Geoneutrino flux measurementwith Borexino detector
Oleg Smirnov, JINR, Dubna
on behalf of Borexino collaboration
International Workshop：Neutrino Research and Thermal
Evolution of the EarthOctober 25 – 27, 2016, Sendai, Japan

MilanoMünchen

Heidelberg

Hamburg

Mainz

Gran Sasso
Perugia

GenovaNapoliTU Dresden

Jagiellonian
Kraków

the Borexino Collaboration
JINRDubna
Virginia Tech

Los Angeles

HoustonPrinceton

ParisUMass
AmherstMoscow

St. Petersburg

Kurchatov
Moscow

BOREXINO started data takingin 2007
•278 t of liquid organic
scintillator PC + PPO(1.5 g/l)• (ν,e)-scattering with200 keV threshold•Outer muon detector

13.7m
18m

Borexino status
Acquires data non-stop from May 2007;(problems with muons identification up to december; potential
contamination of the data sample with muons daughter – excluded
from all analyses)
now in Phase-II (after the calibration campaign and repurification)
Extremely clean LS and well-designed protection against external
backgrounds
Far away from european nuclear power plants (~1000 km averagedistance): only 36 % of the total antineutrino signal in geo-nu
window [0.9-2.6 MeV] Geo/Reactor ratio is 1.8 in Borexino;
Last analysis of geoneutrino:~5½ years (Dec 15, 2007-Mar 8, 2015)
2056 days of live time in total.
1841.9 days after the muons cutLiquid Scintillator Radiopurity
Isotope

Typical abundance(source)

Borexino
goals

Borexino-I

Borexino-II

14C / 12C, g/g

10-12 (cosmogenic)∼10-182.7·10-18

2.7·10-18

238U, g/g

10-6-10-5(dust)∼10-16(1 μBq /t)(1.6±0.1)·10-17<9.7· 10-19 (95%)∼10-16(6.8±1.5)· 10-18<1.2· 10-18 (95%)( 212Bi-212Po)

10-6-10-5(dust)

222Rn (238U),

100 atoms/cm3 (air)

10

10.1

40K, g[K ]/gnat

2·10-6 (dust)∼10-15<1.7·10-15 (95%)---

210Po,

Surfacecontamination∼10-280 (initial),

2In equilibrium with
222Rn or 210Pb

Not
specified

20-70

∼20

1 Bq/m3 (technogenic,air)∼130.4±5 cpd/100t<5 (90% C.L.)
compatible with 017 mBq/m3(cosmogenic in air)

∼1<<85Kr<<85Kr( 214Bi-214Po)
232Th, g/g

ev/d/100 t

ev//d/t
210Bi,

ev/d/100 t
85Kr

ev/d/100 t
39Arev/d/100 t

T1/2=134 days;

Detection of geo(anti)neutrino• Earth (in constrast to the Sun) emits
antineutrino.

• Part of antineutrino in the U and Th decay chainsis emitted with E>1.8 MeV (IBD threshold)• Contributions from U and Th are distinguishable• Oscillations are averaged:

ν + p →n+e+<Pee>=0.548+0.012-0.013

Eν>1.8 MeV

Main backgrounds in
geo-neutrino measurements1)Reactor antineutrinos (81% of the total
antineutrino signal in KamLAND geonu window [0.9-2.6 MeV] and ~36% forthe Borexino case): Geo/Reactor ratio
0.23 in KL vs 1.8 in Borexino;
2)Cosmic muons induced backgrounds,
including cosmogenic production of(βn)-decaying isotopes (at LNGS themuons flux is of about factor 7 lower
than at the Kamioka site)3)Internal radioactive contamination:
accidental coincidences,(αn) reactions

Data selection for geo-neutrino analysis• Total exposition is 907±44 t⋅yr taking into account detection
efficiency• Antineutrino are detected using delayed coincidence tag fromthe inverse beta- decay on proton (~256 µs)

νe + p → e + n+↓ ≈ 250 µs
n + p → d + γ (2.2 MeV )• ~500 p.e./MeV for electrons• 438 p.e./2 x 511 keV γ’s

Set of antineutrino cuts
1.
2.
3.
4.
5.

tuned to select maximumof correlated events inspace and time with max.
suppression ofacc.coincidences

Qprompt>408 p.e. : 3σ(E) above 2me860 <Qdelayed<1300 p.e∆R<1 m;20 <∆t<1280 µs
Pulse shape. gαβ(delayed)<0.015 : selecting e-likeevents (prompt signal from fast n is α-like)
6. Tμ>2 ms : fast neutrons after muon
7. Tμ>2 s for every muon passing through internal
detector. Long-lived cosmogenic (βn) isotopes. ~10% oflive time loss.
8. Multiplicity cut: no n-like events in ±2 ms window
9. RIV(Θ,φ)-Rprompt(Θ,φ)>0.30 m : dynamical, follows IVshape
10. FADC cut : independent check of candidates
features with 400 MHz digitizing system
Total efficiency=84.2±1.5% (MC). 77 candidates selectedSummary of backgrounds
SourceCosmogenic 9Li and 8He
Fast neutrons from μ in Water Tank
Fast neutrons from μ in rockNon-identified muons
Accidental coincidences

events

0.194 ± 0.015(stat)+0.124-0.088 (syst)< 0.01 (90% CL) (measured)< 0.43 (90% CL) (MC)0.12 ± 0.01
0.221 ± 0.004

Time correlated background0.035±0.028(stat)+0.006-0.004 (syst)

Spontaneous fission in PMTs

0.032 ± 0.003(α,n) reactions in the scintillator [210Po]

0.165 ± 0.010 (stat)(α,n) reactions in the buffer [210Po]< 0.66 (90% CL)

214Bi-214Po

0.009±0.013

TOTAL

0.78 +0.13-0.10Selected antineutrino spectrum(77 events)
with chondritic Th/U ratio
Th/U ratio
Th/U ratio

Unbinned likelihood fit using MCenergy spectra for geo and thereactor antineutrinos
2 free parameters Sgeo and Sreact+ backgrounds:
3 nuisance pars SLiH, Sαn and Sacc

Qvis = 438 p.e.(2γ ) + Q( Eν − 1.8MeV )~500 p.e./MeV

Fit results• Predicted reactor signal 87±4 TNU• Ngeo=23.7+6.4-5.7(stat)+0.9-0.6(syst) events
Sgeo=43.5+11.8-10.4(stat)+2.7-2.4(syst) TNU• Nreact=52.6+8.5-7.7(stat)+0.7-0.9(syst) events
Sreact=96.5+15.6-14.2(stat)+4.9-5.0(syst) TNU• Systematics: 4.8% on FV and 1% on the energy scale

Sgeo:Sreact for fixed Th/U=3.9

3.6·10-9 probability of Ngeo=0(5.9 σ)For Th/U=3.9 :Φ(U)=(2.7+0.8-0.7)x106 cm-2s-1
Φ(Th)=(2.3+0.7-0.6)x106 cm-2s-1

1,3 and 5 σ contours for Sgeo:Sreact signals

Unconstrained U/Th analysis

Th/U=3.9

1,2 and 3 σ contours for SU:STh signals

Radiogenic heat

GeodynamicalGeochemical
CosmochemicalSignal from the mantle•Total contribution from the Earth crust (Huang et al.) (LOC + ROC) isSgeo(Crust) = (23.4 ± 2.8) TNU -> 12.75 ±1.53 events (+stat.smearing)•subtraction of probability distributions for the total signal (from the fit)and pdf for crust (normal approximation). Non-physical values ofdifference are excluded and final p.d.f. renormalized to unity.
p.d.f.(Mantle)=p.d.f. (Geo)-p.d.f.(Crust) :Sgeo(Mantle) = 20.9+15.1-10.3 TNU
with a probability of 98% we observe at least 1 event from the mantle•Note:– Mean value is bigger compared to a simple difference <Sgeo>-<S(Crust)>=43.523.5=20.1 as a result of excluding non-physical values from p.d.f.

Antineutrino measurements with Borexino
Year

Live
time,
days

Exposition
t·yr

Ncand

Ngeo

Sgeo TNU

P(0)

537.2

252.6

21

9.9+4.1-3.4

65.2+27.0-22.4

3·10-5(4.2σ)

2013

1363

613 + 2646

14.3±4.438.8±12.0

6·10-6(4.9σ)

2015

2056

907±4477

23.7+6.5-5.7

43.5+12.1-10.7

3.6·10-9(5.9σ)

20102010)G. Bellini, et al. Phys. Lett. B 687 (2010) 2992013)G. Bellini, et al. Phys. Lett. B 722 (2013) 295
2015)M. Agostini, et al, Phys. Rev. D 92, 031101 (2015)

Georeactor•••

In the core (Herndon) on the core/mantle border (Rusovи de Meijer)
5-10 TW will help to explain heating, convection, He3
anomaly, geomegnetism and some other problems.
Both are critisized by geochemists• Easy to test with geoneutrinos, Borexino
excludes georeactor with 4.5 TW power at
95% C.L.
Forming the Moon from a georeactor at the core-mantle boundary4.5 Ga
Forming the Moon from terrestrial
silicate-rich material (2013)
R.J. de Meijer, V.F. Anisichkin, W.
van Westrenen

Another measurement withBorexino?
• We have accumulated another ~1.5 yrs of data
and will run at least 1 yr more in solar mode
before SOX program (+ ~50% statistics)• Tuning of the muon-veto cut will save 9% of livetime• We consider the possibility to perform a spectral fit
in all volume (+ ~50%)• Better understanding of “external” background” isneededNuclear physics for geoneutrino
studiesContribution of elements from U and Th chains in
total geoneutrino signal

214BiCTF (4 tonne Borexino prototype)Experimental spectrum of 214Bi (CTF) withsuperimposed fit

New ToI value:p0=0.1910±0.0017
Phys. Rev. C 81, 034602 (2010) Nuclear physics for geo-neutrino studies G. Fiorentini et al

Deviation from the allowed (universal) shapeResults for signal from 214Bi

With spectral deformations:Geoneutrino with Borexino.
Summary.
• 1)Geoneutrino detection is now extremely robust in
Borexino : 5.9σ (3.6·10-9);• 2) Sgeo(LNGS)=43.5+11.8-10.4(stat)+2.7-2.4(syst) TNU• 3)The precision is still too low: ~25% for U+Th signal
with fixed ratio Th/U=3.9, and much worse for theunconstrained R(U) and R(Th) measurements.
Geological models for the moment can’t bediscriminated;• 3)Radiogenic heat is in 11-51 TW interval at 68% CL• 4)The mantle contribute positive signal at 98% CL:
Smantle=20.9+15.1-10.3 TNU