Status of the BOREXINO experiment Hardy Simgen Max-Planck-Institut für Kernphysik / Heidelberg for the BOREXINO collaboration

Outline BOREXINO physics program The BOREXINO detector Scintillator purification techniques Removal of gaseous impurities 11 C 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): 1 st real-time experiment at low energies

Solar neutrino spectrum BOREXINO  m 2 ≈ 8·10 -5 eV 2 27  <  < 38° Vacuum oscillations Matter effects Transition region

Solar neutrino physics Measurement of 7 Be- -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 SSM + flavour conversion

Supernova neutrinos Main reaction channels N Events Inverse beta decay (anti- e ) ~80 12 C(, ’) 12 C* (E  = 15.1 MeV) ~20 -proton elastic scattering ~55 Galactic supernova: 10 kpc 3  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 results Nature 436 (2005) Expected spectrum: Large fraction of earth’s total heat (40 TW) from radioactivity (U/Th).

e-e-

Radiopurity requirements in the BOREXINO scintillator Expected 7 Be- ν -rate: ~35 events per day Each background contribution ≤1 event per day 14 C/ 12 C ~ nat K ( 40 K)~ g/g ( g/g) 232 Th~ g/g 238 U ( 226 Ra)~ g/g (3· g/g) Ar ( 39 Ar) ~70 Vol.-ppb(STP) Kr ( 85 Kr)~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, 222 Rn emanation studies) e.g. Inner Vessel:U/Th: ~ g/g 222 Rn emanation: <1  Bq/m 2 Scintillator purification: Distillation, H 2 O 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 N 2 /PC flow Gaseous impurities transferred to N 2 Achievable purity determined by N 2 purity Ultrapure N 2 required! N 2 purity requirements PCNitrogen Argon ( 39 Ar)<70 vol-ppb<0.4 vol-ppm Krypton ( 85 Kr)<0.1 vol-ppt 222 Rn ( 210 Pb) <70  Bq/m 3 (STP)<7  Bq/m 3 (STP)

BOREXINO N 2 purification plant Production rate: 100 m 3 /h 222 Rn ≤0.5 Bq/m 3 (STP)  ≤1 222 Rn-atom in 4 m 3 !

Nitrogen tests Nitrogen from different European suppliers investigated. Several plants can produce low Ar/Kr N 2 However, strong deviations after delivery (contamination during storage, transport and refilling)  N 2 delivery chain has to be tested under realistic conditions

SOL LN 2 MPIK Delivery chain succesfully tested: Ar: ~0.01 ppb (Goal: 0.4 ppm) Kr: ~0.02 ppt (Goal: 0.1 ppt)

11 C background reduction Cylindrical cut around muon-track Spherical cut around neutron capture to reject 11 C event 11 C production with neutron (95% prob) PR C 71, (2005) Vetoing the intersection of the 2 volumes for C-lifetimes. 11 C production measured in CTF: PR C 74, (2006) Main background for pep / CNO neutrinos: Cosmogenically produced 11 C 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 CNGS neutrino beam  -beam from CERN Laura Perasso

First neutrino events First CNGS run in August h of data taking 55 t of water (h max ~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), h max ~ 10 m 10 h of running time Expected: 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!