Borexino and Solar Neutrinos

NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano

Borexino and Solar Neutrinos

Emanuela MeroniUniversità di Milano & INFN

On behalf of the Borexino Collaboration


Physics and detection principles

Borexino aims to measure low energy solar neutrinos in real time by elastic neutrino-electron scattering in a volume of highly purified liquid scintillator

Mono-energetic 0.862 MeV 7Be ν is the main target

Pep, CNO and possibly pp ν

Geoneutrinos

Supernova ν

Detection via scintillation light Very low energy threshold Good position recostruction Good energy resolution

Drawbacks: No direction measurements

ν induced events can’t be distinguished from β-decay due to natural radioactivity

Extreme radiopurity of the scintillator

Typical rate

(SSM+LMA+Borexino)


Detector design and layout

Water Tank: and n shield water Č

detector208 PMTs in

water2100 m3

20 legs

Carbon steel plates

Scintillator:270 t PC+PPO in a 150 m thick nylon vessel

Stainless Steel Sphere:

2212 photomultipliers 1350 m3

Nylon vessels:Inner: 4.25 mOuter: 5.50 m

Design based on the principle of graded shielding

Borexino detector at LNGS


Background suppression strategies= 15 years of work

• ’s from rocks, PMT, tank, nylon vessel– Detector design: concentric shells to shield the inner scintillator – Material selection and surface treatment– Clean construction and handling

• Internal background (238U, 232Th, 40K, 39Ar, 85Kr, 222Rn)– Scintillator purification:

• Distillation (6 stages distillation, 80 mbar, 90 °C)• Vacuum Stripping by LAK N2 (222Rn: 8 Bq/m3, Ar: 0.01 ppm, Kr: 0.03

ppt) • Humidified with water vapor 30%

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

– Leak requirements for all systems and plants < 10-8 atm/cc/s• Critical regions (pumps, valves, big flanges, small failures) were

protected with additional nitrogen blanketing


First result in August 2007 During the water filling During the PC filling Finally, May 15th, 2007

We have measured the scattering rate of 7Be solar s on electrons

7Be Rate: 47 ± 7STAT ±

12SYS c/d/100 tAugust 16(2007): PLB 658, August 16(2007): PLB 658, 101(2008)101(2008)

From Aug 2006

From Jan 2007

Hight purity water

Liquid scintillatorLow Ar and Kr N2


47 ± 7stat cpd/100tons for 862 keV 7Be solar Using LMA with:

m122=7.92·10-5 eV2

sin212=0.314

and BPS07(GS98)

Syst. Error: 25%

Expected rate (cpd/100 t)

No oscillation 75 ± 4

BPS07(GS98) HighZ

49 ± 4

BPS07(AGS05)LowZ

44 ± 4


Background: 232Th content Assuming secular equilibrium, 232Th is measured with the delayed

coincidence:

212Bi 212Po 208Pbβ

= 432.8 ns

2.25 MeV ~800 KeV eq.

From 212Bi-212Po correlated events in the scintillator: 232Th: < 6 ×10-18 g(Th)/g (90% C.L.)

Specs: 232Th: 1. 10-16 g/g 0.035 cpd/ton

2 2 2R x y z= + + 2 2cR x y= +

Only fewbulk candidates

212Bi-212Po

Time (ns)

=423±42 ns

Events are mainly in the south vessel surface (probably particulate)

z (m

)

(m) (m)


Background: 238U content

Assuming secular equilibrium, 238U is

measured with the delayed coincidence:214Bi 214Po 210Pb

β = 236 s

3.2 MeV ~700 KeV eq.

214Bi-214Po

=240±8s

Time s

214Bi-214Po

2 2cR x y= +

z (m

)

Setp - Oct 2007

Specs: 238U: 1. 10-16 g/g

< 2 cpd/100 tons

238U: = 6.6 ± 1.7×10-18 g(U)/g


Background: 210PoNOTES• The bulk 238U and 232Th

contamination is negligible• The 210Po background is

NOT related neither to 238U contamination NOR to 210Pb contamination

210Po decay time: 204.6 days

• 210Bi no direct evidence----> free parameter in the total fit

cannot be disentangled, in the 7Be energy range, from the CNO

60 cpd/1ton

• Not in equilibrium with 210Pb !• 210Po decays as expected


Only 4 events (βare selected in the IV in ~120 d.1.4 events were expected from 14C-210Po random coincidences

the 85Kr contamination upper limits <35 counts/day/100 ton (at 90% C.L.)

More statistics is needed---> Taken as free parameter in the total fit

Background: 85Kr

85Kr is studied through :

85Kr β decay :(βdecay has an energy spectrum similar

to the 7Be recoil electron )

85Kr β

85Rb

687 keV

= 10.76 y - BR: 99.56%

85Rb85Kr 85mRb

= 1.46 s - BR: 0.43%

514 keV

β

173 keV


Cosmic are identified by the OD and by the ID• OD eff: ~ 99%• ID analysis based on pulse shape

variables– Pulse mean time, peak position in

time• Estimated overall rejection factor:

– > 104 (still preliminary)

ID efficiency

A muon in OD

Muon flux:(1.21±0.05)h-

1m-2

Muon angular distributions

After cuts, are not a relevant background for 7Be analysis– Residual background: < 1 c/d/100 t


Position reconstruction• Position reconstruction algorithms

– Base on time of flight fit to hit time distribution– developed with MC, tested and validated in CTF– cross checked and tuned in Borexino on selected events (14C,

214Bi-214Po, 11C)

The fit is compatible with the expected r2-like shape with R=4.25m.

The time and the total charge are measured, and the position is reconstructed for each event . Absolute time is also provided (GPS)

14C

Radius (m)

Spatial resolution: 35 cm at 200 keV16 cm at 500 keV (scaling as )

€

N p.e.−1/ 2

214Bi-214Po

β distance(m)


Fiducial volume

Radial distribution

R2

gauss

2 2 2R x y z= + + 2 2cR x y= +

z vs Rc scatter plot

FV

the nominal Inner Vessel radius: 4.25m (278 tons of scintillator)the effective I.V. radius has been reconstructed using: # 14C events # Thoron (=80s) on the I.V. surface (emitted by the nylon) # External background gamma # Teflon diffusers on the IV surface maximum uncertainty : ~ +-12%

FM: by rescaling background components known to be uniformly distributed within the LS and using the known LS mass (278.3 t)

from PMTs that penetrate the buffer

z < 1.8 m, was done to remove gammas from IV endcaps


Light Yield

The 11C sample is selected through the triple coincidence with muon and neutron. We limited the sample to the first 30 min of 11C time profile, which reduces the random coincidence to a factor 1/14.

The Light Yield has been evaluated fitting the 14C spectrum,

(Borex. Coll. NIM A440, 2000)

and the 11C spectrum

14C spectrum (β decay-156 keV, end point)

11C spectrum(β+ decay-960 keV)

Light Yield = 500 +- 12 p.e./MeV

The energy equivalent to the sum of the two quenched 511 keV gammas: E2511) = 0.83 +- 0.03 MeV. Energy resolution: 10% at 200

keV 8% at 400 keV 6% at 1 MeV

The light yield has been evaluated also by taking it as free parameter in a global fit on the total spectrum (14C,210Po, 210Po ,7Be Compton edge)


Final spectrum Final spectrum after all cutsafter all cuts

Understanding the final spectrum:

main components

14C No cutsKr+Be

11C After cut

AfterFV cuts

10C+ ext. bkg

210Po (only, not in eq. with 210Pb!)


Spectral fit to determine the signal

• Stat. error at present includes lack of knowledge of 85Kr

• Syst. uncertainty comes from Fiducial Mass estimation (max error)

47 days of live time (August 2007)• Strategy:

– Fit the shoulder region only– Use between 14C end point and 210Po

peak to limit 85Kr content– pep and 8B neutrinos fixed at SSM-

LMA value– Other backgrounds (U, Th)

negligible with the present radiopurity

– 210Po peak not included in this fit

• Fit components– 7Be, 85Kr– CNO+210Bi combined

• very similar in this limited energy region

– Light yield left free

These bins used to limit 85Kr content in fit

7Be Rate: 47 ± 7STAT ± 12SYS c/d/100 t


Spectral fit in ~200 days

• Improvements:– Better definition of FV (use internal source and diffuser balls deployed on the

IV surface)– PMT charge equalization– LY (but still free parameter in a global fit on the total spectrum)– Better background measurements– Detector stability– Fit in the range 150-2000 keV

• Background issue:– 85Kr– 210Bi - 40K no signature– 11C : reduction by

tagging -induced neutrons identification is in progress


Comments on errors

• Statistical:– Right now, it includes combined the effect of statistics itself, the

lack of knowledge of 85Kr content, and the lack of a precise energy calibration

– These components are left free in the final fit, and contribute to the statistical error

• Systematic (the evaluation is still in progress):– Fiducial volume determination: it is improved due to a

better understanding of the detector response. – Max. range of 7Be flux due to poor knowledge of the

background 210Bi/40K which are in competition with CNO ’s:– Events selection (background subtraction: muons, Rn.. ), energy

scale

7Be Rate: 47 ± 7STAT ±

12SYS c/d/100 t

August (2007)August (2007)

15%

6%


• s may produce 11C by spallation on 12C – n are also produced ~

90% of the times– Untill now, only the

first neutron after a muon can be currently detected

– Events that occur within 2 ms after a are rejected

CCn+

+e++en capture MeV)

Cylindrical cut Around muon-track

Spherical cut around2.2 gamma to reject 11C event

Neutron production

Muon track

Main problem: 11C

pep and CNO fluxespep and CNO fluxes

CNO11C

pep

Simulated


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


What next What next

@ possibly p-p neutrinos

@ seasonal variations of the solar flux due to the eccentricity of the Earth orbit

250-800 keVEn. window


@ search for antineutrinos (from Sun, Earth,reactors) Borex. Coll. Eur. Phys.J. C47,2006

good tagging: +p n+e+ signal > 1 MeV ≈200s neutron capture: signal 2.2 MeV--->> geoneutrinos

Main bckg: from reactorsIn 300 tons: 7- 17 ev/y (BSE)- S/N=1Antineutrinos from Reactors; long base line: ≈1000 kmRate: ≈ 20 ev/y

€

€

What next (cont.)What next (cont.)


CONCLUSIONSCONCLUSIONS

>> Borexino just started the study of the various solarneutrino sources below 2 MeV, with a real time detection ( pp,7Be, pep, CNO)

>> Future goal (in a few months): – try to tag 11C – CNO study

>> The program includes also the study of the antineutrinosfrom Sun, Earth, Reactors)

>> Borexino in also a useful observatory for the Supernova


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


Documents

Borexino and Solar Neutrinos