Measurement of sin22q13 at the Braidwood Nuclear Reactors

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Measurement of sin22q13 at the Braidwood Nuclear
Reactors Jonathan Link Columbia
University Research Techniques Seminar April 13,
2005
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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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Reactor Neutrino Event Signature
The reaction process is inverse ß-decay Two
part coincidence signal is crucial for background
reduction Minimum energy for the primary event
is1.022 MeV from ee- annihilation. Positron
energy spectrum implies the neutrino
spectrum The scintillator is doped with
gadolinium to enhance capture Capture can also
occur on hydrogen (or carbon, rarely)
ne p? en n capture
(prompt)
(delayed)
E? Ee 0.8 MeV
n mGd ? m1Gd gs (8 MeV)
n H ? D g (2.2 MeV)
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Braidwood Nuclear Station Located about 30 miles
due south of the Lab. Operated by Exelon Nuclear.

• 2 Reactors with 7.17 GW thermal power
• Well understood/favorable geology
• Plant construction data
• Site investigation (completed Jan. 05)
• Strong support from owner
• Ability to optimize layout

Far Detector
Near Detector
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Baseline Design Detectors

• Two zone design
• 2.6 meter target radius
• 3.5 meter total radius
• 65 ton target
• 1000 PMTs (coverage gt25)
• 0.2 Gd Loaded Scintillator

2.6 m
3.5 m
More
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Benefits of Gd Loading
Gd has a huge neutron capture cross section ?
faster capture times and smaller spatial
separation. Helps to reduce random coincidence
backgrounds.
No Gd 6.1 cm mean 0.2 Gd 4.3 cm mean
No Gd 142 µs mean 0.2 Gd 15 µs mean
8 MeV capture energy (compared to 2.2 MeV on H)
is distinct from positron energy and rare as
radioactive background.
More
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Baseline Design Site Layout

• Near detectors at a baseline of 286 meters
  (4400 int/day)
• Far detectors at a baseline of 1511 meters (160
  int/day)
• All detector at a depth of 183 meters (600 ft)
• Earth shielding is flat with an average cover of
  464 mwe (as measured by our borehole
  investigation)
• This should be compared to 700 mwe cover under a
  mountain!

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Sensitivity Drivers

• There are four main sensitivity drivers
• Neutrino interaction statistics
• Near/Far detector relative normalization
• Backgrounds
• Detector baseline (varies with ?m2)
• We will look at each of these in turn.

The first 3 are easily translated into
measurement errors
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Simple Sensitivity Model
lt 1 3sR means an effect is observed
Where N is the number of observed signal events,
L is the baseline and e is the relative
efficiency (1). Then
Where
Relative Normalization
Statistics
Background
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Statistics
Top 30 U.S. Sites by Power Performance

• Ways to optimize statistics
• Reactor power
• Braidwood is third most powerful in US
• Detector mass
• 130 ton baseline design
• Planned upgrade to 260 tons
• Run time
• Straight clock time (3 years initial plus 5
  years upgrade)
• Reactor uptime (gt92 at Braidwood)
• Reduced dead time
• Check assumptions about optimal baseline with
  respect to statistical error.

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Relative Normalization

• The use of a near detector eliminates the
  normalization uncertainty due to
• reaction cross section
• neutrino production in the core
• reactor power
• Truly identical detectors would eliminate the
  remaining sources of normalization uncertainty
  which are
• detector efficiency
• gadolinium fraction (neutron detection
  efficiency)
• target mass
• target chemistry free proton (target particle)
  fraction
• The last three are best measured with detector
  cross calibration.

I wont discuss these further
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Relative Normalization II

• Relative detector efficiency can be measured by
  calibrating all detectors with the same long
  lifetime radioactive sources.
• Gd fraction can be measured by looking at Gd to
  H capture ratio (statistical precision to better
  that 0.05).
• Also, control chemistry during detector filling
• Relative free proton count (target mass and H
  fraction) can be controlled by measuring flow
  rates and equally splitting scintillator batches
  during detector filling.
• It can only be measured by detector cross
  calibration with neutrinos
• Requiring Movable Detectors!

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Movable Detectors
Spending just 8 of the run on head-to-head
calibration results in a free proton calibration
precision of better than 0.3. Detector
calibration techniques can be verified against
head-to-head neutrino relative calibration. Detect
ors can be built during shaft civil construction
and commissioned in the near hall while the far
hall is still under construction. Detectors can
be filled simultaneously and in the same location
from the common scintillator batches.
More
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Relative Normalization Final Word
Double CHOOZ hopes to achieve 0.6 (or better)
relative normalization with fixed detectors. With
four detectors there are three independent
relative normalizations. So even if this is the
best we could do on each pair of detectors the
effective relative normalization error is An
effective relative normalization error of 0.2 or
better is very likely.
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Background
The vast majority of backgrounds are directly
related to cosmic rays (cosmogenic).

• There are three types of background
• Random coincidence - where two unrelated events
  happen close together is space and time.
• Fast neutron - where a fast neutron enters the
  detector, creates a prompt signal, thermalizes
  and is captured.
• ßn decays of spallation isotopes - isotopes such
  as 9Li and 8He with ßn decay modes can be created
  in a spallation with µ on 12C.

The background rejection methods are the source
of dead time in the detector.
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Background Notes

• The Collaboration is developing comprehensive
  simulation tools.
• In the mean time background calculations are done
  with a combination of
• Hand calculations
• Numerical integration of functions (i.e. surface
  µ spectrum, µ attanuation in rock)
• Published data tables
• Simple geometries in accepted simulation tools
  (MARS, Fluka and Geant4)
• The Minos near hall, with similar flat
  overburden, may allow us to test the validity of
  these calculation techniques.

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Cosmic Rays and Related Background
Surface spectrum from Gaisser µ attenuation from
Groom, Mokhov Striganov
Neutron production model from Wang et al.
/m2/s/GeV
Isotope production from Hagner et al.
4.5 Hz of µ in target and 21 Hz in veto system
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Random Coincidence Background
The rate of single events is shown on the
left. One can integrate this plot in the positron
and neutron signal regions to get the respective
random rates. 142k/det/day positrons 5070/det/da
y neutrons
Assuming KamLAND concentrations of 40K, 232Th and
238U and 450 mwe
Plot made by Hannah Newfield-Plunkett
The random coincidence of these events in 100 µs
window in the right order is 0.4/detector/day. The
re is no tag for these events ? No dead time.
Reverse order events give a handle on the
background rate
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Fast Neutron Background
There are three main processes for the prompt
positron-like events

• Two neutron captures from the same cosmic - This
  should be tagged the vast majority of the time,
  but it sets the tag window for tagged muons at
  100 µs.
• Proton recoil off fast neutron - dominate effect.
• Fast neutron excitation of 12C - interesting, but
  not significantly different than 2. Energy
  spectrum peaks at particular values (like 4.4
  MeV, first excited state)

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Tagging Muons at Braidwood
The basic idea is to tag muons that pass near the
detector so that we can reject the fast neutron
background. Neutrons from farther away should
mostly be ranged out.
The Collaboration is considering two options for
veto system
Shielding

• An integrated veto shield - 2 meters of liquid
  scintillator
• 1 meter of heavy concrete with surface detectors
  (RPCs?)

6 meters
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Fast Neutron Background in Braidwood

• What eludes the tagging system?
• Veto inefficiency - muon gets inside the veto but
  is not seen by either the veto or the detector.
  Need very efficient veto detector. You get two
  shoots at each muon entering and exiting. (99
  efficiency ? 0.25/detector/day)
• Fast neutron created outside the shielding -
  calculation follows

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Fast Neutron Background in Braidwood II
Using the MARS simulation package I calculate
that 2087 neutrons/day will exit the rock on a
collision course with the detector target. Of
these 0.93 survive 1 meter heavy concrete90 cm
mineral oil and 20 of those have positron-like
prompt event in the detector. 4 background
events In 88 of the original 2087 neutrons, the
parent muon passed through the veto at some
point. 0.5/detector/day Problematic neutrons are
more likely to be vetoed because the colinearity
of the parent and daughter is more likely for the
fast neutrons that survive the shielding.
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Fast Neutron Dead Time
The cosmic ray rate in the veto (generously
assuming 100 m2 veto area) is 21 Hz and the tag
window is 100 µs 0.2 dead time
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Correlated Decay of Spallation Isotopes
t½ 178 ms 49.5 Correlated
t½ 119 ms 16 Correlated
Correlated final state ßn2a
Correlated final state ßn7Li
Used Birks law to quench alphas and protons with
Birks constant of 0.015 cm/MeV (from Borexino PLB
525, 29).
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Correlated Spallation Isotopes in KamLAND
KamLAND tagged correlated events closely
associated with cosmic muons. Their data is
consistent with 9Li being the dominate source of
correlated spallation isotope.
from the thesis of Kevin McKinny
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Correlated Spallation Isotopes KamLAND/Braidwood
9Li Events vs. Minimum Shower Energy
µ Energy Loss Due to Ionization
46 events at threshold of 0.
Kevin McKinnys Thesis
Scaling from KamLAND, a 1.5 GeV shower tag in
Braidwood would capture 85 of all 9Li. A 0.5
second tag window would capture gt85 of the
decays. So this tag would isolate 72 of all 9Li.
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Correlated Spallation Isotopes in Braidwood
Expect 0.078 9Li/ton/day, half decay in ßn modes,
72 are tagged ? 0.7/detector/day. From a
simulation of muon energy loss in the Braidwood
detector (based on Groom, Mokhov Striganov
ref. therein)
Only 3 fail E cut of lt1.5 GeV
µ energy loss in 2.6 m sphere of scintillator.
With 4.5 Hz of µ and 3 tagged at 0.5 seconds ?
7 dead time Can use neutron captures following
cosmic as an additional tag.
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Background Summary
This is where we stand today with conservative
assumptions.
Compare to 160 signal/detector/day at the far
site (S/N85) With a 10-20 uncertainty in the BG
rate, sensitivity to sin22?13 in the simple model
is 0.01 (90 CL) at ?m22.510-3.
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Energy Spectrum Fit
Fitting the energy spectrum takes advantage of
the significant differences between the signal
and backgrounds. With the Phase II upgrade the
experiment is sensitive to the shape deformation
due to oscillations. May extend sensitivity to
0.008-0.006.
gt 3M events/near det
Neutrino Energy
3M events/det
12K events/det Tagged
1800 events/det Tagged
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Baseline Optimization

• The baseline optimization is sensitive to three
  effects
• Oscillation wavelength ( 1/?m2)
• Event statistics which fall off like 1/L2
• To the level of systematic error (S/N is a
  function of L which also varies as 1/L2)

Optimize in terms of the invariant, kinematic
phase, and the percent of systematic error in
terms of statistical error. With ?m22.510-3 and
150 systematic error L1500 meters
Kinematic Phase 1.27?m2L/E
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Braidwood Status

• Completed preliminary MOU with Exelon in
  September 04
• Submitted RD proposal to NSF and DOE in
  September 04
• Completed geological site investigation in
  January 05 which included
• 600 foot bore holes at both the near and far
  site.
• Geophysics tests included audio-televiewer,
  passive gamma, resistance probe, calipers and
  water pressure tests.

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Recent Geological Site Investogation
Audio Televiewer and Geophysics Suite
GZA GeoEnvironmental, Inc.
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Background Radiation at Depth
One of the standard geophysics probes measures
the natural gamma radiation. Measures rock
porosity For us it confirms that the background
radiation in dolomitic limestone is low.
Near Shaft
Far Shaft
GZA GeoEnvironmental, Inc.
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Braidwood Status

• Completed preliminary MOU with Exelon in
  September 04
• Submitted RD proposal to NSF and DOE in
  September 05
• Completed geological site investigation in
  January 05 which included
• 600 foot bore holes at both the near and far
  site.
• Geophysics tests include audio-televiewer,
  passive gamma, resistance probe, calipers and
  water pressure tests.
• Demonstrated a constructive working relationship
  with Exelon and the Braidwood staff.
• We await the guidance of NuSAG.

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Conclusions
The measurement rests on our ability to
understand the near/far relative normalization
and to reduce and understand backgrounds. Each
piece of the relative normalization can be
measured and verified by at least two independent
methods. This redundancy is critical to ensure
that the high precision goals of the experiment
will be meet. Initial, conservative studies
indicate that a background of 1 to 2
events/detector/day is achievable. Significant
improvement is likely.
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Conclusions
Sensitivity studies indicate that the ?m2 region
preferred by Super-K (1.510-3 lt ?m2 lt 3.010-3)
will be covered down to 0.01 (90 CL) in
sin22?13.
Phase II, where two detectors are added at the
far site for an additional 5 years of running,
can extend the reach to 0.007 or better. Phase II
could confirm a rate deficit in the baseline
experiment with an observation of the shape
deformation.
Braidwood is an excellent site to make this
important measurement!
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Question Slides
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CHOOZ and Palo Verde Results

• 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 1 measurement is needed!

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Two Zones Design
With just two zones some of the Gd capture energy
can escape the scintillator undetected. This may
result in relative norm. bias.
Calibrating detectors to the capture peak
virtually eliminates this potential bias.
Back
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Detectors Designed to Move
Use a rail system for easy transport. The
detector needs to carry its electronics and
front-end DAQ as these components also play a
role in detector efficiency. Multiple far
detectors may be used to maintain portability
while increasing total volume. Detectors need to
fit in the shaft (Diameter 7 meters)
Detector
9 meters
8 meters
Back
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