in ßn
modes 72 are tagged 0.7/detector/day.
More
50
Background Summary
Compare to 160 signal/detector/day at the far
site (S/N85)
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Sensitivity and Discovery Potential

• For three years of Braidwood dataand Dm2 gt 2.5 x
  10-3 eV2
• 90 CL limit at sin22q13 lt 0.005
• 3 s discovery for sin22q13 gt 0.013

With two near and two far detectors, the total
uncertainty in the near/far ratio is 0.33
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90 CL Sensitivity vs Years of Data

• Information from both counting and shape fits
• Combined sensitivity for sin22?13 reaches the
  0.005 after three years

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Braidwood Measurement Capability

• For 3 years of data and a combined counting plus
  shape analysis
• Dm2 2.5 x 10-3 eV2 and sin22q13 0.02

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Other Physics Neutrino Electroweak Couplings

• Braidwood experiment can isolate about 10,000?ne
  e events that will allow the measurement of
  the neutrino gL2 coupling to 1
• This is ?4 better than past n-e experiments and
  would give an error comparable to gL2(NuTeV)
  0.3001 ? 0.0014

gL2 - gL2(SM)

• Precision measurement possible since
• Measure elastic scattering relative to inverse
  beta decay
• Can pick a visible energy window (3-5 MeV) away
  from background

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Status of Project

• Engineering/RD proposal
• NuSAG Review
• 2006 Full proposal submission
• 2007 Project approval construction
• start
• 2010 Start datataking
• Cost Estimate
• Civil Costs 34M 8.5M (Cont.)
• 4 Detectors and Veto Systems
  18M 5M (Cont.)

Exelon enthusiastic supporter of project
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Conclusions

• The worldwide program to understand ?
  oscillations and determine the mixing parameters,
  CP violating effects, and mass hierarchy will
  require a broad range of measurements a
  reactor experiment to measure ?13 is a key part
  of this program.
• A reactor experiment will provide the most
  precise measurement
• of ?13 or set the most restrictive limit.
• Reactor experiment with sensitivity of
  sin22???1 will give information needed to
  understand future roadmap of neutrino program.
• Braidwood offers an ideal site to perform an
  experiment with
• the required sensitivity (sin22?13 0.005 at
  90 c.l.)