First Result of NEOSNeutrino Experiment for Oscillation at Short baseline
Kim Siyeon
Chung-Ang University
Seoul, Korea
On behalf of NEOS Collaboration
July 15, 2017 ICRC 2017, Busan 1
First Result of NEOS
• NEOS Publication• Y. J. Ko et al., Sterile neutrino search at the NEOS experiment, Phys. Rev. Lett. 118(12),
121802 (2017)
• Y. J. Ko et al., Comparison of fast neutrino rates for the NEOS experiment, J. Korean Phys. Soc. 69(11), 1651 (2016)
• B. R. Kim et al., Pulse shape discrimination capability of metal-loaded organic liquid scintillators for a short-baseline reactor neutrino experiment, Phys. Scripta 90(5), 055302 (2015)
• B. R. Kim et al., Development and mass production of a mixture of LAB- and DIN-based gadolinium loaded liquid scintillator for the NEOS experiment, J. Radioanal. Nucl. Chem. 310(1), 311 (2016)
• More papers on analysis and instruments are in progress.
• Collaborators: 20 in 7 institutes• (1) Joo, K., Kim, B.R., (2) Ko, Y.J., Jang, C.H., Jeon, (3) E. J., Kim, Y., Lee, J.,
Lee, J.Y., Oh, Y., Park, H.K., Park, K.S., (4) Han B., Sun, K.M., (5) Park, H., (6) Kim, H., Lee, J., (7) Kim, H.S., Kim, J., and Seo, K.M.
July 15, 2017 ICRC 2017, Busan 2
NEOS: Motivation and Purpose
• Reactor Anomalies
• Flux deficit
• 5-MeV excess
July 15, 2017 ICRC 2017, Busan 3
RENO
Mention et al, 2011
NEOS: Motivation and Purpose
• Reactor Anomalies
• Flux deficit
• 5-MeV excess
July 15, 2017 ICRC 2017, Busan 4
• To scan the possibility of the existen
ce of a sterile neutrino of the mass
~1 eV
• To analyze the spectral shape
(not the absolute rate) of react
or antineutrino
NEOS baseline 24 m
• Single detector needs
- an accurate reference reactor a
ntineutrino flux/spectrum
- understanding the detector resp
onse is crucial, especially energ
y-charge relation
Experiment Site:
July 15, 2017 ICRC 2017, Busan 5
Habit Nuclear Power Plant in Younggwang, Korea
- 2.8-GWth commercial reactor
- Core size: 3.1-m diameter and 3.8-m height
- Low enriched Uranium fuel (4.6% 235U)
Detector in Tendon Gallery
- ~24-m baseline and ~20-m.w.e overburden
ICRC 2017
1 2 3 4 5 6✦
Sep '15 Nov '15 Jan '16 Mar '16 May '16
DA
Q L
ivetim
e (
%)
100
80
60
40
20
Neutrino runs Test/Calibration
•46 days’ reactor off data + 180 day’s reactor on data + calibration
- stable reactor operation, ~90% DAQ efficiency
Livetime + IBD event rate
July 15, 2017 ICRC 2017, Busan 6
Sep '15 Nov '15 Jan '16 Mar '16 May '16
DA
Q L
ivetim
e (
%)
100
80
60
40
20
Neutrino runs Test/Calibration
•46 days’ reactor off data + 180 day’s reactor on data + calibration
- stable reactor operation, ~90% DAQ efficiency
July 15, 2017 ICRC 2017, Busan 7
Livetime + IBD event rate
Active Reactor Core
• 177 fuel rods, low enriched (4.65%) uranium-235 (LEU) fuel
• Refueling by changing 1/3 of fuel rods for each burn-up cycle (~1.5 year)
• Active core size: 3.1 m (φ), 3.8 m (h)
10000 15000
Burn-up [MTU/MWD]0 5000
Fis
sio
nF
ractio
n
0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1 U-238
July 15, 2017 ICRC 2017, Busan 8
Mean 23.64
Std Dev 0.8146
26L [m]
22 23 24 25
p.d
.f.
(20 m.w.e) 0
overburden
0.1
0.2
0.3
0.4Mean S
td Dev
23.64
0.8146
Detector Location / Baseline
~8 m
NEOS Detector
Active Core
Tendon Gallery
Entrance / Exit
Neutrino travel distance distribution:
- Beta decays uniformly generated in cyli
ndrical core
- IBD interaction points uniformly distribut
ed in target
- Distance-weighted effect: 1/L2
July 15, 2017 ICRC 2017, Busan 9
July 15, 2017 ICRC 2017, Busan 10
Construction
• Homogeneous LS target
— 1008 L volume
(R 51.5, H 121) cm
— LAB+UG-F (9:1)
— 0.5% Gd loaded for high neutr
on capture efficiency
— 38 8″ PMT in mineral oil buffer
• Shieldings
— 10 cm B-PE (n), 10 cm Pb (γ)
— active muon counter
• Data AcQuisition
— 500 MS/s FADC (waveform)
— 62.5 MS/s SADC (μ veto)
• Source calibration through chi
mney
Linear Alkyl
Benzene
5T PTFE reflectors
Detector Specification
July 15, 2017 ICRC 2017, Busan 11
IBD Reconstruction
Selection rules developed for best S/N ratio:
• Prompt energy: 1-10 MeV
• Delayed energy: 4-10 MeV
• Time coincidence: 1-30 μs
• Multiplicity: no event before (after) 30 (150)
μs from the prompt/delayed pair
• Muon veto: 150 μs
• Pulse shape discrimination
30 40 50 60
Neutron Capture Time [µs]0 10 20
Counts
/day/0
.8µ
s
0
50
100
Reactor ON
Reactor OFF
7 8 9
Delayed Energy [MeV]4 5 6
Counts
/day/0
.1M
eV
0
20
60Reactor ON
Reactor OFF
40
4 5 6 7 8 9
Recontructed Energy [MeV]1 2 3
Co
un
ts/s
ec/1
0ke
V
10−2
10−3
10−4
10−1
1 Target Single Events
All: Reactor OFF
All: Reactor ON
Muon veto cut:Reactor OFF
Muon veto cut:Reactor ON
July 15, 2017 ICRC 2017, Busan 12
S1+S2+background
S2
Pulse Shape Discrimination
• IBD events vs. fast neutron’s scattering events
•Qtail/Qtot or meantime of waveform.
•LAB + DIN (UG-F) mixed LS.
•More than 70 % of IBD background could be reduced after PSD.
5
p ( t )psd
−10 −5 0 10 15 20E
ve
nts
/da
y
10−1
1
10
102 Reactor OFF
Reactor ON
Fast n
e, γ
July 15, 2017 ICRC 2017, Busan 13
Prompt Energy Spectrum
01 2 3 4 5 6
Prompt Energy [MeV]
• Signal / Noise ~ 22
• vs H-M: 5 MeV bump, not so practical to do the oscillation analysis
• vs Daya Bay: bump not totally disappeared, fission fraction difference
• Systematic uncertainties for the shape analysis:
reference spectrum > energy recon. >> background fluc., S2 cut efficiency …
7 ⋅ ⋅ 10
Events
/day/1
00
ke
V
1.0
100.9
Systematic total
20
30
40
50
60Data signal (Sneos=ON-OFF)
Data background (OFF)
MC no oscillation (Φhm⋅σ )Vogel
MC no oscillation (Sdyb)
1 2 3 4 5 6 7 10
Prompt Energy [MeV]
Da
ta/P
red
iction
0.9
1.0
1.1
Sneos / MC(Φhm⋅σ )
Systematic totalVogel
4 5
Prompt Energy [MeV]1 2 3 6 7 10
Data
/Pre
dic
tion
1.1 S / MC(S )dybneos
July 15, 2017 ICRC 2017, Busan 14
• c2 minima are found at
(sin22θ, Δm2)=(0.04, 1.3 eV2),
(0.05, 1.73 eV2)
with Δ c2 = c2 3n− c2 4n = 6.5.
• p-value ~ 22%.
• No strong sign of active-to-sterile
n oscillation.
Prompt Energy [MeV]
1 2
(2.32 eV2, 0.142)
3 4 5 6 7 ⋅ ⋅ 10
Da
ta/P
red
ictio
n
0.9
6000
1.0
4000
(1.73 eV2, 0.050)
1.1NEOS/Daya Bay
Systematic total(c)
July 15, 2017 ICRC 2017, Busan 153ν 4ν
χ2 -χ2
00 2 4 6 8 10 12 14
En
trie
s(a
.u.)
2000
8000
10000
MC pseudo-experiment
no sterile neutrino.
χ2 dist. (NDF=2)
Active-to-Sterile Oscillation
• Exclusion limits
• Raster scan with c2 distribution
• Exclude Δm2 < 3 eV2,
• Rule out models, including RAA (reactor antine
utrino anomaly) and recent global analyses.
July 15, 2017 ICRC 2017, Busan 16
Active-to-Sterile Oscillation
Thank you.
NEOS Collaboration
July 15, 2017 ICRC 2017, Busan 17
Backup
July 15, 2017 ICRC 2017, Busan 18
Non-Linear Charge Response to Energy
• Signal quenched at low energy
• Cerenkov light important at higher energy
Source position
(Φ) target center
(P) PMT
(O) outside
(Ch) calibraion chimney
4 5 6 7 8
True Gamma Energy [MeV]
1 2 3
Cha
rge/E
ne
rgy [
pC
/Me
V]
450
500
550
PoBe(Ch)
137Cs(Φ)
40K(P)
60Co(Φ)
n-H(Φ)
Tl(O)208
PoBe(Ch)
NEOS Preliminary@ Neutrino 2016
July 15, 2017 ICRC 2017, Busan 19
NEOS status•NEOS is over due to regular maintenance of the tendon gallery (once every 5
years)
•No strong evidence of oscillation from active-to-(~1 eV) sterile mixing.
•Tendon gallery is a nice place for reactor neutrino experiment, but
•Severe escaping γ effect with a small volume detecter without γ catcher.
•Measurement of 1 whole burn-up cycle should help to understand the spectrum
in detail.
•Rate analysis, Absolute spectrum,
235U
P1239Pu
238U241Pu
NEOS
P2
21July 15, 2017 ICRC 2017, Busan 20
Dedicated room for neutrino experiments
ground
level
active core
experimental
room (proposed)
NEOS
@ tendon
gallery
Proposed
new room
core diameter 312.4 cm 365 cm
core height 381 cm
thermal power 2815 MW 3983 MW
distance to core 24 m ~18 m *
overburden >20
muon rate ~0.2 cm-2 min-1 *
rectangular space
(WxDxH m3)4 x 2.5 x
3.3
8 x 5 x 4 *
# of IBD events
to be detected (
NEOS detector)
~2000
per day
~5000
per day *
July 15, 2017 ICRC 2017, Busan 21