reactor antineutrinos at short baseline - the nucifer and ... · introduction thenuciferexperiment...

IntroductionThe NUCIFER experimentThe STEREO experiment

Reactor antineutrinos at short baselineThe NUCIFER and STEREO experiments

Maxime Péquignot

CEA SaclayCEA/Irfu - Service de Physique Nucléaire

3 December 2013

Maxime Péquignot - JRJC 2013 1 / 27


Outlook

1 IntroductionNeutrino historyNuclear reactor and neutrinosDetection principleBackground

2 The NUCIFER experimentNon-proliferationNucifer detectorResults

3 The STEREO experimentReactor antineutrinos anomalyStereo detectorLiquid scintillator choiceNeutrons and gammas backgroundSchedule



Neutrino historyNuclear reactor and neutrinosDetection principleBackground

Neutrino history

1930 : hypothesis of "neutrons" by Pauli to explain the β-decay spectrum :

n→ p + e− + ν̄e

Coupling only with weak interaction → hard to detect but ...... 1956 : first detection of electronic antineutrinos by Reines and Cowan with adetector close to a nuclear reactor !1968 : solar neutrino deficit → less detected neutrinos than expected ...Pontecorvo idea : neutrino oscillations.1998 : oscillation observation by Super-Kamiokande.

Pνe→νe = 1− sin2(2θ)sin2(∆m2L4E

)

Three kind of neutrinos observed : electron, muon and tau.




Reactor antineutrinos

Nuclear reactors are pure and intense sources of ν̄e : 1.9× 1020 νe/s/GWth

⇒ small detector at short baseline is possible despite of the small cross-section.

Antineutrino kinetic energy (MeV)2 3 4 5 6 7 8 9

Arb

itrar

y un

its

Cross-section

Emitted spectrum

Detected spectrum




Neutrino detection principle

Inverse β-decay utilisation :

ν̄e + p → n + e+

in Gd-loaded liquid scintillator.Very small cross-section ∼ 10−43 cm2

Prompt and delayed events incoincidence.

Prompt

Time

Delayed

>50 µs >100 µs35 µs5 µs

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




Expected backgrounds

Accidental background

Random coincidencebetween a gamma and aneutron capture or anotherenergetic gamma.Shielding to stop them.Measured reactor ON andsubtracted.

Correlated background

Cosmic µ± ⇒ fast n ⇒ correlated signal.Decreased by overburden and the muon veto.PSD rejection.Measured reactor OFF and subtracted.

⇒ Measured neutrino count rate: Nν̄e = NONcorr − NON

acc − (NOFFcorr − NOFF

acc ).Maxime Péquignot - JRJC 2013 5 / 27



Background rejection with Pulse Shape Discrimination

Time [ns]0

Proton recoil pulse

Total gate Delayed gate

t0 150

e± recoil pulse

tot / Q

tailQ

-10 -8 -6 -4 -2 0 2 4 6 8 100

0.2

0.4

0.6

0.8

1

1.2

Total

Accepted

Rejected

FoM Cut

e+ signal Proton bkg ν̄ prompt, γ, (n,γ) ⇒ low Qtail/Qtotratio.Fast neutron prompt only ⇒ p recoil⇒ high Qtail/Qtot ratio.PSD ⇒ Online rejection of thecorrelated background beforesubstraction of the correlated events.



Non-proliferationNucifer detectorResults

The Nucifer experiment

Nucifer




Antineutrinos for the non-proliferation

Neutrino emission depends on the fuel composition and on the core power :Nν̄e = α ∗ Pth ∗ (1 + k(t)) with k(t) function of the fuel.

Temps (jours)

0 100 200 300 400 500

Tau

x de

rea

ctio

n (f

issi

ons/

s)

0

20

40

60

80

100

120

1810×

Pu (n,fis)239

U (n,fis)235

U initial2353.5% = cte = 3 GWthP

kinetic energy (MeV)ν2 3 4 5 6 7 8

fiss

ions

44 p

er 1

0ν

Det

ecte

d

0

5

10

15

20

25

U235

Pu239Weeks

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

Direct real time informations on reactoroperation.Complementary method to know total burnupreactor.

⇒ IAEA interest for surveillance of reactors.Maxime Péquignot - JRJC 2013 8 / 27



OSIRIS site and detector caracteristics

70MWth “pool type” research reactor,∼7m from the reactor core,⇒ high gamma flux.

Low overburden⇒ muon flux attenuation = 2.7.

Main caracteristics of NUCIFER :850 L of Gd-loaded (0.2%) liquidscintillator (MPIK scintillator),16 PMTs fixed on an acrylic buffer,central calibration tube.

● 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




Reactor spectrum

Single rate ∼200 Hz above 2 MeV, on expectation, but leakage of high energygammas in Edelayed window ⇒ too much accidental background.A lead wall will be added to bring the background on specifications (factor ∼ 30).




Results (PRELIMINARY)

Prompt

Time

Delayed

>50 µs >100 µs35 µs5 µs

Prompt

Time

Delayed

1 ms

40 µs

Simulatedprompt

>100 µs>50 µs

Results (ON : 22 days, OFF : 5 days):

Accidental rate Correlated rateReactor OFF (33.9± 0.3) /day (124± 6) /dayReactor ON (3131± 2) /day (259± 14) /day

⇒ 135 νe/day ± 15.

Optimized cuts → 293 νe/day, publication in preparation.





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.



Reactor antineutrinos anomalyStereo detectorLiquid scintillator choiceNeutrons and gammas backgroundSchedule

The Stereo experiment




Reactor antineutrinos anomalyReevaluation of reactor ν̄e spectra, Th. A. Mueller et al., Phys.Rev.C 83, 054615 (2011)Reanalysis of short baseline experiments ⇒ deficit of 6% (2013 update)

G. Mention et al., Phys. Rev. D 83, 073006 (2011)

atmosphericoscillation

solaroscillation

ReactorAntineutrinosAnomaly

newoscillation ?

KamLAND

Double Chooz,Daya Bay, RENO. . .

Bugey, ROVNO, Goesgen. . .

New oscillations toward a sterile neutrino at very short baselines (∆m2 & 1 eV2).Maxime Péquignot - JRJC 2013 15 / 27



ILL site and detector design

B42

New site

58MWth “pool type” research reactor,∼10m from the reactor core.⇒ High gamma and neutron flux.

Efficient overburden provided by thewater channel (muon attenuation : 4).⇒ Low muon induced.

Main caracteristics of STEREO :6 independent target cells with 4 PMTs,a gamma catcher with 24 PMTs.




STEREO motivationMotivation : to detect a new oscillation pattern in E and L.

Experimental contour covers the 99% C.Lcontour of the reactor anomaly.It includes the best fit at 5σ.




Liquid scintillator choice

Different liquid scintillators are studied based on Petrelad and PXE.

Requested properties :

Attenuation length > 4m in the range of PMTs sensibility (∼ 430nm).

Stable for several years and compatible with detector materials.

Good light yield i.e quantity of photons for one MeV.

Good pulse shape discrimination for background rejection.




Light yield determination

Utilisation of a 60Co source : 2 γ rays (1.15 MeV and 1.35 MeV).Rought calibration using the high E edge of the 60Co spectrum.

All tested liquids have comparable light yields (∼ 7500-8000 photon/MeV).




PSD determination

Same set-up with a 252Cf source (fast neutrons and gammas).

-45000 -40000 -35000 -30000 -25000 -20000 -15000 -10000 -5000 00

0.1

0.2

0.3

0.4

0.5

0.6nTH2D_Qtail_over_Qtail_vs_Qtot_CFD_40ns

Entries 109645Mean x -5937Mean y 0.2252RMS x 4902RMS y 0.09087Integral 1.096e+05 0 22 1 0 109562 0 0 60 0

nTH2D_Qtail_over_Qtail_vs_Qtot_CFD_40ns

Entries 109645Mean x -5937Mean y 0.2252RMS x 4902RMS y 0.09087Integral 1.096e+05 0 22 1 0 109562 0 0 60 0

Qtail/Qtot vs Qtot, time with CFD, delay 40 ns

hQtail_over_Qtot_CFD_40nsEntries 7031Mean 0.2159RMS 0.05688Underflow 0Overflow 0Integral 7020

/ ndf 2χ 2222 / 208Prob 0Constant 0.6± 162 Mean 0.0002± 0.1815 Sigma 0.00015± 0.01703

Qtail/Qtot0.1 0.15 0.2 0.25 0.3 0.35 0.4

# of

ent

ries

0

20

40

60

80

100

120

140

160

180 hQtail_over_Qtot_CFD_40nsEntries 7031Mean 0.2159RMS 0.05688Underflow 0Overflow 0Integral 7020

/ ndf 2χ 2222 / 208Prob 0Constant 0.6± 162 Mean 0.0002± 0.1815 Sigma 0.00015± 0.01703

Qtail/Qtot, time with CFD, delay 40 ns

Very good separation achevied in test cells.Provides margin for an efficient online rejection in the detector..




Site presentation

PN3PN3

Background specifications for STEREO :200 Hz for prompt events (E > 2MeV)1 Hz for delayed events (E > 5MeV)




Thermal neutrons and gammas

Detection of thermal neutrons with a helium tube.High thermal neutron ambiance due to neighbour neutron beam lines.Hotspots close to the wall IN20/PN3.

⇒ Efficient reduction with B4C plate (validated on-site).

Detection of gamma with a HPGe.

hEEntries 5527536

Mean 1417

RMS 1534

Underflow 0

Overflow 0

Integral 1843

Energy (keV)0 1000 2000 3000 4000 5000 6000 7000 8000

)1

Hit

s (

s

310

210

110

1

10

hEEntries 5527536

Mean 1417

RMS 1534

Underflow 0

Overflow 0

Integral 1843

hEEntries 1408647

Mean 1528

RMS 1591

Underflow 0

Overflow 0

Integral 469.5

hEEntries 1408647

Mean 1528

RMS 1591

Underflow 0

Overflow 0

Integral 469.5

IN20 ONOFF, D19 ON

IN20 OFFOFF, D19 ON

Gammas spectrum in PN3

(IN20 ON) spectrum is 4 timeshigher than (IN20 OFF) spectrum80% of gamma comes from IN20.

⇒ Addition of lead.




Neutrons and gammas shieldingAddition of B4C in PN3 and IN20 + lead in PN3.




What about fast neutrons ?

Measurements were done with a PEHD shielding around the helium tube.⇒ Identification of two main sources : IN20 casemate and H7 neutron tube.

MCNPX simulations were done to prove that neutron with an energy higher than6 MeV can not come from the reactor :

6Li produces fast neutrons bythe following reaction :

n(thermal) + 6Li→ 4He + t

t + 6Li→ 8Be + n(fast)

⇒ 6Li will be removed and the plug will stop these fast neutrons coming from H7.




What about fast neutrons from IN20 ?

Addition of CH2 around the IN20collimators to provide enough marginof background reduction.

Tally1_normaliseEntries 207

Mean 0.1946

RMS 0.6032

Underflow 0

Overflow 0

Integral 7.501e+08

Energy (MeV)1 2 3 4 5 6

/s)

2F

lux

(n/c

m

510

610

710

810Tally1_normaliseEntries 207

Mean 0.1946

RMS 0.6032

Underflow 0

Overflow 0

Integral 7.501e+08

Neutrons flux at the end of H13

Tally5_normaliseEntries 120

Mean 0.01661

RMS 0.09361

Underflow 0

Overflow 0

Integral 1.076e+05

Energy (MeV)1 2 3 4 5 6

/s)

2F

lux

(n/c

m

-210

-110

1

10

210

310

410Tally5_normaliseEntries 120

Mean 0.01661

RMS 0.09361

Underflow 0

Overflow 0

Integral 1.076e+05

Neutrons flux before the IN20/PN3 wall

Tally2_normalise

Entries 360

Mean 1.194

RMS 0.9401

Underflow 0

Overflow 0Integral 2064

Energy (MeV)1 2 3 4 5 6

Flu

x (n

/s)

-110

1

10

210

Tally2_normalise

Entries 360

Mean 1.194

RMS 0.9401

Underflow 0

Overflow 0Integral 2064

Neutrons flux in PN3




What are the next steps ?

November 2013 : decommissioning of GAMS5 and H7.

February 2014 : shielding installation in H13 casemate.

Mai 2014 : H7 plug installation.

June 2014 : shielding installation in Stereo casemate.

June-July 2014 : background measurements.

September-October 2014 : detector installation.

November 2014 : Stereo commissioning.




Conclusions

Nucifer sees neutrinos with high stat significance, data from 3 Osiris cycles ondisk.Publication in preparation.Last upgrade of lead shielding planned to reduce accidental background.One full year of data taking with background close to the specifications.

Complete on site background studies, shielding structures being designed.Prototype cell expected early next year to validate the response of the innerdetector.Sequential background cross-checks after reactor restart in July 2014, detectorcommissioning November 2014.

Toward an applied antineutrino physics. . .. . . and a sterile neutrino ?


Back-up


MCNPX Simulations


reactor antineutrinos at short baseline - the nucifer and ... · introduction thenuciferexperiment...

Documents