First real time 7Be solar detection in Borexino

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First real time 7Be solar ? detection in Borexino

• Davide DAngelo
• INFN Sez. Milano
• On behalf of the Borexino Collaboration

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Contents

• Physics goals, detector design
• Detection principles and ? signature
• Detector design
• Radiopurity issues
• The last months
• Detector response, data analysis
• Event selection
• Detector response
• Background content
• Energy calibration and stability
• 11C and neutrons after muons
• Spectral fits
• Comments on errors Conclusions

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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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Detection principles and ? signature
Borexino principal aim mono-energetic 0.862 MeV
7Be ?

• elastic scattering on electrons in highly
  purified liquid scintillator
• Only 7Be ? considered so far.
• pep ?, CNO ? and possibly pp ? will be studied in
  the future
• Detection via scintillation light
• Very low energy threshold
• Good position reconstruction
• Good energy resolution
• BUT
• No direction measurement
• The ? induced events cant be distinguished
  from other ß events due to natural radioactivity
• Extreme radiopurity of the scintillator is a
  must!

previous real-time measurements (SNO, SuperK) lt
1/10.000 of the total solar ? flux
Borexino threshold
Typical ? rate (SSMLMABorexino)
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Abruzzo, Italy 120 Km from Rome
Laboratori Nazionali del Gran Sasso Assergi
(AQ) Italy 3500 m.w.e
External Labs
Borexino Detector and Plants
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Detector layout and main features

• Stainless Steel Sphere
• 2212 PMTs
• 1000 m3 buffer of pcdmp (light queched)

Scintillator 270 t PCPPO (1.4 g/l)
Nylon vessels (125 µm thick) Inner 4.25
m Outer 5.50 m (radon barrier)
Water Tank ? and n shield µ water C detector 208
PMTs in water 2100 m3
Carbon steel plates
20 legs
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Picture gallery
Pmt sealing PC Water proof
2000
Nylon vessels installation (2004)
PMT installation in SSS
2002
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Latest time schedule....

• filling operations
• purging of the SSS volume with LAKN (early 06)
• water filling (Aug. 06 ? Nov. 06)
• replacement of water with PCPPO or PCDMP (Jan.
  07 ? May. 07)
• DATA TAKING with fully filled detector from May
  15, 2007

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Background summary table
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Whats important of previous table

• 238U and 232Th content in the scintillator and in
  the nylon vessels meet specifications or
  sometimes are even below specs
• GOAL lt 10-16 g/g (lt 10 c/d/FV) ACHIEVED lt 10-17
  g/g
• 14C/12C is 10-18 as expected (2.7 10-18
  measured)
• Muon rejection is fine lt 10-4
• Two main backgrounds are still above specs,
  although managable
• Off equilibrium 210Po ?s (no evidence of 210Pb or
  210Bi at that level)
• Some 85Kr contamination a small air leak during
  filling?

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Our first result (astro-ph 0708.2251v2)

• We have detected the scattering rate of 7Be solar
  ?s on electrons
• 7Be ? Rate 47 7STAT 12SYS c/d/100 t

2 approaches... 1 result
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The starting point no cut spectrum
14C dominates below 200 keV
210Po NOT in eq. with 210Pb
Arbitrary units
Mainly external ?s and ?s
Photoelectrons
Statistics of this plot 1 day
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? rejection
ID efficiency

• ? are identified by OD and ID
• OD efficiency gt 99
• ID analysis based on pulse shape
• Deutsch variable fraction of light in the PMTs
  with concentrators
• Pulse mean time time of the peak.
• Overall rejection factor
• gt 104 (still preliminary)
• ? are efficiently tagged for 7Be
• residual background lt 1 c/d/100 t

A muon in OD
A muon in OD
No cuts
After ? cut
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Position reconstruction and fiducial volume
Resolution _at_ 214BiPo (800 keV) 142 cm _at_
14C (100 keV) 414 cm

• Position reconstruction algorythms (4 codes)
• Time-Of-Flight fit to hit time distribution
• developed with MC, tested and validated in CTF
• checked and tuned with 214Bi-Po and 14C events
• External background is large at the periphery of
  the IV
• ? from materials (SSS, PMTs, cones) that
  penetrate the buffer
• They are removed by a fiducial volume cut R lt
  3.276 m (100 t)
• Additionally z lt 1.8 m to remove some Rn events
  introduced during filling

Radial distribution
z vs Rc scatter plot
R2
Preliminary
gauss
FV
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238U and 232Th content
Assuming secular equilibrium, 232Th and 238U are
measured with the delayed concidences
212Bi-212Po
232Th Events are mainly in the south vessel
surface (probably particulate)
212Bi-212Po
214Bi-214Po
Only 3 bulk candidates
238U lt 2. 10-17 g/g
232Th lt 1. 10-17 g/g
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11C and neutrons after muons

• ?s may produce 11C by spallation on 12C
• n are also produced 95 of the times
• Only the first neutron after a muon can be
  currently detected
• Work in progress to try to improve this
• Events that occur within 2 ms after a ? are
  rejected

Preliminary
Neutron Capture Time
Neutron spatial distribution
? 210 ?s
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Final spectrum after all cuts
Understanding the final spectrum main components
210Po (only, not in eq. with 210Pb!)
14C
??Kr?Be ??shoulder
No ?s
11C
After fiducial volume cut (100 tons)
Last cut 214Bi-214Po and Rn daughters removal
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Energy calibration and stability

• We have not calibrated with inserted sources
  (yet)
• Planned for the near (?) future
• So far, energy calibration determined from 14C
  end point spectrum
• Energy stability and resolution monitored with
  210Po ? peak
• Difficult to obtain a very precise calibration
  because
• 14C intrinsic spectrum and electron quenching
  factor poorly known

Light yield monitored with 210Po peak position
Light yield determined from 14C fit
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7Be signal fit without ??? subtraction

• Strategy
• Fit the shoulder region only
• Use between 14C end point and 210Po peak to limit
  85Kr content
• pep neutrinos fixed at SSM-LMA value
• Fit components
• 7Be ?
• 85Kr
• CNO210Bi combined
• very similar in this limited energy region
• Light yield left free

210Po peak not included in this fit
7Be ?
CNO 210Bi
85Kr
These bins used to limit 85Kr content in fit
only 85Kr and no 7Be fit excluded by gt 5?
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?/? discrimination
Full separation at high energy
Small deformation due to average SSS light
reflectivity
? particles
? particles
ns
250-260 pe near the 210Po peak
200-210 pe low energy side of the 210Po peak
2 gaussians fit
2 gaussians fit
??? Gatti parameter
??? Gatti parameter
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7Be signal fit ??? subtraction of 210Po peak
2 gaussians fit

• The large 210Po background is subtracted in the
  following way
• For each energy bin, a fit to the ??? Gatti
  variable is done with two gaussians
• From the fit result, the number of ? particles in
  that bin is determined
• This number is subtracted
• The resulting spectrum is fitted in the energy
  range between 270 and 800 KeV
• A small 210Po residual background is allowed in
  the fit

?
?
The two analysis yield fully compatible results
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Comments on errors
7Be ? Rate 47 7STAT 12SYS c/d/100 t

• 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
• mostly due to the fiducial volume determination
• with 45 days of data taking, and without an
  internal source calibration, we estimate an upper
  limit of 25 for this error
• can be much improved even without internal
  calibration with more statistics and better
  understanding of the detector response

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Conclusions

• Borexino has performed the first real-time
  detection of sub/MeV solar neutrinos
• with just 2 months of data a clear 7Be signal is
  visible after a few cuts
• better results to come in the near future
• ( checks on day/night, seasonal or long term
  effects)
• the central value is well in agreement with
  MSW/LMA
• theoretical prediction with oscillations
• 49 4 counts/day/100t
• measured rate
• 47 7stat 12syst counts/day/100t
• no oscillation expectation
• 75 4 counts/day/100t
• future scientific plans
• pp, pep and CNO neutrinos fluxes
• antineutrinos (earth, reactors, Sun)
• supernova
• neutrino magnetic moment

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Extra stuff

• just for questions...

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15 years of work

• Detector Plants
• All materials carefully and painfully selected
  for
• Low intrinsic radioactivity
• Low Rn emanation
• Good behaviour in contact with PC
• Pipes, vessels, plants
• electropolished, cleaned with detergent(s),
  pickled and passivated with acids, rinsed with
  ultra-pure water down to class 20-50
• The whole plant is vacuum tight
• Leak requirements lt 10-8 atm/cc/s
• Critical regions (pumps, valves, big flanges,
  small failures) with additional nitrogen
  blanketing

• PMTs (2212)
• Sealing PC and water tolerant
• Low radioactivity glass
• Light cones (Al) for uniform light collection in
  fiducial volume
• Time jitter 1.1 ns (for good spatial resolution,
  mu-metal shielding)
• 384 PMTs with no cones for ? id
• Nylon vessels
• Material selection for chemical mechanical
  strength
• Low radioactivity to get lt1 c/d/100 t in FV
• Construction in low 222Rn clean room
• Never exposed to air

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A matter of cleanliness

• Water ( production rate 1.8 m3/h)
• RO, CDI, filters, N2 stripping
• U, Th lt 10-14 g/g
• 222Rn 1 mBq/m3
• 226Ra lt0.8 mBq/m3
• 18.2-18.3 M?/cm typical _at_ 20C
• Scintillator
• IV PCPPO (1.5 g/l)
• OV Buffer PCDMP (5 g/l)
• PC Distillation (all PC)
• 6 stages distillation
• 80 mbar, 90 C
• Vacuum stripping with low Ar-Kr N2
• 222Rn 8 ?Bq/m3
• Ar 0.01 ppm
• Kr 0.02 ppt

• PPO purification
• PPO is solid.
• A concentrated solution (120 g/l) in PC is done
  first (master solution)
• Master solution was purified with
• Water extraction ( 4 cycles)
• Filtration
• Single step distillation
• N2 stripping with LAKN
• Filling operations
• Purging of the SSS volume with LAKN (early 06)
• Water filling (Aug. 06 ? Nov. 06)
• Replacement of water with PCPPO or PCDMP (Jan.
  07 ? May. 07)
• Mixing online

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The Problem of Polonium-210

• 210Po (alpha emitter) is present in Borexino at
  about 60 ev/(day x ton)
• more than 100x the predicted 7Be event rate
• Immediate predecessor, the beta-emitter 210Bi, is
  present (if at all) is less than 1/100th
• Po contaminants may have complicated chemistry
• not as easily removed as Pb or Bi?
• Matches experience of other experiments (e.g.
  KamLAND, cf. Kishimoto talk at TAUP 2007)

Data after fidl vol. and Rn cuts
Fitted 210Po alpha peak
210Bi spectrum if it were inequilibrium with
210Po
gt 2 ordersof magnitude
We see that 210Bi, 210Po are out of equilibrium

• Fortunately, unsupported 210Po goes away quickly!
  (? 200 days)Will be much better in 1-2 years.
• Meanwhile, use ?/? discrimination

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Excluding Radon Daughters

• 214Bi/Po are taggable by their coincidence and so
  can be easily removed from data sample.
• But also, note the progression of mean lifetimes
  in the radon decay chain222Rn ? 218Po ? 214Pb
  ? 214Bi ? 214Po ? 4.4 min 39 min 28
  min 237 ?s
• Excluding events preceding a 214BiPo coincidence
  by3 hours or less, and within 1 m of the 214BiPo
  events spatial locations, lets us eliminate gt
  90 of each of these five species from the data,
  with little sample loss!
• This is particularly useful for the ?-emitting
  isotope 214Pb, whose spectrum has a broad peak
  near the 7Be neutrino shoulder energy.

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What Next pep/CNO neutrinos

• Much harder targets to hit
• rates are lower than 7Be
• by another factor of 5-10
• spectra mostly obscured
• by 7Be neutrino signal
• above the 7Be shoulder, partially
• obscured by cosmogenic 11C (a ß-emitter)
• But also very scientifically desirable
• pep rate is closely tied to that of thepp
  neutrinos that are obscured by 14C
• CNO rate has great theoretical uncertainty (30)
  depending upon unknown factors of the solar
  chemical composition
• And already we have a plan to reduce background
• When a muon produces a 11C atom, 95 of the time
  a neutron is released.
• The neutron is quickly (200 ?s) captured by a
  proton, releasing a 2.2 MeV ?
• 11C has a mean life of 29 min.
• The muon, neutron and 11C decay can be correlated
  by their small spatial and temporal separations.

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What Next Geoneutrinos

• Anti-neutrinos are produced in Earths crust by
  radioactive ? decay (of exactly the isotopes that
  cause problems for us in the detector!)
• We can see them via p ?e ? n e
• first we see the positron annihilation (? 1.02
  MeV)
• then, with a mean life of 200 ?s, we see the
  neutron capture 2.2 MeV ?
• The reaction has a ? energy threshold of mn me
  - mp,or 1.8 MeV
• Expected rate is 10 events/year in 280 tons of
  scintillator (with a background of reactor
  anti-neutrinos on the same order due to European
  nuclear reactors).
• But the amount of radioactivity in the Earths
  crust is not yet very well known, so this data
  will be welcome!

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Supernova signals

• Galactic type-II SN of 3x1053erg at 10kpc

• 80ev of proton elastic scattering (quenching?)