The first real time detection of 7 Be solar neutrinos in Borexino

Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano

The first real time detection of 7Be solar neutrinos in Borexino

Barbara CaccianigaINFN Milano

(an update after 192 days )see also arXiV:0805.3843v1


On behalf of the Borexino Collaboration

Kurchatov Institute(Russia)

Dubna JINR(Russia)

Heidelberg(Germany)

Munich(Germany)Jagiellonian U.

Cracow(Poland)

Perugia

Genova

APC Paris

MilanoPrinceton University

Virginia Tech. University

ItalyUSA

France

Russia

Polland

Germany

SNO, SuperK

Solar neutrinos

•The first idea of Borexino dates back to the 90’s, to perform a direct measurement of solar neutrinos below 1 MeV;

•It took ~ 15 years for it to become reality, due to its technological challenge;

•Still valid: until now no experiments had probed (in real-time) oscillations below 1 MeV;

The Sun shines neutrinos

•LΘ(neutrinos) ~ 7 · 1024 Watts = 4 · 1037 Mev/sec;

•On earth (~ 6x 1010 neutrinos/cm2 s-1)•We now know that solar e oscillates during their path from Sun to Earth;

•The oscillations are matter enhanced through the MSW mechanism;

LMA (Large Mixing Angle)LMA (Large Mixing Angle)mm22 = = 7.6 x 10 7.6 x 10-5-5 eV eV22

sensen22θ =0.87θ =0.87

Borexino

down to 200 keV


Borexino•Borexino is located under the Gran sasso mountain which provides a shield against cosmic rays (residual flux = 1 /m2 hour);

Core of the detector: 300 tons of liquid scintillator contained in a nylon vessel of 4.25 m radius (PC+PPO);

1st shield: 1000 tons of ultra-pure buffer liquid (pure PC) contained in a stainless steel sphere of 7 m radius;

2nd shield: 2000 tons of ultra-pure water contained in a cylindrical dome;

2214 photomultiplier tubes pointing towards the center to view the light emitted by the scintillator;

200 PMTs mounted on the SSS pointing outwards to detect light emitted in the water by muons crossing the detector;

Only the innermost 100tons of scintillator (R<3m) are considered in the analysis, in order to further reduce the external background.


Main goal: detection of 7Be neutrinos E=862 KeVSSM=4.8x109 /s/cm2

xx ee

=10-44 cm2

e

x

Rate(7Be) in LMA hypothesis ~50 counts/day/100tons

•No information on the direction of incoming neutrinos;

•Signal is indistinguishable from natural radioactivity: BX needs to reach extreemely high radiopurity to be able of observing it!

Possibility to detect other sources like CNO, pep, 8B


Radiopurity: the key issue for Borexino

The expected rate of solar neutrinos in 100tons of scintillator is ~50 counts/day;

• Natural water is ~ 10 Bq/Kg in 238U, 232Th and 40K• Air is ~ 10 Bq/m3 in 39Ar, 85Kr and 222Rn• Typical rock is ~ 100-1000 Bq/m3 in 238U, 232Th and 40K

Just for comparison

BX scintillator must be 9/10 order of magnitude less radioactive than anything on earth!

It corresponds to ~ 5 10-9 Bq/Kg;


Background suppression: 15 years of work

• External background: from surrounding materials (PMTs, rock…)– Detector design: concentric shells to shield the inner scintillator;– Material selection and surface treatment;– Clean construction and handling;

• Internal background: contamination of the scintillator itself (238U, 232Th, 40K, 39Ar, 85Kr, 222Rn)

– Solvent purification (pseudocumene): • Distillation (6 stages distillation, 80 mbar, 90 °C);• Vacuum Stripping by Low Argon/Kripton N2 (LAKN);

– Fluor purification (PPO): • Water extraction ( 5 cycles);• Filtration;• Single step distillation;• N2 stripping with LAKN;

– Leak requirements for all systems and plants < 10-8 mbar· liter/sec;• Critical regions (pumps, valves, big flanges) were protected with

additional nitrogen blanketing;


A long story

in few pictures


PMTs installation (2001-2002)


Nylon vessels installation (2004)


Nylon vessels installed and inflated (May 2004)


Final closure of the SSS (june 2004)


Low Ar and Kr Nitrogen

Filling with water (fall 2006)

Ultra-pure water


Ultra-pure water

Liquid scintillator

Filling with pseudocumene (started Jan 2007)


Filling with pseudocumene completed Borexino starts taking data on 15th May 2007


…and now…

DATA!

How does Borexino spectrum look like?

14C dominant below 200 KeV

210Po NOT in equilibrium with 210Pb

Mostly ’s e ’s

7Be shoulder

Total spectrum (192 days) no cuts

The crucial information collected for each event are•Number of photons energy of the event•Time of arrival of each photon at each PMT position of the event

•14C is an unavoidable background in an organic scintillator;

•It is responsible for most of the counting rate in Borexino (~20cnts);

~ 1 MeV

•210Po belongs to the 238U but it is NOT in equilibrium with 238U nor 210Pb;

•It decays away with its lifetime 200 days;

Energy scale and energy resolution• The energy scale is currently determined by means of internal contaminants;• The most precise information come from the fit to the 14C beta-decay spectrum

(156 keV end-point), taking into account form factor and light quenching;

• Other internal contaminants are also used like 11C (cosmogenic);• WARNING: life is not simple. Light quenching introduces non-linearity in the

energy scale which MUST be taken into account in any global analysis of the Borexino spectum;

• Light quenching is taken into account via the Birks parametrization and the light yield is left as a free parameter in the global fit to the energy spectrum;

• Obviously, the insertion of calibration sources will be important to reduce systematics associated with the energy scale.

The light yield is found to be500 ± 12 p.e./MeV

• This high light-yield leads to a good energy resolution

• (E)/E = 5% at 1MeV;

Position reconstruction and FV definition• It is obtained by a maximum-likelihood fit to the photon arrival time distribution;• It relies on the precise time-alignment of all the 2200 PMTs (within 1.5nsec);• The algorithm was tuned on the BX prototype data (CTF) and on MC simulations;• Its behavior is tested on internal contamination events;

• Position reconstruction is needed to select an inner fiducial region of the detector for the neutrino analysis free of external background

Radial distribution of ev in the region

R2

gauss

2 2 2R x y z

----- external

----- internal

z < 1.7 m,to remove from IV endcaps

2 2cR x y

z vs Rc scatter plot

from PMTs that penetrate in the IV

FV

• Maximum systematic error on FV definition ±6%;• Willl be reduced with source calibration;

the

BACKGROUND


Radioisotopes Typical Goal Achieved in BX Counts/day/100t

238U ~10-5 - 10-6 g/g 10-16 g/g 1.9 x 10-17 g/g ~30

232Th ~10-5 - 10-6 g/g 10-16 g/g 6.8 x 10-18 g/g ~2

Background suppression: achievements

There are two backgrounds which are out of specifications:

•210Po: started out at 6000 counts/day/100t (the origin of the contamination is not known);

• It is NOT in equilibrium with 238U nor 210Pb;• It decays away with its lifetime (200d) now it is 1500 counts/day/100t;• Can be rejected by pulse/shape discrimination methods;

•85Kr : ~29 counts/day/100tons (probably because of a few liters air leak which happened during filling);

• It decays beta with an end-point of ~687 keV very annoying for neutrino analysis;

• It is long-lived (31 years);• Its amount can be estimated indipendently via a rare channel decay;• Its amplitude is also left free in the global fit;

the 7Be ANALYSIS

after 192 days

All data (scaled to fiducial volume sizeAfter analysis cuts

Analysis cuts1. rejected;2. Everything 2ms after

rejected;3. Rn daughters before Bi-Po

coincidences vetoed;4. FV cut;

All data (scaled to fiducial volume sizeAfter analysis cutsAfter statistical subtraction of events

Analysis cuts1. rejected;2. Everything 2ms after

rejected;3. Rn daughters before Bi-Po

coincidences vetoed;4. FV cut;

Fit to the spectrum •Fit between 100-800p.e.•Light yield is a free parameter of the fit;•Light quenching is included according to the Birks’ parametrization;• 14C, 11C and 85Kr are left as free parameters of the fit

•Fit to the spectrum with and without alpha subtraction is performed giving consistent results

7Be: 49±3 cpd/100 tons

Expected rate (cpd/100 t)

No oscillation 75 ± 4

BPS07(GS98) HighZ 48 ± 4

BPS07(AGS05) LowZ 44 ± 4

Systematic uncertainties Source Syst.error (1)

Tot. scint. mass ± 0.2%Live Time ± 0.1%Efficiency of Cuts ± 0.3%Detector Resp.Function ± 6%Fiducial Mass ± 6%TOT ± 8.5%

49 ± 3stat ± 4sys cpd/100 tons

No-oscillation hypothesis rejected at 4 level

Under the assuptions of High-Z SSM the measurement corresponds to

Pee = 0.56 ± 0.1 (1) at E=862 keV

which is consistent with the number derived from the global fit to all solar and reactor experiments

Constraints on pp and CNO fluxes •It is possible to combine the results obtained by Borexino on 7Be flux with those obtained by other experiments to constraint the fluxes of pp and CNO ;

•The measured rate in Clorine and Gallium experiments can be written as:

wherek,i

k,ieek,iik PRfR

B,Be,CNO,pep,ppi

Gallex,Homestakek87

"k"erimentexpin"i"sourceforyprobabilitsurvivalaverageP

.)oscillno("k"erimentexpin"i"sourceofrateectedexpR)predicted()measured(

f

k,eee

k,i

i

ii

•Ri,k and Pi,k are calculated in the hypothesis of high-Z SSM and MSW LMA, ;

•Rk are the rates actually measured by Clorine and Gallium experiments;

• f8B is measured by SNO and SuperK to be 0.87 ±0.07;

• f7Be=1.02 ±0.10 is given by Borexino results;

•Performing a based analysis with the additional luminosity constraint;

.)L.C%90(90.3f

)1(004.1f

CNO

008.0020.0pp

Which is the best determination of pp

flux (with luminosity constraint)

What next?

What can BX say aboutother solar sources?

ppCNO, pep 8B

above~ 3MeV


CNO11C

pep

pep and CNO fluxesThe main background for pep and CNO analysis is 11C

+ 12C +11C + n

n + p d +

~260sec

11C 11B + e+ + e

~30 min

•Possibility of using the three-fold coincidence to tag this background;•Spatial cuts around the neutron position and muon track can be performed to reduce dead-time;

Muon Spherical cut around2.2 to reject 11C event

Cylindrical cut Around -track n

11C

Detecting supernova neutrinos

-expected signal rate in 300 tons (d~10kpc) 1) elastic scattering ~ 5 events (all flavors) 2) inverse decay ~ 80 events (all flavors) 3) NC on 12C ~ 20 events (all n flavors) 4) CC on 12C ~ 7 events (only e and e) 5) -p scattering ~ 50 events (all flavors)

Measuring anti-neutrinos from Earth

-spectroscopy of geo-neutrinos would provide information on the radiochemical composition of Earth;

-main contribution to radiogenic heat are expected to come from U,Th chains and K;

-the expected rates in Borexino are around ~10-30 events/year;

-the reactor background is ~20 ev/year;

Other Borexino potentials

Total number of events

~160 events in 20 secs

Conclusions

PRESENT (after 192 live days of data-taking)• Borexino has provided the first direct measurement of the 7Be solar

neutrino flux;• This measurement probes the survival probability for solar e in the

transition region between matter-enhanced and vacuum oscillations;• The result of Borexino combined with the results from other solar

neutrino experiments also improves the experimental determination of pp and CNO neutrino fluxes;

FUTURE (~1 year )• Further improvement in the 7Be flux measurement (error~5%) by

reducing systematic errors: calibration with sources;• 8B neutrinos down to ~3 MeV;• Study of CNO neutrinos (tag on background from cosmogenic 11C) ;• pp neutrinos?• Study of the anti-neutrinos from earth;• Ready for detection of SuperNova neutrinos;

Backup slides


Background: 238U

214Bi-214Po

=240±8sTime s

214Bi-214Po

2 2cR x y

z (m

)

Setp - Oct 2007

238U: = (1.9 ± 0.3)×10-17 g(U)/g

BETTER THAN DESIGN GOAL!(10-16 g/g)

In the 238U chain there is a sequence of two decays which is easily tagged with the fast-coincidence method: the 214Bi – 214Po sequence

214Bi 214Po + + (Q=3.23MeV)

214Po 210Pb +

after =236.6 sec

Limits on neutrino magnetic moment• Neutrino-electron scattering is the most sensitive test for neutrino

magnetic moment search.• The differential cross-section is the sum of weak and

electromagnetic terms

2

222

0νee

weak

214E,T

dTdσ

EmT

ggET

gg eeRL

eRL

νe

2υ

20νe

e

em

E1

T1μrπE,T

dTdσ

• At low energy (Te << E) their ratio is proportional to 1/Te and the sensitivity to the increases;

• A fit is performed to the energy spectrum including contributions from 14C, leaving as free parameter of the fit;

EXP. Method 10-11 B

SuperK 8B <11

Montanino et al. 7Be <8.4

GEMMA Reactor <5.8

Borexino 7Be <5.4

Background: 232Th

232Th: (6.8 ±1.5)×10-18 g(U)/g (90%C.L.)

BETTER THAN DESIGN GOAL!(10-16 g/g)

In the 232Th chain there is a sequence of two decays which is easily tagged with the fast-coincidence method: the 212Bi – 212Po sequence

212Bi 212Po + + (Q=2.2MeV)

212Po 208Pb +

after =432.8 nsec212Bi-212Po

Time (ns)

=423±42 ns

2 2cR x y

Only fewbulk candidates

z (m

)

(m)

238U chain

Background: 210Po

• 210Po contamination at the beginning was ~6000 cnts/day/100ton definitely NOT in equilibrium with 238U!

• It decays away with its expected lifetime (200 days);

• Currently the 210Po rate is ~1500cnts/day/100tons;

• 210Po is NOT in equilibrium with 210Pb!

• Otherwise, we would see also 210Bi (annoying for neutrino analysis!!)

• 210Bi content is left as free parameter in the fit and found to be <20cnts/day/100tons);


• Only 10 events are selected in the IV in ~220 d.• 2.5 events were expected from 14C-210Po random coincidences• The corresponding 85Kr contamination is (29±14) counts/day/100 ton • MORE STATISTICS NEEDED!• 85Kr contribution is put as a free parameter in the global spectrum fit; it

is one of the main source of error for the 7Be neutrino analysis;

85Kr decays

Background: 85Kr

= 10.76 y

85Kr 85Rb + (Q=0.687MeV)

Spectrum similar to the 7Be- recoil electron !

99.56% of the times

85Kr 85Rb* + (Q=0.173MeV)

85Rb* 85Rb + (Q=0.514MeV)after =1.46 sec

0.44% of the times

Strong signature to tag events


11C and neutrons after muons

• electronics improvement to detect all the neutrons produced by a muon– Implementation of the

main electronics– FADC in parallel to the

main electronics


System description: Glove-box mounted in the clean-room on the top of

the external water tank; The sources will be mounted on bars made of

stainless electropolished steel;The bars will be inserted in the detector via a feed-

through flushed with LAKN nytrogen; Sources will be of differenbt types: LEDs, quartz bulb;It will be also possible to take scintillator samples via a

teflon tube;

Motivations: Position reco calibration; reduction of the

systematic error on the Fiducial Volume;

Energy calibration;

Study of light quenching effect at different energies

Study of α / β discrimination efficiencies;

Calibrating Borexino

Documents

The first real time detection of 7 Be solar neutrinos in Borexino