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Measuring sin 2 2 q 13 with the Daya Bay nuclear power reactors

Measuring sin 2 2 q 13 with the Daya Bay nuclear power reactors. Yifang Wang Institute of High Energy Physics. Neutrino oscillations: Pontecorvo-Maki-Nakagawa-Sakata Matrix. Mass eigenstates  Weak eigenstates. Atmospheric. CP phase. Solar. Sub-dominant q 13 oscillations.

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Measuring sin 2 2 q 13 with the Daya Bay nuclear power reactors

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  1. Measuring sin22q13 with the Daya Bay nuclear power reactors Yifang Wang Institute of High Energy Physics

  2. Neutrino oscillations: Pontecorvo-Maki-Nakagawa-Sakata Matrix Mass eigenstates  Weak eigenstates Atmospheric CP phase Solar Sub-dominant q13 oscillations cij=cosqij, sij=sinqij Majorana phases Only appear in 0nbb decays A total of 6 parameters: 2 Dm2, 3 angles, 1 phases + 2 Majorana phases

  3. Evidence of Neutrino Oscillations Unconfirmed: LSND: Dm2 ~ 0.1-10 eV2 Confirmed: Atmospheric: Dm2 ~ 210-3 eV2 Solar: Dm2 ~ 8 10-5 eV2 2 flavor oscillation in vacuum: P(n1->n2) = sin22qsin2(1.27Dm2L/E)

  4. A total of six n mixing parameters： Known： |D m232|, sin22q32 --Super-K D m221, sin22q21 --SNO,KamLAND Unknown：sin22q13，d, sign of Dm232 at reactors: Pee  1 sin22q13sin2 (1.27Dm213L/E)  cos4q13sin22q12sin2 (1.27Dm212L/E) at LBL accelerators: Pme ≈ sin2q23sin22q13sin2(1.27Dm223L/E) + cos2q23sin22q12sin2(1.27Dm212L/E)  A(r)cos2q13sinq13sin(d)

  5. Why at reactors • Clean signal, no cross talk with d and matter effects • Relatively cheap compare to accelerator based experiments • Can be very quick • Provides the direction to the future of neutrino physics Small-amplitude oscillation due to 13 Large-amplitude oscillation due to 12

  6. Current Knowledge of 13 Global fit fogli etal., hep-ph/0506083 Direct search PRD 62, 072002 Sin2(213) < 0.18 Sin2(213) < 0.09 Allowed region

  7. No good reason(symmetry) for sin22q13 =0 • Even if sin22q13 =0 at tree level, sin22q13 will not vanish at low energies with radiative corrections • Theoretical models predict sin22q13 ~ 0.1-10 % model prediction of sin22q13 Experimentally allowed at 3s level An experiment with a precision for sin22q13 less than 1% is desired

  8. How to do this experiment

  9. How Neutrinos are produced in reactors ? The most likely fission products have a total of 98 protons and 136 neutrons, hence on average there are 6 n which will decay to 6p, producing 6 neutrinos Neutrino flux of a commercial reactor with 3 GWthermal : 6 1020 /s

  10. Fission rate evolution with time in the Reactor Neutrino rate and spectrum depend on the fission isotopes and their fission rate

  11. Neutrino energy spectrum K. Schreckenbach et al., PLB160,325 A.A. Hahn, et al., PLB218,365 F  exp(a+bEn+cEn2) P. Vogel et al., PRD39,3378

  12. Reactor thermal power Fission rate depend on the thermal power Typically Known to ~ 1%

  13. Prediction of reactor neutrino spectrum • Three ways to obtain reactor neutrino spectrum: • Direct measurement • First principle calculation • Sum up neutrino spectra from 235U, 239Pu, 241Pu and 238U 235U, 239Pu, 241Pu from their measured b spectra 238U(10%) from calculation (10%) • They all agree well within 3%

  14. Inverse-β reaction of neutrinos in liquid scintillator t  180 or 28 ms(0.1% Gd) n + p  d + g (2.2 MeV) n + Gd  Gd* + g (8 MeV) Neutrino Event: coincidence intime, space and energy Neutrino energy: 10-40 keV 1.8 MeV: Threshold

  15. Reactor Experiment: comparing observed/expected neutrinos: • Palo Verde • CHOOZ • KamLAND Typical precision: 3-6%

  16. Palo Verde • 32 mwe • 12 ton, Gd loaded, scintillating target • 3 reactors: 11.6 GW • Baselines 890 m and 750 m • Expected rate of ~50 evts/day Palo Verde

  17. Chooz • • 5 ton, Gd loaded scintillator • 300 mwe • 2 reactors: 8.5 GWl • Baselines 1115 m and 998 m • Expected ~25 evts/day

  18. KamLAND 1000t scintillators Shielding: 3000 MWE/3m Water 180 km baseline Signal: ~0.5/day Eff. ~40% BK: corr.: ~0.001/day uncorr. ~0.01/day

  19. Lessons learned in order to have small systematic error • Energy threshold less than 0.9 MeV • Homogeneous detector • target mass well determined  rigid container • Target scintillator all from one batch, mixing procedures well controlled • Not too large detector (<50t ?) • Comprehensive calibration program • Background well controlled  good shielding • Be able to measure everything(Veto ineff., background, energy/position bias, …) • A lot of unforeseen effects will occur when looking at 0.1% level

  20. How to reach 1% precision ? • Three main types of errors: reactor related(~2-3%), background related (~1-2%) and detector related(~1-2%) • Use far/near detector to cancel reactor errors • Optimize baseline to have best sensitivity and reduce reactor related errors • Movable detectors, near far, to cancel part of detector systematic errors • Sufficient shielding to reduce backgrounds • Comprehensive calibration to reduce detector systematic errors • Careful design of the detector to reduce detector systematic errors • Large detector to reduce statistical errors

  21. Systematic error comparison

  22. xperi Proposed Reactor Neutrino Experiments Krasnoyasrk, Russia Chooz, France Braidwood, USA Kashiwazaki,Japan Young Gwang, South Korea Diablo Canyon, USA Daya Bay, China Angra, Brazil

  23. Currently Proposed sites/experiments

  24. Daya Bay nuclear power plant • 4 reactor cores, 11.6 GW • 2 more cores in 2011, 5.8 GW • Mountains near by, easy to construct a lab with enough overburden to shield cosmic-ray backgrounds

  25. DYB NPP region - Location and surrounding Convenient Transportation,Living conditions, communications 60 km

  26. Cosmic-muons at sea level: modified Gaisser formula

  27. Cosmic-muons at the laboratory

  28. Baseline optimization and site selection • Neutrino spectrum and their error • Neutrino statistical error • Reactor residual error • Estimated detector systematical error: total, bin-to-bin • Cosmic-rays induced background (rate and shape) taking into mountain shape: fast neutrons, 9Li, … • Backgrounds from rocks and PMT glass

  29. Best location for far detectors Rate only Rate + shape

  30. The Layout Far: 80 ton 1600m to LA, 1900m to DYB Overburden: 350m Muon rate: 0.04Hz/m2 • Total Tunnel length • 3200 m • Detector swapping • in a horizontal tunnel cancels most detector systematic error. Residual error ~0.2% • Backgrounds • B/S of DYB,LA ~0.5% • B/S of Far ~0.2% • Fast Measurement • DYB+Mid, 2008-2009 • Sensitivity (1 year) ~0.03 • Full Measurement • DYB+LA+Far, from 2009 • Sensitivity (3 year) <0.01 LA: 40 ton Baseline: 500m Overburden: 98m Muon rate: 0.9Hz/m2 0% slope 0% slope Mid: Baseline: ~1000m Overburden: 208m Waste transport portal 0% slope DYB: 40 ton Baseline: 360m Overburden: 97m Muon rate: 1.2Hz/m2 Access portal 8% slope

  31. Geologic survey completed, hole boring will start soon far Faults(small) near mid Weathering bursa (风化囊） near

  32. From mid site to far site: a fault From Daya near site to mid point: Weathering bursa

  33. Tunnel construction • The tunnel length is about 3000m • Local railway construction company has a lot of experience (similar cross section) • Cost estimate by professionals, ~ 3K $/m • Construction time is ~ 15-24 months • A similar tunnel on site as a reference

  34. How large the detectorshould be ?

  35. Detector: Multiple modules • Multiple modules for cross check, reduce uncorrelated errors • Small modules for easy construction, moving, handing, … • Small modules for less sensitive to scintillator aging • Scalable • Higher cost • More trouble with calibration Two modules at near sites Four modules at far site: Side-by-side cross checks

  36. Central Detector modules • Three zones modular structure: I. target: Gd-loaded scintillator II. g-ray catcher: normal scintillator III. Buffer shielding: oil • Reflection at two ends • 20t target mass, ~200 8”PMT/module sE = 5%@8MeV, ss ~ 14 cm I III II Oil buffer thickness g Catcher thickness

  37. Simulated energy resolution Simulated position resolution

  38. Why three zones ? Capture on Gd Capture on H • Three zones: • Complicated acrylic tank construction • g backgrounds on walls • Less fiducial volume • Two zones: • Neutrino energy spectrum distorted • Neutron efficiency error due to energy scale and resolution: two zones: 0.4%, three zones 0.2% • Using 4 MeV cut can reduce the error by a factor of two, but backgrounds from b+g do not allow us to do so 3 zone 2 zone cut cut

  39. Gd-loaded liquid Scintillator ---Technical chanllenge • It can reduce significantly backgrounds(short capture time, high capture energy) • Problem: Aging Palo Verde: 0.03%/day Chooz: 0.4%/day • Light yield: 55% antracene • Attenuation length: 11m

  40. Water Buffer & VETO • 2m water buffer to shield backgrounds from neutrons and g’s from lab walls • Cosmic-muon VETO Requirement: • Inefficiency < 0.5% • known to <0.25% • Solution: Two active vetos • active water buffer, Eff.>95% • Muon tracker, Eff. > 90% • RPC • scintillator strips • total ineff. = 10%*5% = 0.5% Neutron background vs water shielding thickness 2m water

  41. Water pool as the buffer • Safe • cheap tunnel

  42. Two tracker options : • RPC outside the steel cylinder • Scintillator Strips sink into the water RPC from IHEP Scintillator Strips from Ukraine Contribution of JINR,Dubna

  43. Background related error • Need enough shielding and an active veto • How much is enough ?  error < 0.2% • Uncorrelated backgrounds: U/Th/K/Rn/neutron single gamma rate @ 0.9MeV < 50Hz single neutron rate < 1000/day 2m water + 50 cm oil shielding • Correlated backgrounds: n  Em0.75 Neutrons:>100 MWE + 2m water Y.F. Wang et al., PRD64(2001)0013012 8He/9Li: > 250 MWE(near) & >1000 MWE(far) T. Hagner et al., Astroparticle. Phys. 14(2000) 33

  44. Precision to determine the 9Li background in situ Spectrum of accidental background Fast neutron spectrum

  45. Background estimated by GEANT MC simulation

  46. Calibration • Radioactive Source 137Cs, 22Na, 60Co, 54Mn, 65Zn , 68Ge, Am-Be 252Cf, Am-Be • Gamma generator p+19F→ α+16O*+6.13MeV; p+11B→ α+8Be*+11.67MeV • Backgrounds 40K, 208Tl, cosmic-induced neutrons, Michel’s electrons, … • LED calibration KI & CIAE Hong Kong

  47. Sensitivity to Sin22q13 • Reactor-related correlated error: sc ~ 2% • Reactor-related uncorrelated error: sr ~ 1-2% • Calculated neutrino spectrum shape error: sshape ~ 2% • Detector-related correlated error: sD ~ 1-2% • Detector-related uncorrelated error: sd ~ 0.5% • Background-related error: • fast neutrons: sf ~ 100%, • accidentals: sn ~ 100%, • isotopes(8Li, 9He, …) : ss ~ 50-60% • Bin-to-bin error: sb2b ~ 0.5% Many are cancelled by the near-far scheme and detector swapping

  48. Sensitivity to Sin22q13 Other physics capabilities: Supernova watch, Sterile neutrinos, …

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