1Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano

Borexino: A Real Time Liquid Scintillator Detector for Low Energy Solar Neutrino StudyLino Miramonti

Milan University & I.N.F.N

(7Be)) = 862 keV

2Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano

LaboratoriNazionaliGran Sasso

~ 3500 meters water equivalent

Borexino

3Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano

The Borexino conceptual design

ν are detected via the:

),,( exee xx

The signature of the ν event is the scintillation light produced by the recoil e-

The signature of the ν event is the scintillation light produced by the recoil e-

σ(νe) 5σ(νμ,τ)

4Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano

eLieBe 77

),,( exee xx

Eν = 862 keV (monochromatic)ΦSSM = 4.8 · 109 ν s-1 cm2

σ 10-44 cm2 (@ 1 MeV)

Recoil electron energy

νe

νx

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

6

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

The Scintillator

300 tons of liquid scintillatorPC + 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• λmax 365 nm

alphagamma/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 = 107

Energy Resolution: FWHM 12% @ 1 MeVTime Resolution: 10 cm @ 1 MeV

7Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano

Inner Vessel

Outer Vessel

PMTs on the Stainless Steel Sphere

8Calor02 Pasadena (USA) 25-29 March 2002 Lino Miramonti - University and INFN Milano

forΦν= 4.8 · 109 cm-2 s-1

σ ~ 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 “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 PMTsRadioactivity from the rockMuon 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 FacilityCounting Test Facility (Borexino prototype) in 1997

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

Borexino expected performancesIn 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

10Calor02 Pasadena (USA) 25-29 March 2002 Lino 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.