1. 1 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano Borexino: A Real Time Liquid Scintillator Detector for Low Energy Solar Neutrino Study Lino Miramonti Milan University & I.N.F.N  (7Be) ) = 862 keV

2. 2 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano L aboratori N azionali G ran S asso ~ 3500 meters water equivalent Borexino

3. 3 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano The Borexino conceptual design ν are detected via the: The signature of the ν event is the scintillation light produced by the recoil e - σ( ν e )  5σ( ν μ,τ )

4. 4 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano E ν = 862 keV (monochromatic) Φ SSM = 4.8 · 10 9 ν s -1 cm 2 σ  10 -44 cm 2 (@ 1 MeV) Recoil electron energy νeνe νxνx

5. 5 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano

6. 6 Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano The Scintillator 300 tons of liquid scintillator PC + PPO (1,5 g/l)  = 0.88 g cm -3 n = 1.505 Characteristics of the scintillator: High photon yield (  11000 ph/MeV) Fast response (τ  3.6 ns) (τ  5.5 ns in large volumes) Good α/β discrimination High transparency Can be purified λ max  365 nm alpha gamma/beta Normalized emission spectra of pseudocumene (PC) and of the scintillator mixture (PC+PPO 1.5 g/l) Time decay distribution of the scintillator for emission excited by α or β-γ radiation Characteristics of the PMs: 8” Thorn EMI 9351 Efficiency = 26 % (@ 420 nm) Transit time spread σ = 1 ns Peak/valley = 2.5 Dark noise = 1 kHz Gain = 10 7 Energy Resolution: FWHM  12% @ 1 MeV Time Resolution:  10 cm @ 1 MeV

7. 7 Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano Inner Vessel Outer Vessel PMTs on the Stainless Steel Sphere

8. 8 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano for Φ ν = 4.8 · 10 9 cm -2 s -1 σ ~ 10 -44 cm 2 ~ 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 “ν 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 MaterialU & 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, N 2 stripping, distillation and silica gel For water – reverse osmosis, deionization, N 2 bubbling and stripping Counting Test Facility The capability to reach the requested radiopurity levels has been proved by the Counting Test Facility (Borexino prototype) in 1997

9. 9 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano 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

10. 10 Calor02 Pasadena (USA) 25-29 March 2002Lino Miramonti - University and INFN Milano 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. 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.