Real-time Solar neutrino detection with Borexino

Oleg Smirnov (JINR, Dubna)

on behalf of Borexino collaboration

5-th International Workshop on Low energy neutrino physics

19  - 21 October 2009, Reims, France

- Borexino goal, 5%

50 events/d/100t expected (νe and vμ elastic scattering on e-)

Low energy->no Cherenkov light->No directionality,no other tags-> extremely pure scintillator is needed

Standard Solar Model predictions.

measuring neutrino fluxes one can discriminate between different models.

Kurchatov Institute(Russia)

Dubna JINR(Russia)

Heidelberg(Germany) Munich

(Germany)

Jagiellonian U.Cracow(Poland)

Perugia

Genova

APC Paris

MilanoPrinceton University

Virginia Tech. University

BOREXINO Collaboration

University of Massachusetts

Reducing external background with “graded shielding"

Neutrons and external gammas(ultrapure water layer, 2.15 m, 2400 tones)

γ-s from construction materials(outer layer of scintillator, 1.25 m or 200 tones)

Software-defined active volume of scintillator(fiducial volume, 3m, 100 tones)

γ-s from construction materials(PC buffer, 700 tones, 2.5 m)

Cosmic muons(LNGS underground labs:

rocks, 3200 m.w.e.)

Position reconstructionneeded

Increasing radiopurity of m

aterials

BOREXINO

18m

13.7m

•278 t of liquid organic scintillator PC + PPO (1.5 g/l)• (ν,e)-scattering with 200 keV threshold•Outer muon detector

LS radiopurity in Borexino: results of 15 yrs work

Background Typical abundance

(source)

Borexino

goals

Borexino

measured

14C / 12C [g/g] 10-12 (cosmogenic) 10-18 2·10-18

238U [g/g]

(by 214Bi-214Po)

2·10-5 (dust) 10-16

(1 μBq / t)

(1.6±0.1)·10-17

232Th [g/g]

(by 212Bi-212Po)

2·10-5 (dust) 10-16 (5±1)· 10-18

222Rn (238U) [g/g]

(by 214Bi-214Po)

100 atoms/cm3 (air)

(emanation from materials)

10-16 10-17

( 1 cpd/100 ton)

40K [g/g] 2·10-6 (dust) 10-18 <3·10-18 (90%)

210Po[cpd / t] (surface contamination) 10-2 70 (initial, T1/2=134 d;

not in equilibrium with parent 210Bi);

<5 after 2 yr

85Kr [cpd / 100 t] 1 Bq/m3 (air) 1 28±7 cpd/100t

39Ar [cpd / 100 t] 17 mBq/m3(air) 1 <<85Kr

Borexino technical data

1.Light yield: >500 p.e./MeV/2000 PMTs (31% of 4π);

2.Mass: full 278 t; FV (R<3 m && |Z|<1.67 m) mass 78.5 tones (used in 7Be analysis);

3.Energy resolution (1σ) within the FV: ~5% @ 1 MeV;

4.Practical threshold on the electrons recoil is 180 keV (corresponds to 380 keV

neutrino);

5.Muons registering efficiency close to 100%;

6.Triggers rate: 11 cps (mainly 14C, 2.7 ± 0.6 x 10-18 g/g 14C/12C )

7.Spatial resolution 14 cm @ 1 MeV

8

Active shielding effectively suppress

external gamma background

Kr+Be

14C

210Po (not in equilibrium with 210Pb)

11C

214Bi-214Po

No s

R<3.0 m(100 t)

Spectral components in the Borexino spectrum (model)

210Po & 210Bi

Energy scale• Calibrated using “internal uniformly distributed sources” taking into account the CTF

calibration experience: 14C (β-,E0=156 keV), 11C (β+ decay), 210Po (α, Eα=5.3 MeV)

• Monoenergetic line of 210Po has been used to fit the detector’s response width and shape

(non-gaussian shape is used)

• Careful modeling of the Birks’ ionization quenching at low energies (worked out with the

CTF data); kB~0.017 cm/MeV

• Two quasi-independent energy variables are used: the total number of registered p.e. (Q)

and the number of triggered PMTs (Npm)

E, keV RR(Q)%

RR(Npm) %

250 11.1 9.8

400 (210Po) 8.8 7.8

660 (7Be) 7.0 6.2

1000 5.8 5.2

A first calibration campaign with on axis and off axis radioactive sources has been performed (Oct 08 on axis, Jan-Feb 09 off axis).115 points inside the sphere: ,γ,α,n sources.The model used is in a good agreement with measurements.Also the position reconstruction has been tuned (source is localized within 2 cm precisionthrough red laser light and CCD camera).

100 Bq 14C+222Rn source diluted in PC: 115 points inside the sphere: : 14C, 222Rn : 222Rn : 8 sources from 122 keV to 1.4 MeV (54Mn, 85Sr, 222Rn in air)AmBe source (protons recoil study) :

Source localization within 2 cmthrough red laser light and CCD camera;

Accurate handling and manipulationof the source and of the materialsinserted in the scintillator;

Calibration campaigns 2008-2009A first calibration campaign with on axis and off axis sources has been performed (Oct 08 on axis, Jan-Feb09 off axis)

accurate position reconstruction

precise energy calibration

detector response vs scintillation position

Laser ball: check of PMT allignment

Model used to fit the experimental data (7Be analysis)

Normalization of main backround components are free:

14C (with fixed form-factor α);

85Kr free; in principle can be bounded (correlated with 7Be);

210Po; (in another approach is removed using α/β statistical subtraction)

210Bi; 11C;

214Pb fixed at the number of registered events of 222Rn (anyway negligible).

Other background sources (40K; isotopes from decay chains of 238U and 232Th in secular equilibrium) are found

to give negligible contributions.

Electrons recoil spectra for solar neutrino are calculated assuming MSW(LMA) scenario:

7Be;

CNO fixed @ SSM+MSW(LMA) (strongly correlated with free 210Bi component);

pp and other solar neutrino fluxes are fixed @ SSM+MSW(LMA);

Energy scale parameters:

Light yield + 1 energy resolution parameter vT+ 210Po peak position;

Two other parameters pt=0.13 and gc=0.105 (found using MC simulation) for Npm variable are fixed;

For Q variable calibration parameter c is free; parameter feq is fixed (calculated) for both variables;

Birks’ parameter kB fixed at the value found with CTF

“Direct Measurement of the 7Be Solar Neutrino Flux with 192 Days of Borexino Data” PRL 101, 091302 (2008).

49±3stat±4syst cpd/100 t

Fit to the spectrum with -subtraction gives consistent results

Main source of systematic uncertainty in this measurent is error in FV definition (significantly reduced after position reconstruction code tuning using calibration data).

210Po and α/β - discriminationOptimal Gatti filter

E. Gatti, F. De Martini, A new linear method of discrimination between elementary particles in scintillation counters, in: Nuclear Electronics, vol. 2, IAEA, Wien, 1962, pp. 265–276.

H.O. Back et al. / NIM A 584 (2008) 98–113Pulse-shape discrimination with the Counting Test Facility

ii

ii

iii

SPG

P

Works also for p(n)/ discrimination. Fine tuning in progress

Comparison with theory, 7Be

The survival probability of the 0.862 MeV 7Be neutrinos (assuming the BS07(GS98) SSM) is 0.56±0.10.

• Borexino exp. result: 49 ± 3(stat) ± 4 (syst) cpd/ 100t49 ± 3(stat) ± 4 (syst) cpd/ 100t• High metallicity Solar model MSW/LMA:

48 ± 4 cpd / 100t48 ± 4 cpd / 100t• Low metallicity Solar model , MSW/LMA

44 ± 4 cpd / 100t44 ± 4 cpd / 100t• High metallicity Solar model,

nonoscillating neutrino (inconsistent with measurement at the 4 σ C.L.)

74 ± 4 cpd / 100t74 ± 4 cpd / 100t

Constraints on pp and CNO neutrino fluxes with 192 days of Borexino data

)1(04.1 13.019.0 SSMppf

.).%90(80.3

)1(005.1 008.0020.0

LCf

f

CNO

pp

)1(04.1 13.019.0 SSMppf

with luminosity constraint

pp vs CNO 7Be vs CNO

[Ga+Cl+8B]

%4.5)(%)90(3.6 CNOlumf SSMCNO

.).%90(80.3

)1(005.1 008.0020.0

LCf

f

CNO

pp

=>Lum(CNO)<3.3%

ВэВ

m

1102.3 19

From the theoretical point of view, there is no magnetic moment for Dirac massless neutrino, as well as for Majorana neutrino, massive or massless. Massive Dirac neutrino should have small m.m.:

Neutrino magnetic moment

2

2

222

12

E

Tmgg

E

Tgg

mG

dT

d eRLRL

eF

W

ETmdT

d

e

em

EM

112

22

“flat”

1/T behaviour

m.m. can be searched for by studying the deviations from the weak shape

Limit on effective solar neutrino magnetic moment

• with 192 days of live-time statistics the 90% c.l. limit is: µeff<5.4·10-11 µB

• stronger limits with the same statistics can be obtained bounding some spectral contributions (i.e. 85Kr);

• The limit is model-independent, defined only by the shape of the spectra, also no systematics is attributed to the uncertainty of the FV.

• The best up-to-date existing limit comes from the measurements with high purity 1.5 kg Ge detector at Kalinin Nuclear Power Plant, GEMMA experiment (arXiv:0906.1926):

µ<3.2·10-11 µB • For flavour components one can write [D.Montanino et al. PRD 77, 093011 (2008)]:

)sin)(cos1()( 223

2223

222 eeeeeMSWeff PP

where Pee=0.552±0.016 is the survival probability at Earth for electronic neutrino at E=0.863 MeV, sin2θ23=0.5+0.07

-0.06

New limits on μ and τ neutrino magnetic moments

• Present limits on the neutrino magnetic moments are:• μe < 3.2×10-11 μB by GEMMA (elastic scattering)• μμ < 68×10-11 μB by LSND (elastic scattering)• μτ < 39000×10-11 μB by DONUT (elastic scattering)

1210 11B

12.510 11B

Applying constraints on μνe of Gemma experiment:

8B neutrino flux meaurementMeasurement of the solar 8B neutrino flux with 246 live days of Borexino and observation of the MSW vacuum-matter transition by Borexino coll. arXiv:astro/ph 0808.2868v1[see also Nucl.Phys.Proc.Suppl. 188:127-129, 2009]

0.26±0.04stat±0.02 syst cpd/100 t

Energy spectrum after statistical 208Tl subtraction.

The 8B mean electron neutrino survival probability, assuming the BS07(GS98) SSM, is 0.35±0.10 at the effective energy of 8.6 MeV in agreement with water Cherenkov detectors.

The ratio between the measured survival probabilities for 7Be and 8B neutrinos is 1.60±0.33, 1.8σ different from 1.

Borexino is the first LS experiment observing 8B neutrinos.

Update of 8B analysis• Principal sources of systematic error on measured 8B flux: energy threshold, fiducial

volume, detector stability

• Statistical error remains the limit: 250 days (stat error 17%) -> 500 days analyzed (12%) -> 600 days collected (11%).

• Preliminary analysis of 500 days data has been performed, the results are in agreement with published ones.

• Improved understanding of energy scale: energy calibration with 12 sources with energy from 120 keV up to 9.3 MeV; PRELIMINARY: uncertainty in energy threshold <1%. Monte Carlo code tuned to take into account non- linearities of the energy scale (ionization quenching, electronics);

• Improved position reconstruction (calibrated with sources). PRELIMINARY: error on FV could be as low as 3% (FV: R<3 m @ E>2.8 MeV red).

• Currently finalizing impact of stability and overall systematic error.

• The study in progress: tagging of 208Tl events in coincedence with 212Bi-208Po (b.r. 36%).

• 11Be contribution in E>2.8 MeV (Q=11.5 MeV, τ=19.9 s): Hagner et al measurements N(11Be)<0.02 cpd (90%), scaling the value measured by KamLAND N(11Be)=0.02±0.004 cpd in Borexino. Preliminary analysis shows no significant presence of 11Be in Borexino (about 10 times lower than scaled KamLAND value), while other important cosmogenic backgrounds are in agreement with KamLAND data.

Borexino provided measurement of electron neutrino survival probability

in two different energy ranges

Time variations of 7Be neutrino flux

±3.5% variations due to the seasonal variation of Earth-Sun distance: need more statistics, feasibility of measurement depends on stability of backgrounds and strategy chosen for (possible) repurification. For the moment no statistically significant measurement is available.

Preliminary “negative” result on day/night assimetry (see G.Testera’s talk at Neutrino Telescopes in March 2009) with 422 days statistics (213 “nights” + 209 “days”) is in agreement with MSW/LMA predictions:

ADN N D

N D0.02 0.04

Solar CNO- neutrino cycle: Solar CNO- neutrino cycle: a clue to the chemical composition of the Suna clue to the chemical composition of the Sun

dominates in massive stars

“bottle-neck” N(p,γ) reaction, slower than expected (LUNA result)

A direct test of the heavily debated solar C, N andO abundances would come from measuring the CNO neutrinos.

The feasibility of the CNO neutrino detection in Borexino is under study(depends on the possibility of background reduction)

Spectral components in the experimental spectrum (model)

CCn+

+e++en capture MeV)

Cylindrical cut Around muon-track

Spherical cut around2.2 gamma to reject 11C event

Neutron production

Muon track

11C background suppression

Borexino collaboration: “CNO and pep neutrino spectroscopy in Borexino: Measurement of the deep-undergroundproduction of cosmogenic 11C in an organic liquid scintillator” PHYSICAL REVIEW C 74, 045805 (2006)

Detecting antineutrino

• Inverse beta-decay [high c.s. ~10-42 cm2]

• Evisible = E – 0.78MeV [EeV]

)2.2(

s250

MeVdpn

nepe

28 April 2009 Milan

Reactor antineutrino

207 Nucl. power plants in 17 countries.

13 Plants give 40% of total signal.

3 most powerful power plants in France give 13% of the total signal.

in Borexino: ~15 ev/yr are expected for 100% reactors duty cycle.

15 ev/yr

28 April 2009 Milan

Geoneutrinos study is promising due to the location of the Borexino far away from the European reactors.

Emax(U) = 3.26 MeV

Emax(Th) = 2.25 MeV

Emax(K) = 1.3 MeV

Energy “window”:

1.81-3.26 MeV

Expected 6 ev/yr in the geoneutrino region.

•Radiogenic heat (HR) is connected with the antineutrino number (Lν):

• H [TW] ; M [1017kg] ; L[1024 1 /с]

•M(U), M(Th) and M(K)M(U), M(Th) and M(K)

HR = 9.5 M(U) + 2.7 M(Th) + 3.6 M(40K)

L = 7.4 M(U) + 1.6 M(Th) + 27 M(40K)

Earth heat flow Φ≈Φ≈ 60 mW/m 60 mW/m22

Full flux: HHEE = (30- 4 = (30- 444))ТТWW44±1 TW (Pollack 93) 31 ±1 TW (Hofmeister & Criss 04)

Cosmochemistry (meteorites) estimates of radiogenic heat give from 19 to 31 ТW : only limiting values are consistent with heat balance, existing estimates shows the lack of heat up to 25 TW

Expected antineutrino signal for 1 yr of the data taking

0.00.20.3Random 8.55.63.8Total 8.53.30.5Reactor

02.32.1Geo 238U001.2Geo 232Th

2.6-10 1.5-2.61-1.5

For reactor neutrino 0.8 duty cycle has been used.

13C(α,n)16O background is negligible.

Other (from random) backround sources are muon-induced -n decaying isotopes (8He+9Li) and fast neutrons induced by muons missed by MVS are effectively removed applying 2 seconds cut after each muon crossing the LS, the introduced dead time is about 11%

no FV cut (278 t), detection efficiency about 85%

Borexino potential on supernovae neutrinos

Detection channel Expected number of events in 300 t LS for standard SN @ 10kpc

ES(E > 0.25 MeV)

5

Electron anti-neutrinos(E > 1.8 MeV)

78

-p ES (E > 0.25 MeV)

52

12C()12C* (E = 15.1 MeV)

18

12C(anti-,e+)12B(Eanti- > 14.3 MeV)

3

12C(,e-)12N(E > 17.3 MeV)

9

Borexino has entered SNEWS (Super Nova Early Warning System)

Summary/What’s next? Borexino operates at purity levels never achieved before, it demonstrated the feasibility of the

neutrino flux measurement in sub-MeV region, under the natural radioactivity threshold (4.2 MeV);

Solar 7Be- flux has been measured with 10% accuracy; a first measurement of 8B- in LS with threshold below 5 MeV (2.8 MeV); Borexino results are compatible with MSW/LMA; strong limit on neutrino effective magnetic moment is obtained; extremely high sensitivity to electron antineutrino has been experimentally confirmed, waiting

for more statistics. Further calibration and reduction of the error on the 7Be flux down to 5% (further improvements

if constraining 85Kr, in this case also the limits on the effective magnetic moment will be improved);

Seasonal variations of the neutrino fluxes (detector stability, more statistics); other time variations

More precise measurement of the oscillation probability in the transition region (either due to the higher statistics or due to increase of the FV);

The CNO and pep-neutrino fluxes measurement (requires cosmogenic 11C tagging); The feasibility of the pp-neutrino flux measurement is under study (better understanding of the

detector at low energies and the precise spectral shape of 14C is needed);

Antineutrino studies: geo, reactor, supernova.