Borexino and Solar Neutrinos

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Borexino and Solar Neutrinos

• Emanuela Meroni
• Università di Milano INFN
• On behalf of the Borexino Collaboration

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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 ? is the main
  target
• Pep, CNO and possibly pp ?
• Geoneutrinos
• Supernova ?
• Detection via scintillation light
• Very low energy threshold
• Good position recostruction
• Good energy resolution
• Drawbacks
• No direction measurements
• ? induced events cant be distinguished from
  ß-decay due to natural radioactivity

Typical ? rate (SSMLMABorexino)
Extreme radiopurity of the scintillator
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Detector design and layout
Borexino detector at LNGS
Stainless Steel Sphere 2212 photomultipliers
1350 m3
Scintillator 270 t PCPPO in a 150 ?m thick
nylon vessel
Water Tank g and n shield m water C detector 208
PMTs in water 2100 m3
Nylon vessels Inner 4.25 m Outer 5.50 m
20 legs
Carbon steel plates
Design based on the principle of graded
shielding
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Background suppression strategies 15 years of
work

• gs 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 lt
  10-8 atm/cc/s
• Critical regions (pumps, valves, big flanges,
  small failures) were protected with additional
  nitrogen blanketing

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First result in August 2007
During the water filling
Finally, May 15th, 2007
During the PC filling
Liquid scintillator
Low Ar and Kr N2
Hight purity water
From Aug 2006
From Jan 2007

• We have measured the scattering rate of 7Be solar
  ns on electrons
• 7Be n Rate 47 7STAT 12SYS c/d/100 t
• August 16(2007) PLB 658, 101(2008)

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47 7stat cpd/100tons for 862 keV 7Be solar n

Syst. Error 25
Using LMA with dm1227.9210-5
eV2 sin2q120.314 and BPS07(GS98)
Expected rate (cpd/100 t)
No oscillation 75 4
BPS07(GS98) HighZ 49 4
BPS07(AGS05) LowZ 44 4
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Background 232Th content
Specs 232Th 1. 10-16 g/g 0.035
cpd/ton
Assuming secular equilibrium, 232Th is measured
with the delayed coincidence
212Bi-212Po
?42342 ns
Time (ns)
Events are mainly in the south vessel surface
(probably particulate)
z (m)
Only few bulk candidates
(m)
(m)
From 212Bi-212Po correlated events in the
scintillator 232Th lt 6 10-18 g(Th)/g (90
C.L.)
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Background 238U content
Specs 238U 1. 10-16 g/g
Assuming secular equilibrium, 238U is measured
with the delayed coincidence
214Bi-214Po
?2408?s
Time ?s
Setp - Oct 2007
214Bi-214Po
z (m)
lt 2 cpd/100 tons 238U 6.6 1.710-18 g(U)/g
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Background 210Po

• NOTES
• The bulk 238U and 232Th contamination is
  negligible
• The 210Po background is NOT related neither to
  238U contamination NOR to 210Pb contamination

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

• 210Po decay time 204.6 days

60 cpd/1ton

• 210Bi
• no direct evidence----gt free parameter in the
  total fit
• cannot be disentangled, in the 7Be energy
  range, from the CNO

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Background 85Kr
85Kr ? decay (b decay has an energy spectrum
similar to the 7Be recoil electron )
85Kr is studied through

• Only 4 events (?????are selected in the IV in
  120 d.
• 1.4 events were expected from 14C-210Po random
  coincidences
• the 85Kr contamination upper limits lt35
  counts/day/100 ton (at 90 C.L.)
• More statistics is needed---gt Taken as free
  parameter in the total fit

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Cosmic m

• ? 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
• gt 104 (still preliminary)

A muon in OD
Muon angular distributions
ID efficiency

• After cuts, m are not a relevant background for
  7Be analysis
• Residual background lt 1 c/d/100 t

Muon flux(1.210.05)h-1m-2
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Position reconstruction
The time and the total charge are measured, and
the position is reconstructed for each event .
Absolute time is also provided (GPS)

• 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)

14C
214Bi-214Po
Radius (m)
??? distance(m)
The fit is compatible with the expected r2-like
shape with R4.25m.
Spatial resolution 35 cm at 200 keV 16 cm at
500 keV (scaling as )

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Fiducial volume

• 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

Radial distribution
z vs Rc scatter plot

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

FV
R2
gauss
g from PMTs that penetrate the buffer
FM by rescaling background components known to
be uniformly distributed within the LS and using
the known LS mass (278.3 t)
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Light Yield
The light yield has been evaluated also by taking
it as free parameter in a global fit on the total
spectrum (14C,210Po, s 210Po ,7Be n Compton edge)
14C spectrum (b? decay-156 keV, end point)
The Light Yield has been evaluated fitting the
14C spectrum, (Borex. Coll. NIM A440, 2000) and
the 11C spectrum
11C spectrum(? decay-960 keV)
Light Yield 500 - 12 p.e./MeV The energy
equivalent to the sum of the two quenched 511 keV
gammas E2??511) 0.83 - 0.03 MeV.

• 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.

Energy resolution 10 at 200 keV
8 at 400 keV
6 at 1 MeV
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Final spectrum after all cuts
Understanding the final spectrum main components
210Po (only, not in eq. with 210Pb!)
14C
No cuts
85Kr7Be n
11C
After m cut
10C ext. bkg
After FV cuts
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Spectral fit to determine the ? signal
7Be n Rate 47 7STAT 12SYS c/d/100 t

• 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
• CNO210Bi combined
• very similar in this limited energy region
• Light yield left free

These bins used to limit 85Kr content in fit

• Stat. error at present includes lack of knowledge
  of 85Kr
• Syst. uncertainty comes from Fiducial Mass
  estimation (max error)

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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

Preliminary

• Background issue
• 85Kr
• 210Bi - 40K no signature
• 11C reduction by tagging ?-induced neutrons
  identification is in progress

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Comments on errors

• 7Be n Rate 47 7STAT 12SYS c/d/100 t
• August (2007)

? ?6

• 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

? ?15
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pep and CNO fluxes
Simulated
CNO
Main problem 11C
11C
pep

• ms 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 m are
  rejected

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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

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What next

• _at_ possibly p-p neutrinos
• _at_ seasonal variations of the solar n flux due to
  the
• eccentricity of the
• Earth orbit
• 250-800 keV
• En. window

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What next (cont.)
_at_ search for antineutrinos (from Sun,
Earth,reactors)
Borex. Coll. Eur. Phys.J.
C47,2006 good tagging p
ne signal gt 1 MeV

200ms
neutron capture signal 2.2
MeV ---gtgt geoneutrinos Main bckg from
reactors In 300 tons 7- 17 ev/y (BSE)-
S/N1 Antineutrinos from Reactors long base
line 1000 km Rate 20 ev/y
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CONCLUSIONS

• gtgt Borexino just started the study of the various
  solar neutrino sources below 2 MeV, with a real
  time detection ( pp,7Be, pep, CNO)
• gtgt Future goal (in a few months)
• try to tag 11C
• CNO study
• gtgt The program includes also the study of the
  antineutrinos (from Sun, Earth, Reactors)
• gtgt Borexino in also a useful observatory for the
  Supernova

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Borexino collaboration
Genova
Princeton University
APC Paris
Virginia Tech. University
Munich (Germany)
Dubna JINR (Russia)
Kurchatov Institute (Russia)
Jagiellonian U. Cracow (Poland)
Heidelberg (Germany)
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