the nucifer and stereo experiments...introduction nuciferexperiment stereoexperimentconclusions...

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


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)



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



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)



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


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



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)



The Nucifer experimentat the Osiris reactor (Saclay)

Nucifer



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



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


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 .



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.



Nucifer simulation with GEANT4

Cobalt source in the detector centre

nQtot_calEntries 399204

Mean 675.1


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


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



Results (PRELIMINARY)

Still work to get a more accurate prediction of the neutrino rateNeed a good precision measurement for the distance from the core



The Stereo experimentat the ILL reactor (Grenoble)



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.



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.



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.



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.



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.



Positron and neutron response

Complete GEANT4 model to simulate the detector response.

h1Entries 100000

Mean 2.819


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


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.



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 @ 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



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


Back-up


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



Best Fit

Exp. contour @ 95% CL nominal

Exp. contour @ 95% CL bump at 5MeV



Best Fit

Exp. contour @ 95% CL nominal

Exp. contour @ 95% CL bump at 5MeV


the nucifer and stereo experiments...introduction nuciferexperiment stereoexperimentconclusions...

Documents