1. The Daya Bay Reactor Neutrino Experiment
Jonathan Link
Virginia Tech
On behalf of the Daya Bay Collaboration
October 20, 2011

2. Nuclear Reactors as a Neutrino Source
Nuclear reactors are a very intense sources of νe coming from the b-decay of the neutron-rich fission fragments.
Arbitrary
Flux
Cross Section
Observable ν Spectrum 
From Bemporad, Gratta and Vogel
A typical commercial reactor, with 3 GW thermal power, produces 6×1020 νe/s
The observable ne spectrum is the product of the flux and the cross section.
Jonathan Link, Virginia Tech Seminar
12/10/2018

3. Reactor Neutrino Event Signature
The reaction process is inverse β-decay (Used by Reines and Cowan in the neutrino discovery experiment)
Two part coincidence signal is crucial for background reduction
Minimum energy for the primary signal is MeV from e+e− annihilation at threshold
Positron energy implies the anti-neutrino energy
Neutron capture on Gd provides a secondary burst of light approximately 30 μs later
ne p→ e+n
n capture
Eν = Ee MeV ( =mn-mp+me-1.022)
Jonathan Link, Virginia Tech Seminar
12/10/2018

4. Reactor Oscillation Experiment Basics
Well understood, isotropic source of electron anti-neutrinos
Oscillations observed as a deficit of νe
Detectors are located underground to shield against cosmic rays.
Unoscillated flux observed here
1.0
sin22θ13
πEν /2Δm213
Probability νe
Distance (L/E)
~1800 meters
(at 3 MeV)
Jonathan Link
10/21/11

5. The Daya Bay Nuclear Power Plant
Located in Guangdong Province, China, about one hour from Hong Kong.
6 reactors on site for a total of 17.4 GW of thermal power.
It is among the most powerful nuclear power plants in the world
The mountainous terrain is well suited for shielding underground detectors
The utility company (China Guangdong Nuclear Power Group) has joined the collaboration
The Daya Bay Nuclear Power Plant
Jonathan Link
10/21/11

6. Daya Bay Design Principles
Identical near and far detectors cancel many systematic error.
Multiple modules boost statistics while reducing systematic errors with multiple independent measurements and direct comparisons of detector counting rates in a common ν flux.
Three zone detector design eliminates the need for spatial cuts which can introduce systematic uncertainties.
Shielding from cosmic rays and natural radioactivity reduces background rates and provides measurable handles on remaining background.
Movable detectors allows for concurrent civil and detector construction, early detector commissioning at the near site, and possible cross calibration between near and far detectors to further reduce systematic errors.
Jonathan Link
10/21/11

7. Experimental Setup Total tunnel length ~ 3000 m Ling Ao II Reactors
Far site
Overburden: 355 m
900 m
Ling Ao Near
Overburden: 112 m
Filled detectors are transported between halls via horizontal tunnels.
465 m
Ling Ao II
Reactors
Water
hall
(Starting 2011)
Construction
tunnel
810 m
Ling Ao
Reactors
Liquid
Scintillator
hall
Entrance
295 m
Daya Bay Near
Overburden: 98 m
Daya Bay
Reactors 7

8. Experimental Setup 9 different baselines Ling Ao II Reactors Ling Ao
8 identical anti-neutrino detectors (two at each near site and four at the far site) to cross-check detector efficiency
Two near sites sample flux from reactor groups
Far site
Overburden: 355 m
9 different baselines
Ling Ao Near
Overburden: 112 m
Ling Ao II
Reactors
(Starting 2011)
Ling Ao
Reactors
Daya Bay Near (m)
Ling Ao Near (m)
Far (m)
Daya Bay
363
1347
1985
Ling Ao I
857
481
1618
Ling Ao II
1307
526
1613
Halls
Reactors
Daya Bay Near
Overburden: 98 m
Daya Bay
Reactors 8

9. The Daya Bay Detector Design
Gd-Loaded LS
(20 tons) LS
Mineral Oil
5 meters
1.55 m
1.99 m
2.49 m
Three zone, cylindrical design
0.1% wt Gd-Loaded LS target
LS gamma catcher
Mineral oil buffer
Reflectors at top and bottom
196 PMT’s arrayed around the barrel of the cylinder
5 meter total diameter
Designed to sit in a pool of ultrapure water
Jonathan Link
10/21/11

10. The Daya Bay Detector Design
Jonathan Link
10/21/11

11. Water Shield and Muon Tagging System
The water pool shields the detectors from energetic γ-rays from the decay chains of 238U, 232Th and 40K in surrounding the rock
It also detects the Čerenkov light produced by cosmic ray muons which pass near the detectors
Water Pool
RPCs
The pool is lined with white Tyvek and sparsely populated with PMTs
The pool is optically separated into two zones (inner and outer)
The two zones allow a better measurement of efficiency
The top is covered with 4 layers of RPC
Minimum 2.5 m water shielding in all directions.
Jonathan Link
10/21/11

12. Water Shield and Muon Tagging System
Jonathan Link
10/21/11

13. Water Controls Radioactive Backgrounds
in air
in water
Singles events verses height in the partially filled pool show the suppression of radioactive backgrounds by water.
Jonathan Link
10/21/11

14. Filled Water Pool in First Near Hall
Jonathan Link
10/21/11

15. Completed Near Hall Ready for Data Taking
Jonathan Link
10/21/11

16. Muon Induced Correlated Backgroundsp n
Tag muons that pass near the detectors.
Range out fast neutrons from muons that are further away.
Jonathan Link
10/21/11

17. Muon Spallation Backgrounds
Isotopes like 9Li and 8He are produced in the detectors in the spallation of 12C nuclei by muons.
9Li
Antineutrino Rate
9Li and 8He decay with half-lives of tenths of seconds to β+n.
Can be identified by their time correlation with muons in the detector.
Jonathan Link
10/21/11

18. Signal to Background
(a)
(d)
(c)
(b)
(1%)
After all filters the background rates are small compared to a disappearance due to oscillations with sin22θ13 of 1%.
In addition, each background has a characteristic and distinct energy spectrum.
Jonathan Link
10/21/11

19. Ratio of Detector Efficiencies
Measuring sin22θ13
The measurement is a ultimately a ratio of observed inverse β-decay events in near and far detectors in initially one, but ultimately many energy bins (sampling a broad range of oscillation phases).
Proton Number Ratio
Ratio of Detector Efficiencies
sin22θ13
±0.3%
Calibration
±0.2%
Jonathan Link
10/21/11

20. Systematic and Statistical Errors
Source of Uncertainty
Chooz (absolute)
Daya Bay (relative)
Baseline
Goal
Goal w/Swapping
Number of Protons
0.8%
0.3%
0.1%
0.006%
Detector Efficiency
Energy Cuts
0.2%
Position Cuts
0.32%
0.0%
Time Cuts
0.4%
0.03%
H/Gd Ratio
1.0%
N Multiplicity
0.5%
0.05%
Trigger 0%
0.01%
Live Time
<0.01%
Total Detector Related Uncertainty
1.7%
0.38%
0.18%
0.12%
Background (per detector)
0.85%
<0.4%
Neutrino Flux
2.7%
0.13%
Signal Statistics
1.8%
Sensitivity to sin22θ13 (at 90% CL)
~13%
0.7%
0.6%
Total Detector Related Uncertainty
1.7%
0.38%
0.18%
0.12%
Jonathan Link
10/21/11

21. Sensitivity
Jonathan Link
10/21/11

22. Project Schedule and Status
October 2007: Official Ground Breaking
4 out of 8 detectors completed
August 2011: Hall 1 data taking begins
Detector installation underway in hall 2
Jonathan Link
10/21/11

23. Installation in Second Near Hall
Jonathan Link
10/21/11

24. Project Schedule and Status
October 2007: Official Ground Breaking
4 out of 8 detectors completed
August 2011: Hall 1 data taking begins
Detector installation underway in hall 2
Muon system installation underway in hall 3.
Jonathan Link
10/21/11

25. Muon Installation in the Far Hall
Jonathan Link
10/21/11

26. Project Schedule and Status
October 2007: Official Ground Breaking
4 out of 8 detectors completed
August 2011: Hall 1 data taking begins
Detector installation underway in hall 2
Muon system installation underway in hall 3.
Summer 2012: Start of data with full installation
Three years of data taking to reach sensitivity goal.
Jonathan Link
10/21/11

27. The Daya Bay Collaboration
Europe.:
Charles U., JINR, Kurchatov Institute
Asia:
Beijing Normal, Chengdu U. of Tech., CGNPE, CIAE, CUHK, Dongguan Polytech, IHEP Beijing, Nankai, Nanjing, National Chiao-Tung U., National Taiwan U., National United U., Shangdong U., SJTU, Shenzhen U., Tsinghua U., HKU, USTC, Zhongshan U.
U.S.:
BNL, Caltech, Cincinnati, George Mason, Houston, IIT, Iowa State, LBNL, Princeton, RPI, UC Berkeley UCLA, UIUC, Virginia Tech, William and Nary, Wisconsin