Status of the BOREXINO experiment

Hardy SimgenMax-Planck-Institut für Kernphysik / Heidelberg

for the BOREXINO collaboration

Outline

BOREXINO physics program The BOREXINO detector Scintillator purification techniques

Removal of gaseous impurities 11C background reduction Water and scintillator filling First neutrino events: The CNGS beam Conclusions

The Borexino Collaboration Italy (INFN & University of Milano and

Genova, Perugia Univ., LNGS) USA (Princeton Univ., Virginia Tech) Russia (RRC KI, JINR, INP MSU, INP

St. Petersburg) Germany (MPIK Heidelberg,

TU München) France (APC Paris) Hungary (Research Institute for

Particle & Nuclear Physics) Poland (Institute of Physics,

Jagiellonian University, Cracow)

BOREXINO physics program

Solar neutrinos

Supernova neutrinos

Reactor anti-neutrinos

Geological anti-neutrinos

Rare decay search

Solar neutrino physics

Two types of solar neutrino experiments Radiochemical experiments (low energy

threshold, integrated flux) Water experiments (real-time information, higher

energy threshold: Only ~10-4 of total flux)

BOREXINO (and KamLAND solar phase): 1st real-time experiment at low energies

Solar neutrino spectrum

BOREXINO

m2 ≈ 8·10-5 eV2

27< < 38°Vacuum

oscillations

Matter effects

Transition region

Solar neutrino physics

Measurement of 7Be--flux (~35 per day) 10% measurement yields pp--flux with <1%

uncertainty (Gallium experiments!) Measurement of pep--flux (~1 per day)

directly linked with pp--flux Measurement of CNO--fluxes (~1 per day)

Energy production in heavy stars SS

M +

fla

vour

con

vers

ion

Supernova neutrinos

Main reaction channels

NEvents

Inverse beta decay (anti-e)

~80

12C(,’)12C*(E= 15.1 MeV)

~20

-proton elastic scattering

~55

Galactic supernova: 10 kpc 31053 ergs

threshold: 250 keV

Anti-neutrino physics: European reactors

Gran Sasso laboratory

≥ 800 km baseline

Averaged oscillation signal expected.

Anti-neutrino physics:Geo-neutrinos from U/Th

KamLAND resultsNature 436 (2005) 499-503.

Expected spectrum:

Large fraction of earth’s total heat (40 TW) from radioactivity (U/Th).

e-

Radiopurity requirements in the BOREXINO scintillator

Expected 7Be-ν-rate: ~35 events per day Each background contribution ≤1 event per day

14C/12C ~10-18

natK (40K) ~10-14 g/g (10-18 g/g)232Th ~10-16 g/g

238U (226Ra) ~10-16 g/g (3·10-23 g/g)

Ar (39Ar) ~70 Vol.-ppb (STP)

Kr (85Kr) ~0.1 Vol.-ppt (STP)

Suppression of radioactive background

15 years of R&D: Development of new purification and detection techniques

Careful material selection (-spectrometry, mass spectrometry, 222Rn emanation studies) e.g. Inner Vessel: U/Th: ~10-12 g/g

222Rn emanation: <1 Bq/m2

Scintillator purification: Distillation, H2O extraction, Silicagel column,

nitrogen sparging

BOREXINO purification columns

Counting Test Facility (CTF)

Experimentally proven:Purity requirements can be fulfilled!

Example: Nitrogen sparging of scintillator

Countercurrent N2/PC flow

Gaseous impurities transferred to N2

Achievable purity determined by N2 purity

Ultrapure N2 required!

N2 purity requirements

PC Nitrogen

Argon (39Ar) <70 vol-ppb <0.4 vol-ppm

Krypton (85Kr) <0.1 vol-ppt <0.1 vol-ppt222Rn (210Pb) <70 Bq/m3 (STP) <7 Bq/m3 (STP)

BOREXINO N2 purification plant

Production rate: 100 m3/h222Rn ≤0.5 Bq/m3 (STP)

≤1 222Rn-atom in 4 m3!

Nitrogen tests

Nitrogen from different European suppliers investigated.

Several plants can produce low Ar/Kr N2

However, strong deviations after delivery (contamination during storage, transport and refilling)

N2 delivery chain has to be tested under realistic conditions

SOL LN2-tank @ MPIK

Delivery chain succesfully tested:

Ar: ~0.01 ppb (Goal: 0.4 ppm)Kr: ~0.02 ppt (Goal: 0.1 ppt)

11C background reduction

Cylindrical cut around muon-track

Spherical cut aroundneutron capture to reject 11C event

11C production with neutron (95% prob)

PR C 71, 055805 (2005)

Vetoing the intersection of the 2 volumes for 5-10 11C-lifetimes.11C production measured in CTF: PR C 74, 045805 (2006)

Main background for pep / CNO neutrinos: Cosmogenically produced 11C muon track

BOREXINO filling

Long stop after spill accident in 2002 Improvement of Gran Sasso safety and

environmental standards Operations with liquid resumed in 2006 BOREXINO filling strategy:

1: Filling inner detector with pure water

2: Replacing water by scintillator

3: Using same (+new) water to fill outer detector

PC procurement

Since January: Fresh-PC trucking from Sarroch to LNGS

Background data taking started

The CNGSneutrino beam

-beam from CERN

Laura Perasso

First neutrino events

First CNGS run in August 200630 h of data taking

55 t of water (hmax ~1.8 m) No reconstruction, only

time difference used Expectation:

5 -events (neutrino interactions in the rock)

seen 5 events

Second CNGS run in October

Detector filled with 1120 t of water (80% full), hmax ~ 10 m

10 h of running timeExpected:

10 CNGS events

seen: 12 events

A CNGS event

from CERN

Conclusions

After a long forced stop: BOREXINO water filling started in August 2006 Scintillator filling since end 2006 Detector is alive: Background data taking has

started (not yet fully shielded) First -events from CNGS beam detected BOREXINO detector expected to be in its final

configuration around May

Physics data taking in 2007!