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Borexino A Real Time Liquid Scintillator
Detector for Low Energy Solar Neutrino Study
Lino Miramonti Milan University I.N.F.N
E(n(7Be)) 862 keV
Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano
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Laboratori Nazionali Gran Sasso
Borexino
3500 meters water equivalent
Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano
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The Borexino conceptual design
? are detected via the
s(?e) ? 5s(?µ,t)
The signature of the ? event is the scintillation
light produced by the recoil e-
Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano
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E? 862 keV (monochromatic) FSSM 4.8 109 ?
s-1 cm2
?e
?x
Recoil electron energy
s ? 10-44 cm2 (_at_ 1 MeV)
Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano
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Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano
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The Scintillator

• Characteristics of the scintillator
• High photon yield (? 11000 ph/MeV)
• Fast response (t ? 3.6 ns)
• (t ? 5.5 ns in large volumes)
• Good a/ß discrimination
• High transparency
• Can be purified
• ?max ? 365 nm

300 tons of liquid scintillator PC PPO (1,5
g/l) r 0.88 g cm-3 n 1.505
Energy Resolution FWHM ? 12 _at_ 1 MeV Time
Resolution ? 10 cm _at_ 1 MeV
alpha
gamma/beta

• Characteristics of the PMs
• 8 Thorn EMI 9351
• Efficiency 26 (_at_ 420 nm)
• Transit time spread s 1 ns
• Peak/valley 2.5
• Dark noise 1 kHz
• Gain 107

Time decay distribution of the scintillator for
emission excited by a or ß-? radiation
Normalized emission spectra of pseudocumene (PC)
and of the scintillator mixture (PCPPO 1.5 g/l)
Lino Miramonti - University and INFN Milano
Calor02 Pasadena (USA) 25-29 March 2002
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PMTs on the Stainless Steel Sphere
Outer Vessel
Inner Vessel
Lino Miramonti - University and INFN Milano
Calor02 Pasadena (USA) 25-29 March 2002
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for F? 4.8 109 cm-2 s-1 s 10-44 cm2
55 events per day in 100 tons of fiducial
volume! (250 keV 800 keV)
Reducing the background!
The background which could give signal in the so
called ? window (250 keV 800 keV) can be
divided in two main classes Internal background
(scintillator itself) Mainly natural
radioactivity External background (coming from
outside the IV) Radioactivity from
PMTs Radioactivity from the rock Muon induced
background
Material U Th content
Scintillator ? 10-16 g/g
Nylon Vessel ? 10-12 g/g
PC buffer ? 10-14 g/g
Water buffer ? 10-10 g/g

• Radiopurity levels obtained by
• For scintillator water extraction, N2
  stripping, distillation and silica gel
• For water reverse osmosis, deionization, N2
  bubbling and stripping

The capability to reach the requested radiopurity
levels has been proved by the Counting Test
Facility (Borexino prototype) in 1997
Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano
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Borexino expected performances
In non-oscillation frame Signal 55 events per
day (in 100 tons, 250-800 keV ) Bkg 15
events per day (in 100 tons, 250-800 keV)
Oscillation frame Events/day (in 100 t, 250-800 keV)
SMA 12
LMA 31
LOW 29
Vaccum 23
Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano
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Present status of the detector

• The 92 of PMTs are installed.
• The PC procurement is in progress.
• Water filling summer-fall 2002.
• Borexino will take data (background) during the
  filling of the detector.
• Borexino will take data (physics events) from
  the spring 2003.

Calor02 Pasadena (USA) 25-29 March 2002
Lino Miramonti - University and INFN Milano