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Introduction NUCIFER experiment STEREO experiment Conclusions
The NUCIFER and STEREO experimentsSearch for light sterile neutrino state at short baseline
Maxime Péquignoton behalf of the Nucifer and Stereo collaborations
CEA Saclay - IRFU/SPhN
September 12, 2014
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 1 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Reactor antineutrino anomaly
Reevaluation of reactor ν̄e spectra, Th. A. Mueller et al., Phys. Rev.C 83, 054615.Reanalysis of short baseline experiments, G. Mention et al., Phys. Rev. D 83, 073006
⇒ neutrino deficit of 6.5%
Error on the flux prediction ?New oscillations toward a sterile neutrino ? (∆m2 & 0.5 eV2, sin2(2θ) ∼ 0.1)
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 2 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Reactor antineutrinos and detection
Beta decay of the fission products :
n→ p + e− + ν̄e
Intense and pure production of electronicantineutrinos→ ∼ 1,9 x 1020 νe/s/GWth
Delayed signal (µs): Edelayed ~ 8 MeV on Gadolinium
Prompt signal (ns): Eprompt a En (1 - 8 MeV)
ne
p n
e+ e-
Gd
g
g
g
g Detection with the inverse beta decayreaction in a liquid scintillator :
ν̄e + p → e+ + n
Selection of the events with time and energy
Very small cross-section :∼ 10−43 cm2 at 1 MeV
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 3 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Background
ACCIDENTALS
Random coincidence between twoindependent events :
• Gamma + neutronic capture• Gamma + high energy gamma
Lead and polythene shieldings
Reactor ON measurement andsubtraction
CORRELATED
Coincidence because of a same event• Cosmic muon → fast neutron
Decreased by overburden and anactive muon veto
Reactor OFF measurement andsubtraction
Online pulse shape discrimination(PSD)
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 4 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Pulse shape discrimination
Time [ns]0
Proton recoil pulse
Total gate Delayed gate
t0 150
e± recoil pulse
hQtail_over_Qtot_Th_65nsEntries 15669Mean 0.1277
RMS 0.03745Underflow 0Overflow 0
Integral 1.566e+04 / ndf
2χ 255.7 / 228Prob 0.1008
Constant_1 4.1± 386.4 Mean_1 0.0001± 0.1152 Sigma_1 0.0001± 0.0156
Constant_2 1.17± 38.47 Mean_2 0.0005± 0.2132 Sigma_2 0.00049± 0.02233
Qtail/Qtot0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
# of
eve
nts
0
50
100
150
200
250
300
350
400
hQtail_over_Qtot_Th_65nsEntries 15669Mean 0.1277
RMS 0.03745Underflow 0Overflow 0
Integral 1.566e+04 / ndf
2χ 255.7 / 228Prob 0.1008
Constant_1 4.1± 386.4 Mean_1 0.0001± 0.1152 Sigma_1 0.0001± 0.0156
Constant_2 1.17± 38.47 Mean_2 0.0005± 0.2132 Sigma_2 0.00049± 0.02233
Qtail/Qtot distribution
STEREO liquid in small cell
FAST NEUTRON SIGNALPOSITRON SIGNAL
FoM = 1.1
Neutrino prompt signal : positronionisation and annihilation⇒ small Qtail/Qtot
Fast neutron prompt : proton recoil⇒ high Qtail/Qtot
⇒ FoM = µ2 − µ12.35 ∗ (σ1 + σ2)
Correlated background subtraction with the PSD (before the offline subtraction)
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 5 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
The Nucifer experimentat the Osiris reactor (Saclay)
Nucifer
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 6 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Motivations and detector caracteristics
Weeks
0 10 20 30 40 50 60 70 80
/week
ν
8000
10000
12000
14000
Weeks
0 10 20 30 40 50 60 70 80
/week
ν
8000
10000
12000
14000
from 235Uν
from 239Puν
rateνTotal
Design for reactor monitoring :Nν̄e = α ∗ Pth ∗ (1 + k(t))
with k(t) function of the fuelcomposition.
Direct real time informations onreactor operation.⇒ IAEA interest for non-proliferation.
Main caracteristics of NUCIFER :850 L of Gd-loaded (0.2%) liquidscintillator (MPIK scintillator),16 PMTs fixed on an acrylic buffer,central calibration tube,∼7m from a 70MWth “pool type”research reactor.
● Liquid scintillators are fragile:
– Damaged by oxygen
– Only compatible with quartz,acrylic and Teflon
10 cmof lead
14 cm of borondoped polyethylene
2,5
m
Active muon veto: plastic scintillator + PMT
Nitrogen atmosphere16 PMTsAcrylic buffer
Target liquid: 850 L
Stainless steel tank + Teflon
Total: ~ 50 tonnes3 m
3 m
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 7 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Energy spectrum for all detected events
nQtotEntries 1.35329e+09Mean 1125RMS 499.5Underflow 1909Overflow 0Integral 1.15e+07
Energy (p.e)1000 1500 2000 2500 3000
Rat
e (/
day)
210
310
410
nQtotEntries 1.35329e+09Mean 1125RMS 499.5Underflow 1909Overflow 0Integral 1.15e+07
nQtotEntries 1.299346e+08Mean 1077RMS 461.5Underflow 1.122e+05Overflow 0Integral 8.274e+06
nQtotEntries 1.299346e+08Mean 1077RMS 461.5Underflow 1.122e+05Overflow 0Integral 8.274e+06
nQtotEntries 6.208563e+08
Mean 951.4
RMS 424.4Underflow 5931
Overflow 0
Integral 1.943e+06
nQtotEntries 6.208563e+08
Mean 951.4
RMS 424.4Underflow 5931
Overflow 0
Integral 1.943e+06ON before East wall
ON after East wall
OFF
Energy spectrum of all detected events
Prompt
Delayed
2 MeV4.2 MeV
7.1 MeV 9.6 MeV
Installation on site since April 2012 - Four runs of ∼3 weeks in 2013.
Liquid scintillator change in November 2012.
Shielding upgrades to mitigate the reactor induced background.
Since July 2014 SBacc
∼ 110 , S
Bcorr∼ 1
1 .
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 8 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Results (april-may 2013)
Results (april-may 2013) :
Accidental rate Correlated rateReactor OFF (66± 1) /day (360± 8) /dayReactor ON (8511± 3) /day (665± 28) /day
→ 305 ± 29 ν/day
Correlated events excess = fastneutrons coming from the core ?
High Qtail/Qtot : fast neutrons.Low Qtail/Qtot : γ, neutrino.
No more fast neutrons when thereactor is ON than when it is OFF
⇒ excess at low Qtail/Qtot isdue to neutrinos.
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 9 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Nucifer simulation with GEANT4
Cobalt source in the detector centre
nQtot_calEntries 399204
Mean 675.1
RMS 208.7Underflow 0
Overflow 0Integral 3.99e+05
Energy (pe)0 200 400 600 800 1000 12000
1000
2000
3000
4000
5000
nQtot_calEntries 399204
Mean 675.1
RMS 208.7Underflow 0
Overflow 0Integral 3.99e+05
nhSubEntries 785993
Mean 684.4
RMS 186.8
Underflow -1.653
Overflow 0
Integral 3.812e+05
nhSubEntries 785993
Mean 684.4
RMS 186.8
Underflow -1.653
Overflow 0
Integral 3.812e+05
GEANT4
Data
Neutron captures in target and shieldings
simuEntries 255163
Mean 1508
RMS 711.2
Underflow 35.45
Overflow 0.02001
Integral 2028
Reconstructed Qtot [pe]500 1000 1500 2000 2500 3000
Hits
[]
0
10
20
30
40
50
simuEntries 255163
Mean 1508
RMS 711.2
Underflow 35.45
Overflow 0.02001
Integral 2028
simuEntries 255163
Mean 1508
RMS 711.2
Underflow 35.45
Overflow 0.02001
Integral 2028
simuEntries 255163
Mean 1508
RMS 711.2
Underflow 35.45
Overflow 0.02001
Integral 2028
simuEntries 255163
Mean 1508
RMS 711.2
Underflow 35.45
Overflow 0.02001
Integral 2028
simuEntries 255163
Mean 1508
RMS 711.2
Underflow 35.45
Overflow 0.02001
Integral 2028
dataEntries 98903Mean 1567RMS 690.4Underflow 0Overflow 0Integral 1496
dataEntries 98903Mean 1567RMS 690.4Underflow 0Overflow 0Integral 1496
G4 simulation with neutrons
Reactor OFF data
Detector efficiency with optimized analyse cuts :
hDeltaT_2.0<Ep<7.1_4.2<Ed<9.6_R<650
Entries 168996Mean 21.15
RMS 16.96
Underflow 0Overflow 0
Integral 1.674e+05
s]µDelta T [0 20 40 60 80 100
Hits
210
310
410
hDeltaT_2.0<Ep<7.1_4.2<Ed<9.6_R<650
Entries 168996Mean 21.15
RMS 16.96
Underflow 0Overflow 0
Integral 1.674e+05
Simulated time distribution
sµ6 sµ40
hpromptEntries 354321
Mean 1076
RMS 490.4
Underflow 0
Overflow 0.9506
Integral 3.564e+05
Qtot [pe]0 500 1000 1500 2000 2500 3000
Hits
[]
0
1000
2000
3000
4000
5000
6000
hpromptEntries 354321
Mean 1076
RMS 490.4
Underflow 0
Overflow 0.9506
Integral 3.564e+05
Simulated prompt event spectrum
2MeV 7.1MeV
hdelayed
Entries 354309Mean 1527RMS 788.9Underflow 0
Overflow 12.93Integral 3.565e+05
Qtot [pe]0 500 1000 1500 2000 2500 3000 3500 4000
Hits
[]
0
1000
2000
3000
4000hdelayed
Entries 354309Mean 1527RMS 788.9Underflow 0
Overflow 12.93Integral 3.565e+05
Simulated delayed event spectrum
4.2 MeV 9.6 MeV
⇒ Efficiency determination : ε(detection) = 0.31 ± 0.02
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 10 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Results (PRELIMINARY)
Still work to get a more accurate prediction of the neutrino rateNeed a good precision measurement for the distance from the core
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 11 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
The Stereo experimentat the ILL reactor (Grenoble)
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 12 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Motivation and specifications of the Stereo experiment
MOTIVATION :Observe an unambiguous new oscillationpattern in energy and distance.
SPECIFICATIONS :Close to a compact and high flux nuclear reactor core : 10 m from the ILLreactor.
Relative distortions among identical detector cells : independent from reactornormalization and history (pure 235U spectrum).
Accurate detector response : mature technology of Gd-Loaded liquidscintillators.
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 13 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
ILL site
ADVANTAGES :58 MW research reactor.High 235U enrichment and compact core.Overburden with the transfer water channel.Strong structure of floor (500 t /slab).
B42
New site DRAWBACK :
High neutron and gamma flux because ofexperimental beam lines.→ Requires heavy shielding.
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 14 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Shieldings against muons, gammas and neutrons
SpectreGamma_Corr
Entries 10000
Mean 3938
RMS 1666Underflow 0
Overflow 0
Integral 2242
Energy (keV)2000 3000 4000 5000 6000 7000 8000 9000
Hits
(/s
)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
SpectreGamma_Corr
Entries 10000
Mean 3938
RMS 1666Underflow 0
Overflow 0
Integral 2242
Gamma spectrum in the STEREO casemate
n-capture gamma rays
Front wall shielding and H13 reinforcement :• Dedicated beam plug.• Additional lead wall.• Additional neutron shielding : B4C.
Shielding surrounding the target :• 10 cm thick lead.• 15 cm thick polyethylene.• 30 cm thick surrounding crown.
"On-site" protection :• Water channel.
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 15 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Detector design
Six cells (40x90x90) cm3 filled withGd-loaded liquid scintillator.
Surrounding crown filled with unloadedliquid scintillator.
• Containment of energy leakage• Active veto of external background
Light collection :• Four PMTs per cell and acrylic buffer.• Acrylic walls and optical segmentations
with VM200.
Validation with prototype in September2014.
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 16 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Calibration system
OBJECTIVES :
Energy scale and neutron efficiency.
Fine-tuned simulation.
TOOLS :Gamma and neutron sources :
• Inside cells along vertical axis.• Underneath the detector vessel.• Around the detector vessel.
Tagged source :
• Circulated around the detector vesselat different Z.
• Absolute calibration at 511 keV.
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 17 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Positron and neutron response
Complete GEANT4 model to simulate the detector response.
h1Entries 100000
Mean 2.819
RMS 0.3256Underflow 0
Overflow 0
Integral 1e+05
Detected Energy (MeV)0 0.5 1 1.5 2 2.5 3 3.5 4
# o
f ev
ents
0
500
1000
1500
2000
2500
3000
3500
4000
4500
h1Entries 100000
Mean 2.819
RMS 0.3256Underflow 0
Overflow 0
Integral 1e+05
Center
Border
Response to 2 MeV positrons
RMS/Peak = 11.5%
Similar response between centerand border cell :
• RMS/Peak(center cell) = 11.5%• RMS/Peak(border cell) = 11.7%.
h1Entries 24946Mean 5.543RMS 2.304Underflow 0Overflow 7Integral 2.494e+04
Detected Energy (MeV)0 1 2 3 4 5 6 7 8 9 10
0
50
100
150
200
250
300
350
400
450
500
h1Entries 24946Mean 5.543RMS 2.304Underflow 0Overflow 7Integral 2.494e+04
Center
Border
Response to 20 keV neutrons
Neutron efficiency above 5 MeV :
• 64.5% ± 0.5% for center cell• 60.1% ± 0.5% for border cell.
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 18 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Stereo contours
)newθ(22sin-110 1
)2 (
eV2 ne
wm∆
-110
1
10
RAA contour @ 95 % CL
RAA contour @ 99 % CLBest FitExp. contour @ 95 % - Shape
Exp. contour @ 95 % - Norm+Shape
RAA contour @ 95 % CL
RAA contour @ 99 % CLBest FitExp. contour @ 95 % - Shape
Exp. contour @ 95 % - Norm+Shape
300 days data taking (6 reactorcycles)
Energy reconstructionsystematics : δEscale = 2%
Systematics of the emittedneutrino spectra includedNormalisation : 4%Signal / background = 1.5
Prompt signal energy > 2 MeV
Delayed signal energy > 5 MeVExpected detection rate = 410νe / day
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 19 / 2
Introduction NUCIFER experiment STEREO experiment Conclusions
Conclusions
NUCIFERNow in its final configuration after two major shielding upgrades
About 300 neutrinos per day are detected (∼30% of efficiency)
The accuracy is limited by the accidental rate of high energy γ-rays
STEREOThe sensitivity covers the contour of the reactor anomaly
Several prototypes are under test to validate the detector response
Challenging mitigation of reactor induced background
Beginning of data taking in Spring 2015
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 20 / 2
Back-up
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 1 / 2
Bump at 5 MeV
Neutrino excess at 5 MeV in several experiments : DC, RENO, Chooz, Rovno ...
Scale with reactor power so• bias in the conversion procedure ?• bias in the reference electron spectrum ?• another neutrino interaction ?
Which impact on Stereo sensibility ?
Bump amplitude = 10% of neutrino signal.Incertainty = 50% of bump amplitude. →No significative impact on the contour.
)newθ(22sin-110 1
)2 (
eV2 ne
wm∆
-110
1
10
RAA contour @ 95 % CL
RAA contour @ 99 % CL
Best Fit
Exp. contour @ 95% CL nominal
Exp. contour @ 95% CL bump at 5MeV
RAA contour @ 95 % CL
RAA contour @ 99 % CL
Best Fit
Exp. contour @ 95% CL nominal
Exp. contour @ 95% CL bump at 5MeV
Maxime Péquignot - NOW 2014 The NUCIFER and STEREO experiments 2 / 2