Observation of Reactor Antineutrinos
at RENO
Soo-Bong Kim for the RENO Collaboration
KNRC, Seoul National University
March 29, 2012Outline Introduction
 Experimental setup & detector
 Data-taking & data set
 Calibration
 Event selection
 Efficiency & Background Reactor antineutrino prediction
 Results
 Systematic uncertainties
 SummaryNeutrino Oscillation
PMNS Neutrino Mixing
Angles and CP Violation

Reactor Antineutrino Oscillation

ν1

θ12

θ23

θ13(?)

ν2

ν3

Past Efforts for Finding θ13
 Chooz (2003) & Palo Verde (2000): No signalsin2(2θ13) < 0.12 at 90% C.L.
 T2K : 2.5 σ excess (2011)
0.03 < sin2(2θ13) < 0.28 at 90% C.L. for N.H.

 Daya Bay (03. 08. 2012)
5.2 σ observationsin2(2θ13) = 0.092 ±0.016(stat.)±0.005(syst.)

0.04 < sin2(2θ13) < 0.34 at 90% C.L. for I.H.
 MINOS : 1.7 σ excess (2011)
0 < sin2(2θ13) < 0.12 at 90% C.L. for N.H.
0.04 < sin2(2θ13) < 0.19 at 90% C.L. for I.H.
 Double Chooz : 1.7 σ measurement (2011)
sin2(2θ13) = 0.086 ± 0.041(stat.) ± 0.030(syst.)

RENO Collaboration(12 institutions and 40 physicists)
 Chonbuk National University
 Chonnam National University
 Chung-Ang University
 Dongshin University
 Gyeongsang National University
 Kyungpook National University
 Pusan National University
 Sejong University
 Seokyeong University
 Seoul National University
 Seoyeong University
 Sungkyunkwan University

RENO Experimental Setup
Near Detector

Far Detector

Contribution of Reactor to Neutrino Fluxat Near & Far Detectors
Reactor #Far ( % )Near (% )

1

13.73

6.78

2

15.74

14.93

3

18.09

34.19

4

18.56

27.01

517.8011.50616.08

5.58

 Accurate measurement of baseline distances to a precision of10 cm using GPS and total station
 Accurate determination of reduction in the reactor neutrino
fluxes after a baseline distance, much better than 0.1%

RENO Detector

 354 ID +67 OD 10” PMTs
 Target : 16.5 ton Gd-LS, R=1.4m, H=3.2m
 Gamma Catcher : 30 ton LS, R=2.0m, H=4.4m
 Buffer : 65 ton mineral oil, R=2.7m, H=5.8m
 Veto : 350 ton water, R=4.2m, H=8.8m

Summary of Detector Construction
 2006. 03 : Start of the RENO project 2008. 06 ~ 2009. 03 : Civil construction including tunnel excavation
 2008. 12 ~ 2009. 11 : Detector structure & buffer steel tanks
completed 2010. 06 : Acrylic containers installed 2010. 06 ~ 2010. 12 : PMT test & installation
 2011. 01 : Detector closing/ Electronics hut & control room built
 2011. 02 : Installation of DAQ electronics and HV & cabling 2011. 03 ~ 06 : Dry run & DAQ debugging
 2011. 05 ~ 07 : Liquid scintillator production & filling 2011. 07 : Detector operation & commissioning 2011. 08 : Start data-taking

PMT Mounting (2010. 8~10)

PMT Mounting (2010. 8~10)

Detector Closing (2011. 1)

Near : Jan. 21, 2011

Far : Jan. 24, 2011

Detection of Reactor Antineutrinos(prompt signal)

γ (0.511MeV)e- Neutrino energy measurementγ (0.511MeV)e+ν e(delayed signal)~180 µs+ p  D + γ (2.2 MeV)
~28 µs
(0.1% Gd) + Gd  Gd + γ‘s (8 MeV)

prompt signal

pn
Delayed signal

30μ sγγ∑ Eγ ~ 8MeVGdγγ

CnH2n+1-C6H5 (n=10~14)

Gd Loaded Liquid Scintillator
 Recipe of Liquid Scintillator
Aromatic Solvent &FlourWLS

Gd-compound

LAB

PPO +
Bis-MSB

0.1% Gd+(TMHA)3(trimethylhexanoic acid)* Stable light yield over the time period : ~250 pe/MeV* Measured cosmic induced
neutron’s Gd capture timeLiquid(Gd-LS/LS/MO/Water) Production & Filling(May-July 2011)

Gd-LS filling for Target
Gd Loaded Liquid Scintillator

Water filling for Veto

Loaded Liquid
Scintillator
LS filling forGdGammaCatcher

1D/3D Calibration System Calibration system to deploy
radioactive sources in 1D &3D directions
 Radioactive sources :137Cs, 68Ge, 60Co, 252Cf
 Laser injectors

Data Acquisition System• 24 channel PMT input to ADC/TDC• 0.1pC, 0.52nsec resolution• ~2500pC/ch large dynamic range• No dead time (w/o hardware trigger)• Fast data transfer via Ethernet R/WData-Taking & Data Set
 Data taking began on Aug. 1, 2011
with both near and far detectors.

 Data-taking efficiency

 Data-taking efficiency > 90%.
 Trigger rate at the threshold energyof 0.5~0.6 MeV : 80 Hz Data-taking period : 213 days
Aug. 11, 2011 ~ Mar. 10, 2012
Event rate before reduction
208Tl

A candidate for aneutron capture
by Gd

40K

2 MeV

n capture by Gd

6 MeV

10 MeVPMT Threshold & Gain Matching PMT gain : set 1.0x107 using a Cs source at center
 Gain variation among PMTs : 3% for both detectors.

discri. thr.
Gain (10 7)

 PMT threshold : determined bya single photoelectron responseusing a Cs source at the center-0.4mV-0.5mV-0.6mV-0.7mV-1.0mVCharge(counts)

Energy Calibration
Cs 137(662 keV)Co 60(2,506 keV)

-Near Detector-Far Detector

Ge 68(1,022 keV)

Cf 252(2.2/7.8 MeV)

Energy Scale Calibration
Near Detector

Far Detector
CfCfCfCsCo

CfGeCsδE =CoGe

5.9%+ 1.2%E ( MeV ) ~ 250 pe/MeV (sources at center) Identical energy response (< 0.1%) of ND & FD
 Slight non-linearity observedDetector Stability & Identity- Near Detector- Far Detector

 IBD candidate’s delayed signals (capture on Gd)

Delayed Energy

Capture Time Cosmic muon induced neutron’s capture by H- Near Detector- Far Detector

IBD Event Signature and Backgrounds
 IBD Event Signature Prompt signal (e+) : 1 MeV 2γ’s + e+ kinetic energy (E = 1~10 MeV)
 Delayed signal (n) : 7.8 MeV γ’s from neutron’s capture by Gd~28 µs (0.1% Gd) in LS
Prompt Energy

Delayed Energy Backgrounds Random coincidence between prompt and delayed signals (uncorrelated) 9Li/8He β-n followers produced by cosmic muon spallation
 Fast neutrons produced by muons, from surrounding rocks and insidedetector (n scattering : prompt, n capture : delayed)

IBD Event Selection
 Reject flashers and external gamma rays : Qmax/Qtot < 0.03
 Muon veto cuts : reject events after the following muons(1) 1 ms after an ID muon with E > 70 MeV, or with 20 < E < 70
MeV and OD NHIT > 50
(2) 10 ms after an ID muon with E > 1.5 GeV
 Coincidence between prompt and delayed signals in 100 µs- Eprompt : 0.7 ~ 12.0 MeV,Edelayed : 6.0 ~ 12.0 MeV- coincidence : 2 µs < ∆te+n < 100 µs
 Multiplicity cut :100 ms window

reject pairs if there is a trigger in the preceding

Random Coincidence Backgrounds
 Calculation of accidental coincidence(N accidental = N delayed × 1 − exp[ − R prompt ( Hz )×∆T ( s )])N accidental±N delayed ∆T = 100 ms time window Near detector :Rprompt = 8.8 Hz, Ndelay = 5100/day → BG
 Far detector :Rprompt = 10.7 Hz, Ndelay = 674/day → BG

near= 4.51 ± 0.06 / dayaccidental

far= 0.72 ± 0.03 / day
accidental

9Li/8He β-n Backgrounds
 Find prompt-delay pairs after muons, and obtain their time interval
distribution with respect to the preceding muon.
 9Li time interval distribution

Time interval (ms)

 Near detector :near
BG= ± / dayLi / He Far detector :far
BG= ± / dayLi / He

 9Li energy spectrum

Energy (MeV)Fast Neutron Backgrounds
 Obtain a flat spectrum of fast neutron’s scattering with proton,
above that of the prompt signal.
Far detector

Near detector

near=neutron±/ dayfar=neutron±/ day Near detector :BG

 Far detector :BG

Spectra & Capture Time of Delayed Signals
Near Detector
 Observed spectra of IBD delayed signals

τ = 27.8 ± 0.2 µsec

Near Detector
Far Detector

Far Detector
τ = 27.6 ± 0.4 µsec

Measured Spectra of IBD Prompt Signal

Near Detector
148387

Far Detector
16306

Expected Reactor Antineutrino Fluxes Reactor neutrino flux

PthΦ ( Eν ) = isotopes∑ fi ⋅ Eiisotopes∑ f ⋅ φ ( Eν )iiii- Pth : Reactor thermal power provided by the YG nuclear power plant- fi : Fission fraction of each isotope determined by reactor coresimulation of Westinghouse ANC- φi(Eν) : Neutrino spectrum of each fission isotope[* P. Huber, Phys. Rev. C84, 024617 (2011)
T. Mueller et al., Phys. Rev. C83, 054615 (2011)]- Ei : Energy released per fission[* V. Kopeikin et al., Phys. Atom. Nucl. 67, 1982 (2004)]

Observed Daily Averaged IBD Rate

Reduction of Systematic Uncertainties
 Detector related :- “Identical” near and far detectors- Careful calibration
 Reactor related :- Relative measurements with near and far detectors

Neutrino
flux

1/r2

Numberofprotons

Detection
efficiency

Yield of sin2(2θ13)

Efficiency & Systematic Uncertainties

Summary RENO started data taking with both near and far detectors from
August 1, 2011..
 RENO will published their first result soon.
- Rapid detector construction, data-taking & data-analysis- Satisfactory detector performance.
- Detector calibration and comparison of ND & FD are performed.
- IBD selection and background estimation completed.
- Detection efficiency and systematic uncertainty understood.
 RENO started to get an oscillation signal from the late 2011, andhas been under a sanity check and an accurate backgroundestimation based on the reported schedules of the otherexperiments.
A bright future for the neutrino physics due to a large value of θ13 !!!