1. 2nd Sino-French Workshop on the Dark Universe Changgen Yang Institute of High Energy Physics, Beijing for the Daya Bay Collaboration Daya Bay Reactor Neutrino Experiment 2nd Sino-French Workshop on the Dark Universe August 31, 2006

2.  13  The Last Unknown Neutrino Mixing Angle U MNSP Matrix ? atmospheric, K2K reactor and accelerator 0  SNO, solar SK, KamLAND  12 ~ 32°  23 = ~ 45°  13 = ? ? What is  e fraction of 3 ? U e3 is a gateway to CP violation in neutrino sector: P(   e ) - P(     e )  sin(2  12 )sin(2  23 )cos 2 (  13 )sin(2  13 )sin 

3. Current Knowledge of  13 Direct search At  m 2 31 = 2.5  10  3 eV 2, sin 2 2   < 0.15 allowed region Fogli etal., hep-ph/0506083 Sin 2 (2  13 ) < 0.09 Sin 2 2  13 < 0.18 Best fit value of  m 2 32 = 2.4  10  3 eV 2 Global fit

4. Daya Bay: Goals And Approach Utilize the Daya Bay nuclear power facilities to: - determine sin 2 2  13 with a sensitivity of 1% - measure  m 2 31 Adopt horizontal-access-tunnel scheme: - mature and relatively inexpensive technology - flexible in choosing overburden and changing baseline - relatively easy and cheap to add experimental halls - easy access to underground experimental facilities - easy to move detectors between different locations with good environmental control. Employ three-zone antineutrino detectors.

5. How To Reach A Precision of 0.01 ? Powerful nuclear plant Larger detectors “Identical” detectors Near and far detectors to minimize reactor-related errors Optimize baseline for best sensitivity and smaller residual reactor-related errors Interchange near and far detectors – cancel many detector systematic errors Sufficient overburden/shielding to reduce background Comprehensive calibration/monitoring of detectors

6. Ling Ao II NPP: 2  2.9 GWth Ready by 2010-2011 Ling Ao NPP: 2  2.9 GW th Daya Bay NPP: 2  2.9 GW th 1 GW th generates 2 × 10 20  e per sec 55 km 45 km The Daya Bay Nuclear Power Facilities 12th most powerful in the world (11.6 GW) Top five most powerful by 2011 (17.4 GW) Adjacent to mountain, easy to construct tunnels to reach underground labs with sufficient overburden to suppress cosmic rays

7. Where To Place The Detectors ? Place near detector(s) close to reactor(s) to measure raw flux and spectrum of  e, reducing reactor-related systematic Position a far detector near the first oscillation maximum to get the highest sensitivity, and also be less affected by  12 Since reactor  e are low-energy, it is a disappearance experiment: Large-amplitude oscillation due to  12 Small-amplitude oscillation due to  13 integrated over E near detector far detector Sin 2   = 0.1  m 2 31 = 2.5 x 10 -3 eV 2 Sin 2   = 0.825  m 2 21 = 8.2 x 10 -5 eV 2

8. Daya Bay NPP Ling Ao NPP Ling Ao-ll NPP (under const.) Entrance portal Empty detectors: moved to underground halls through access tunnel. Filled detectors: swapped between underground halls via horizontal tunnels. Total length: ~2700 m 200 m (46% slope) 296 m (~10% slope) 730 m 570 m 910 m Daya Bay Near 360 m from Daya Bay Overburden: 97 m Ling Ao Near 500 m from Ling Ao Overburden: 98 m Far site 1600 m from Ling Ao 2000 m from Daya Overburden: 350 m Mid site ~1000 m from Daya Overburden: 208 m Baseline and site selection

9. Geophysical profile (Daya–mid--far)

10. Bore Samples Zk4 (depth: 133 m) Zk2 (depth: ~180 m) Zk3 (depth: ~64 m) Zk1 (depth: 210 m) At tunnel depth

11. Findings of Geotechnical Survey No active or large fault Earthquake is infrequent Rock structure: massive and blocky granite Rock mass: most is slightly weathered or fresh Groundwater: low flow at the depth of the tunnel Quality of rock mass: stable and hard Good geotechnical conditions for tunnel construction

12. Detecting Low-energy  e  e  p  e + + n (prompt)  + p  D +  (2.2 MeV) (delayed)  + Gd  Gd*  Gd +  ’s (8 MeV) (delayed) Time- and energy-tagged signal is a good tool to suppress background events. Energy of  e is given by: E   T e+ + T n + (m n - m p ) + m e+  T e+ + 1.8 MeV 10-40 keV The reaction is the inverse  -decay in 0.1% Gd-doped liquid scintillator: Arbitrary Flux Cross Section Observable Spectrum From Bemporad, Gratta and Vogel 0.3b 50,000b

13. What Target Mass Should Be? Systematic error Black : 0.6% DYB: B/S = 0.5% LA: B/S = 0.4% Far: B/S = 0.1%  m 2 31 = 2  10 -3 eV 2 tonnes (3 year run) Red : 0.25% (baseline goal) Blue : 0.12%

14. Design of Antineutrino Detectors Three-zone structure: I. Target: 0.1% Gd-loaded liquid scintillator II. Gamma catcher: liquid scintillator, 45cm III. Buffer shielding: mineral oil, ~45cm Possibly with diffuse reflection at ends. ~200 PMT’s around the barrel: Isotopes (from PMT) Purity (ppb) 20cm (Hz) 25cm (Hz) 30cm (Hz) 40cm (Hz) 238 U(>1MeV)502.72.01.40.8 232 Th(>1MeV)501.20.90.70.4 40 K(>1MeV)101.81.30.90.5 Total5.74.23.01.7 Oil buffer thickness buffer 20 tonne s Gd-LS gamma catcher

15. Why three zones ? 3-ZONE2-ZONE n capture on Gd yields 8 MeV with 3-4  ’ s Chooz background 3 zones provides increased confidence in systematic error associated with detection efficiency and fiducial volume 2 zones implies simpler design/construction, some cost reduction but with increased risk to systematic error

16. Gd-loaded Liquid Scintillator For Daya Bay Absorbance at 430 nm Calendar Date 507 days (1.2% Gd in PC) 455 days (0.2% Gd in PC) 367 days (0.2% Gd in 20% PC + 80% C 12 H 26 ) 130 days (0.2% Gd in LAB) Require stable Gd-loaded liquid scintillator with - high light yield - long attenuation length BNL/IHEP/JINR nuclear chemists study on metal-loaded liquid scintillator (~1% Gd diluted to ~0.1% Gd) for Daya Bay: - technology of 1% Gd in pseudocumene (PC) is mature - need R&D for 1% Gd in mixture of PC and dodecane, and with linear alkyl benzene (LAB) Attenuation lengths > 15 m BNL samples

17. Design of Active Shield-Muon Veto Detector modules enclosed by 2 m of water to shield neutrons produced by cosmic-ray muons and gamma-rays from the surrounding rock Water shield also serves as a Cherenkov veto for tagging muons Augmented with a muon tracker: scintillator or RPCs Combined efficiency of Cherenkov and tracker > 99.5% 2 m of water Neutron background vs thickness of water Fast neutrons per day water thickness (m) 0.05 0.10 0.15 0.20 0.25 0.30 0. 1.2. water muon tracker rock a conceptual design

18. Design of Active Shield Muon Veto Reduce ambient radiation, e.g. 222Rn, in the air or dust from entering the detector Compact muon veto systems Easier to perform calibration with more open space. Large volume of water helps to keep the temperature of the detector relative stable PMT's for water Cherenkov tunnel

19. Experimental Hall (Conceptual Design) Gas room Counting room Water Purif.

20. Tunnel

21. Main Portal (CD)

22. Prototype setup at IHEP LED Cables Flange to put Source Purposes: Test reflection, energy resolution, LS performance … Inner acrylic vessel: 1m in diameter and 1m tall, filled with normal liquid scintillator(70% mineral oil + 30% mesitylene). Outer stainless steel vessel: 2m in diameter and 2m tall, filled with mineral oil. PMTs mounted and immerged in oil. 45 MACRO PMT, 15 PMT/Ring

24. 137 Cs 0.662MeV gamma, 164 p.e.; Energy resolution 10%. 60 Co 2.506MeV gamma, 666 p.e.; Energy resolution 5.6%. Preliminary

25. ~350 m ~97 m ~98 m ~210 m Cosmic-ray Muon Apply modified Gaisser parametrization for cosmic-ray flux at surface Use MUSIC and mountain profile to estimate muon flux & energy DYBLAMidFar Elevation (m)9798208347 Flux (Hz/m 2 )1.20.730.170.045 Mean Energy (GeV) 556097136

26. Summary of Background Near SiteFar Site Radioactivity (Hz)<50 Accidental B/S<0.05% Fast neutron background B/S0.15%0.1% 8 He/ 9 Li B/S 0.41% ± 0.18% 0.02% ± 0.08% Use a modified Palo Verde-Geant3-based MC to model response of detector: (neutrino signal rate 560/day 80/day) Further rejection of background may be possible by cutting showering muons.

27. Summary of Systematic Errors Baseline: currently achievable relative uncertainty without R&D Goal: expected relative uncertainty after R&D Absolute measurement Relative measurement → 0 → 0.006 → 0.06% w/Swapping → 0 Swapping: can reduce relative uncertainty further 3-zone design Multiple, identical detectors/siteOverburden/shielding

28. 90% confidence level 2 near + far (3 years) near (40t) + mid (40 t) 1 year Near-mid Use rate and spectral shape Sensitivity of Daya Bay in sin 2 2  13 Daya Bay near hall (40 t) Tunnel entrance Ling Ao near hall (40 t) Far hall (80 t)

29. Summary The Daya Bay nuclear power facility in China and the mountainous topology in the vicinity offer an excellent opportunity for carrying out a reactor neutrino program using horizontal tunnels. The Daya Bay experiment has excellent potential to reach a sensitivity of 0.01 for sin 2 2  13. The Daya Bay Collaboration continues to grow. Will complete detailed design of detectors, tunnels and underground facilities in 2006. Plan to commission the Fast Deployment scheme in 2009, and Full Operation in 2010.