# Presentation: 103230

## Measurement of sin2213 at the Braidwood Nuclear Reactors

- Measurement of sin2213 at the Braidwood Nuclear Reactors
- Jonathan Link
- Columbia University
- Research Techniques Seminar
- April 13, 2005

**Notes:**
Today, I’m going to try something different.
Before this talk, I’ve given at least 5 talks on short baseline reactor oscillation experiments, here at the lab, at least two in completely open forums.
In addition several other speakers have talked about θ13 mediated neutrino oscillations and in particular reactor measurements of sin22θ13, including Dave Reyna in the last Research Techniques Seminar.
So today I’m going to skip the neutrino phenomenology, and the measurement history and focus, instead on the design of the Braidwood experiment and why we think we can achieve a sensitivity in sin22θ13 of better than 0.01.

## Sin22θ13 Reactor Experiment Basics

- Distance

- Probability νe

- 1.0

- Eν ≤ 8 MeV

- 1200 to
- 1800 meters

- Sin22θ13 Reactor Experiment Basics

- Unoscillated flux observed here

- Well understood, isotropic source of electron anti-neutrinos

- Oscillations observed as a deficit of νe

- Survival Probability

## Reactor Neutrino Event Signature

- _Two part coincidence signal is crucial for background reduction_
- Minimum energy for the primary event is1.022 MeV from *e**+**e**− *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)

- e* *p*→ e****+***n
-                     n* *capture

- n* *H *→ *D* *  (2.2 MeV)

- n* **m*Gd *→ **m+1*Gd* *’s (8 MeV)

- Eν = E*e *+ 0.8 MeV

- (prompt)

- (delayed)

## 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

- Near Detector

- Far Detector

## Baseline Design: Detectors

- 2.6 m

- 3.5 m

-  Two zone design
-  2.6 meter target radius
-  3.5 meter total radius
-  65 ton target
-  1000 PMT’s (coverage >25%)
-  0.2% Gd Loaded Scintillator

- _[More](slide39.xml)__..._

## Benefits of Gd Loading

- No Gd: 6.1 cm mean
- 0.2% Gd: 4.3 cm mean

- No Gd: 142 μs mean
- 0.2% Gd: 15 μs mean

- Gd has a huge neutron capture cross section → faster capture times and smaller spatial separation.  Helps to reduce random coincidence backgrounds.

- 8 MeV capture energy (compared to 2.2 MeV on H) is distinct from positron energy and rare as radioactive background.

- _[More](slide41.xml)__..._

## 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!

## Sensitivity Drivers

- There are four main sensitivity drivers...
-  Neutrino interaction statistics
-  Near/Far detector relative normalization
-  Backgrounds
-  Detector baseline (varies with Δm2)
- We will look at each of these in turn.

- The first 3 are easily translated into measurement errors...

## Simple Sensitivity Model

- Where *N* is the number of observed signal events, *L* is the baseline and ε is the relative efficiency (≈1).  Then...

- Where...

- < 1– 3*σ**R** *means an effect is observed

- Statistics

- Relative Normalization

- Background

## 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 (>92% at Braidwood)
-  Reduced dead time
-  Check assumptions about optimal baseline with respect to statistical error.

## 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 won’t discuss these further

## 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!

## 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.
- Detectors 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](slide40.xml)__..._

## 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.

## Background

- 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.
- βn decays of spallation isotopes ─ isotopes such as 9Li and 8He with βn decay modes can be created in a spallation with μ on 12C.

- The vast majority of backgrounds are directly related to cosmic rays (cosmogenic).

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

## 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.

**Notes:**
The general principle of the calculations is to make conservative assumptions and only add complexity, approaching the complexity of a real analysis as needed to demonstrate that the sensitivity is achievable.
Above all else, don’t fool yourself.

## Cosmic Rays and Related Background

| Muons | Average E | Neutrons | 9Li & 8He |
| --- | --- | --- | --- |
| 0.21/m2/s | 84 GeV | 178/ton/day | 0.078/ton/day |

- Surface spectrum from Gaisser
- μ attenuation from Groom, Mokhov & Striganov

- Neutron production model from Wang *et al.*

- Isotope production from Hagner *et al.*

- 4.5 Hz of μ in target and 21 Hz in veto system

**/**m2**/**s**/**GeV

## Random Coincidence Background

- Assuming KamLAND concentrations of 40K, 232Th and 238U and 450 mwe

- Plot made by Hannah Newfield-Plunkett

- 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/day “neutrons”

- The random coincidence of these events in 100 μs window in the right order is _0.4/detector/day_.
- There is no tag for these events → _No dead time_.
- Reverse order events give a handle on the background rate

## Fast Neutron Background

- 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)

- There are three main processes for the prompt “positron-like” events

## 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

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

- 6 meters

- Shielding

## 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...

## 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 concrete+90 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.

## 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_

## Correlated Decay of Spallation Isotopes

- Correlated final state: β+n+2α

- Used Birks’ law to quench alphas and protons with Birks constant of 0.015 cm/MeV (from Borexino PLB 525, 29).

- Correlated final state: β+n+7Li

- τ1⁄2 = 178 ms 49.5% Correlated

- τ1⁄2 = 119 ms 16% Correlated

## 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

## Correlated Spallation Isotopes KamLAND/Braidwood 

| Eμ  (GeV) | KamLAND 1300 cm | Braidwood 520 cm |
| --- | --- | --- |
| Min I (0.33) | 2.5 GeV | 1.0 GeV |
| AvgBraidwood (83) | 3.4 GeV | 1.3 GeV |
| AvgKamLAND (215) | 3.5 GeV | 1.4 GeV |
| 1000 | 3.7 GeV | 1.5 GeV |

- 9Li Events vs. Minimum Shower Energy

- ~46 events at threshold of 0.

- Kevin McKinny’s Thesis

- Correlated Spallation Isotopes KamLAND/Braidwood

| AvgKamLAND (215) | 3.5 GeV | 1.4 GeV |
| --- | --- | --- |

| AvgBraidwood (83) | 3.4 GeV | 1.3 GeV |
| --- | --- | --- |

| Min I (0.33) | 2.5 GeV | 1.0 GeV |
| --- | --- | --- |

| 1000 | 3.7 GeV | 1.5 GeV |
| --- | --- | --- |

- 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 >85% of the decays.
- So this tag would isolate ~72% of all 9Li.

- μ Energy Loss Due to Ionization

## Correlated Spallation Isotopes in Braidwood

- Expect 0.078 9Li/ton/day, half decay in βn 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)

- μ energy loss in 2.6 m sphere of scintillator.

- Only 3% fail E cut of <1.5 GeV

- 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.

## Background Summary

| Background | Rate (<) | Dead Time |
| --- | --- | --- |
| Random | 0.4/detector/day | none |
| Fast Neutron (inside) | 0.3/detector/day | none |
| Fast Neutron (outside) | 0.5/detector/day | 0.2% |
| 9Li | 0.7/detector/day | 7% |
| Total | 1.9/detector/day | ~7% |

- This is where we stand today with conservative assumptions.

- Compare to 160 signal/detector/day at the far site (S/N~85)
- With a 10-20% uncertainty in the BG rate, sensitivity to sin22θ13 in the simple model is 0.01 (90% CL) at Δ*m*2=2.5×10-3.

## Energy Spectrum Fit

- Neutrino Energy

- 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.

- > 3M events/near det

- ~3M events/det

- ~12K events/det Tagged

- 1800 events/det Tagged

## Baseline Optimization

- The baseline optimization is sensitive to three effects:
-  Oscillation wavelength (   1/Δ*m*2)
-  Event statistics which fall off like 1/*L*2
-  To the level of systematic error (S/N is a function of L which also varies as 1/*L*2)

- Kinematic Phase = 1.27Δ*m*2L/E

- Optimize in terms of the invariant, kinematic phase, and the percent of systematic error in terms of statistical error.
- With Δ*m*2=2.5×10-3 and 150% systematic error...
- L≈1500 meters

## Braidwood Status

-  Completed preliminary MOU with Exelon in September 04
-  Submitted R&D 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.

## Recent Geological Site Investogation

- GZA GeoEnvironmental, Inc.

- Audio Televiewer and Geophysics Suite

## Background Radiation at Depth

- GZA GeoEnvironmental, Inc.

- 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

## Braidwood Status

-  Completed preliminary MOU with Exelon in September 04
-  Submitted R&D 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.

## 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.

## Conclusions

- Sensitivity studies indicate that the Δ*m*2 region preferred by Super-K (1.5×10-3 < Δ*m*2 < 3.0×10-3) will be covered down to 0.01 (90% CL)  in sin22θ13.

- 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!

## Question Slides

## CHOOZ and Palo Verde Results

-  sin2213< 0.18 at 90% CL (at m2=2.0×10-3)
-  Future experiments should try to improve on these limits by *at least* an order of magnitude.
- Down to sin2213    0.01
- In other words, a 1% measurement is needed!

## 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.

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## Detectors Designed to Move

- Detector

- 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)

- 8 meters

- 9 meters

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