1. The first year of Borexino data
LIP
July 9, 2008 – Lisbon
Davide Franco
Milano University & INFN

2. Outline The role of neutrinos in particle physics and astro-particle
The physics of Borexino
The Borexino detector
The “radio-purity” challenge
The reached goals (7Be and mn)
Near and far future goals
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

3. Neutrinos: cosmic messengers
High energy protons 5·10Mpc
Astrophysical
source
neutrinos
High energy gammas
10 Mpc
Low energy protons deflected
Ideal messenger:
neutral
stable
weakly interacting
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

4. Messengers from the Sun’s core
Core ( Rs)
Nuclear reactions: T~ °K
energy chains pp e CNO (neutrino production)
Radiative region ( Rs)
Photons carry energy in ~ 105 y
Convective region ( Rs)
Strong convection and turbulence
Complex surface phenomena
Corona (> 1 Rs)
Complex magneto-hydrodynamic phenomena
Gas at T~ 106 °K
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

5. The Standard Solar Model
It is an evolution model f the Sun, starting from the origin up to now:
Includes:
Hydrostatic equilibrium equations
Energetic and mass balance
Nuclear Reactions
Energy transport
Depends on several parameters and/or assumptions:
Initial chemical composition
Nuclear cross sections
Magnetic field, rotation
The model predicts the actual temperature and hence neutrino production
Main uncertainties due to chemical compound and nuclear cross sections
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

6. Solar nuclear reactions
pp chain
Fusion of 4 protons in a 4He nucleus with emission of 27 MeV energy
Main chain for “small-medium size” stars, like the Sun
CNO cycle
Carbon-Nitrogen-Oxygen or CNO cycle converts hydrogen to helium
It is dominant in population I “hot” stars
In the Sun, the CNO cycle contributes to 1-2 %, but there is no any experimental evidence
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

7. Neutrino Production In The Sun
pp chain: pp, pep, 7Be, and 8B n
CNO cycle: 13N, 15O, and 17F n
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

8. Solar Neutrinos Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

9. Solar Neutrino Spectra
Gallex
GNO
Homestake
Sage
SNO
SuperK
(real time)
Borexino (real time)
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

10. The Standard Solar Model before 2004
One fundamental input of the Standard Solar Model is the metallicity of the Sun - abundance of all elements above Helium:
The Standard Solar Model, based on the old metallicity derived by Grevesse and Sauval (Space Sci. Rev. 85, 161 (1998)), was in agreement within 0.5 in % with the solar sound speed measured by helioseismology.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

11. Reconstructed speed of sound
The Sun Structure
The study of the Sun structure is supported by the “helium-seismology”
Pressure wave propagation in the Sun
Sun’s vibration modes depend on the internal structure
The Standard Solar Model can be tested by looking at the p-mode (acoustic waves), g-mode (density or gravity waves) and f-mode (surface density waves)
Reconstructed speed of sound
Surface waves
Reconstruction
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

12. The Standard Solar Model after 2004
Latest work by Asplund, Grevesse and Sauval (Nucl. Phys. A 777, 1 (2006)) indicates a metallicity lower by a factor ~2. This result destroys the agreement with helioseismology
[cm-2 s-1]
pp (1010)
pep (1010)
hep (103)
7Be (109)
8B (106)
13N (108)
15O (108)
17F (106)
BS05
AGS 98
6.06
1.45
8.25
4.84
5.69
3.07
2.33
5.84
BS05 AGS 05
5.99
1.42
7.93
4.34
4.51
2.01
3.25 D
-1%
-2%
-4%
-12%
-23%
-42%
-47%
-57%
Solar neutrino measurements can solve the problem!
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

13. Borexino goals: solar physics
First ever observations of sub-MeV neutrinos in real time
Check the balance between photon luminosity and neutrino luminosity of the Sun
CNO neutrinos (direct indication of metallicity in the Sun’s core)
pep neutrinos (indirect constraint on pp neutrino flux)
Low energy (3-5 MeV) 8B neutrinos
Tail end of pp neutrino spectrum?
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

14. Borexino goals: neutrino physics
Test of the matter-vacuum oscillation transition with 7Be, pep, and low energy 8B neutrinos
7Be
pep 8B
Check of the mass varying neutrino model (Barger et al., PRL 95, (2005))
Limit on the neutrino magnetic moment by analyzing the 7Be energy spectrum and with Cr source
Moreover: geoneutrinos and supernovae
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

15. Huge active masses and extremely low background are required
Detecting neutrinos
Solar neutrino flux on Earth F ~ 1010 cm-2 s-1 with energy [0.1-10] MeV
Weakly interacting : s ~ – cm2
At 1 MeV (F ~ 108 cm-2 s-1 ) in water (s ~2 x10-46 cm2) in 30 days (T = s) to see 1 event we need:
Huge active masses and extremely low background are required
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

16. First requirement: underground
Second requirement: ultra high purity materials and proper design to minimize contaminations
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

17. Borexino Collaboration
Genova
Milano
Princeton University
Perugia
APC Paris
Virginia Tech. University
Munich
(Germany)
Dubna JINR
(Russia)
Kurchatov
Institute
(Russia)
Jagiellonian U.
Cracow
(Poland)
Heidelberg
(Germany)
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

18. Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e
Abruzzo
120 Km da Roma
Laboratori
Nazionali del
Gran Sasso
Assergi (AQ)
Italy
~3500 m.w.e
Laboratori esterni
Borexino – Rivelatore e impianti
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

19. Detection principles and n signature
Borexino detects solar n via their elastic scattering off electrons in a volume of highly purified liquid scintillator
Mono-energetic MeV 7Be n are the main target, and the only considered so far
Mono-energetic pep n , CNO n and possibly pp n will be studied in the future
Detection via scintillation light:
Very low energy threshold
Good position reconstruction
Good energy resolution
BUT…
No direction measurement
The n induced events can’t be distinguished from other b events due to natural radioactivity
Extreme radiopurity of the scintillator is a must!
Typical n rate (SSM+LMA+Borexino)
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

20. n-spectrum in Borexino
NW-1 = 0.25 – 0.8 MeV (7Be)
NW-2 = 0.8 – 1.4 MeV (pep & CNO)
LMA –BP2004 – LUNA
3 years statistics
100 tons
Events/3years/100 tons/0.05 MeV
NW-1
NW-2
Source
Number of events in 3 y
NW-1
NW-2
7Be
28470
pep
1095
986 pp 8B 44 33
13N
1260 66
15O
1533
593
All
33497
1634
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

21. Borexino Background
Expected solar neutrino rate in 100 tons of scintillator ~ 50 counts/day (~ Bq/Kg)
Just for comparison:
Natural water
~ 10 Bq/kg in 238U, 232Th and 40K
Air
~ 10 Bq/m3 in 39Ar, 85Kr and 222Rn
Typical rock
~ Bq/kg in 238U, 232Th and 40K
BX scintillator must be 9/10 order of magnitude less radioactive than anything on earth!
- Low background nylon vessel fabricated in hermetically sealed low radon clean room (~1 yr)
- Rapid transport of scintillator solvent (PC) from production plant to underground lab to avoid cosmogenic production of radioactivity (7Be)
- Underground purification plant to distill scintillator components.
- Gas stripping of scintlllator with special nitrogen free of radioactive 85Kr and 39Ar from air
- All materials electropolished SS or teflon, precision cleaned with a dedicated cleaning module
Throughout the experiment we developed special methods to achieve extremely low levels of background in the detector.
[See text in slide.]
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

22. Detector layout and main features
Stainless Steel Sphere:
2212 PMTs
1350 m3
Scintillator:
270 t PC+PPO in a 150 mm
thick nylon vessel
Nylon vessels:
Inner: 4.25 m
Outer: 5.50 m
Water Tank:
g and n shield
m water Č detector
208 PMTs in water
2100 m3
Carbon steel plates
20 legs
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

23. Nylon vessel installation
PMTs: PC & Water proof
Nylon vessel installation
Installation of PMTs on the sphere
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

24. Storage area and Plants
Water Plant
Storage area and Plants
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

25. Counting Test Facility
CTF è un prototipo su piccola scala di Borexino:
~ 4 tons of scintillator
100 PMTs
Buffer of water
Muon veto
Vessel radius: 1 m
CTF ha dimostrato la fattibilità di Borexino
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

26. May 15, 2007 Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

27. Strategy for Reduction
Borexino background
RadioIsotope
Concentration or Flux
Strategy for Reduction
Name
Source
Typical
Required
Hardware
Software
Achieved m
cosmic
~200 s-1 m-2
~ 10-10
Underground
Cherenkov signal
<10-10
at sea level
Cherenkov detector
PS analysis
(overall)
Ext. g
rock
Water Tank shielding
Fiducial Volume
negligible
Int. g
PMTs, SSS
Material Selection
Water, Vessels
Clean constr. and handling
14C
Intrinsic PC/PPO
~ 10-12
~ 10-18
Old Oil, check in CTF
Threshold cut
238U
Dust
~ g/g
< g/g
Distillation, Water Extraction ~
232Th
Organometallic (?)
(dust)
(in scintillator)
Filtration, cleanliness ~
7Be
Cosmogenic (12C)
~ Bq/t
< 10-6 Bq/ton
Fast procurement, distillation
Not yet measurable ?
40K
Dust,
~ g/g
< g/g scin.
Water Extraction
PPO
< g/g PPO
Distillation
210Pb
Surface contam.
Cleanliness, distillation
from 222Rn decay
(NOT in eq. with 210Po)
210Po
Spectral analysis
~ 14
a/b stat. subtraction
~ 0.01 c/d/t
222Rn
air, emanation from
~ 10 Bq/l (air)
< 1 c/d/100 t
Water and PC N2 stripping,
Delayed coincidence
< 0.02 c/d/t
materials, vessels
~100 Bq/l (water)
(scintillator)
cleanliness, material selection
39Ar
Air (nitrogen)
~17 mBq/m3 (air)
Select vendor, leak tightness
85Kr
~ 1 Bq/m3 in air
Spectral fit
= 25±3
(learn how to measure it)
fast coincidence
= 29±14
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

28. “Few” details Detector & Plants
All materials carefully and painfully selected for:
Low intrinsic radioactivity
Low Rn emanation
Good behaviour in contact with PC
Pipes, vessels, plants:
electropolished, cleaned with detergent(s), pickled and passivated with acids, rinsed with ultra-pure water down to class 20-50
The whole plant is vacuum tight
Leak requirements < 10-8 atm/cc/s
Critical regions (pumps, valves, big flanges, small failures) were protected with additional nitrogen blanketing
PMTs (2212)
Sealing: PC and water tolerant
Low radioactivity glass
Light cones (Al) for uniform light collection in fiducial volume
Time jitter: 1.1 ns (for good spatial resolution, mu-metal shielding)
384 PMTs with no cones for m id
Nylon vessels
Material selection for chemical & mechanical strength
Low radioactivity to get <1 c/d/100 t in FV
Construction in low 222Rn clean room
Never exposed to air
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

29. Expected Spectrum
How did Borexino do? Let’s first state what is the ultimate goal. The ideal experiment looking at low energy solar neutrinos with organic liquid scintillators faces two main sources of irreducible background.
14C - in yellow - is an intrinsic low energy contaminant for the scintillator and determines the lowest possible threshold of sensitivity to solar neutrinos.
11C - in green - is a cosmogenic radionuclide. It is produced in situ by the residual muon flux. It was long thought to be irreducible background, covering the region between 1 and 2 MeV. Some recent on cosmogenics will allow a very significant reduction of this background.
All other source of background - including 85Kr, 210Po, 210Bi, that impede observation of Solar Neutrinos in other contexts - can be removed.
In between 14C and 11C is the expected signal of low energy solar neutrinos: 7Be - in red - and CNO + pep - in pink -. The 7Be neutrinos produce a box-like spectrum of recoil electrons with end point at 667 keV.
I remark that this is the ideal spectrum: what you get if you are able to remove all possible contaminants that can be removed.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

30. The starting point: no cut spectrum
How did Borexino do?
The black curve shows the all data collected in Borexino, with no cuts.
You can notice background from 14C at low energy and a peak around 400 keV due to alphas from 210Po, which are heavily quenched in organic liquid scintillator. The portion of the spectrum above 400 keV is dominated by external gammas and muons.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

31. Detecting (and rejecting) cosmic muons
m pulses
m crossing the buffer only
m crossing the scintillator
m are identified by ID and OD
OD eff: ~ 99%
ID based on pulse shape analysis
Rejection factor
> 103 (conservative)
A muon
in OD
m track
ID efficiency
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

32. Detecting (and rejecting) cosmogenic neutrons
t = 255 ± 5 ms
n + p
d + g
t ~ 250 ms
A dedicated trigger starts after each muon opening a gate for 1.6 ms.
An offline clustering algorithm identifies neutron in high multiplicity events
A muon
in OD
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

33. Residual background : << 1 c/d/100 t
Muon and neutron cuts
No cut
m cut
Residual background : << 1 c/d/100 t
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

34. Position reconstruction
Position reconstruction algorythms (we have 4 codes right now)
time of flight fit to hit time distribution
developed with MC, tested and validated in CTF
cross checked and tuned in Borexino with 214Bi-214Po events and 14C events
z vs Rc scatter plot
Resolution
214Bi-214Po (~800 KeV)
14±2 cm
14C (~100 KeV):
41±4 cm
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

35. Vessel Radius and Shape
Vessel radius and origin calibrated exploiting fast coincidences of 222Rn and 220Rn chain segments emanated from vessel nylon
212Bi-212Po
214Bi-214Po
t = ns
t = 236 ms
The two plots confirms the CCD camera calibration results: vessel origin is shifted along positive z-axis by about 5 cm.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

36. 7Be energy region (mainly 210Po)
Spatial resolution
22 ~400 keV FV
220Rn-216Po
7Be energy region (mainly 210Po)
214Bi-214Po (~800 KeV)
14±2 cm
14C (~100 KeV):
41±4 cm
m-induced neutrons FV
15 ~2200 keV
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

37. Spectrum after FV cut (100 tons)
The blue curve shows what happens when applying a fiducial cut to select the innermost 100 tons as the fiducial mass of the detector.
The peak at 400 keV becomes more prominent, because 210Po is intrinsic to the scintillator.
The background above 400 keV is reduced very substantially. A first evidence of the 7Be shoulder appears. You will also notice that the background above 1 MeV is limited to 11C.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

38. 238U content 214Bi 214Po 210Pb b a t = 236 ms 3.2 MeV ~700 keV eq.
Assuming secular equilibrium and looking in the FV only:
0.02 cpd/tons corresponding to
238U = (1.9 ± 0.3)×10-17 g(U)/g
214Bi
214Po
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

39. 232Th content 212Bi 212Po 208Pb b a t = 432.8 ns 2.25 MeV
~1000 keV eq.
t = 442 ± 57 ns
Events are mainly on the south vessel surface (probably particulate)
Assuming secular equilibrium and looking in the FV only :
cpd/ton corresponding to
232Th = (6.8±1.5)×10-18 g(Th)/g
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

40. a/b discrimination Full separation at high energy a particles
Small deformation due to average
SSS light reflectivity
a particles
b particles ns
pe; near the 210Po peak
pe; low energy side of the 210Po peak
2 gaussians fit
2 gaussians fit
a/b Gatti parameter
a/b Gatti parameter
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

41. 210Po contamination Not from 210Pb FV (R < 3 m) No radial cut
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

42. a/b statistical subtraction
The red curve shows what happens after application of pulse shape discrimination to remove alphas from 210Po.
The 210Po peak disappears.
The 7Be shoulder now appears in its full glory.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

43. Energy calibration and stability
We have not calibrated with inserted sources (yet)
Planned for the near future
So far, energy calibration determined from 14C end point spectrum
Energy stability and resolution monitored with 210Po a peak
Difficult to obtain a very precise calibration because:
14C intrinsic spectrum and electron quenching factor poorly known
Light yield determined from 14C fit
Light yield monitored with 210Po peak position
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

44. Energy scale
14C
MC vs data comparison of photoelectron time distributions from 14C
MC-G4Bx
Data
11C
m-induced neutrons
LY = 500 (1%) p.e./MeV
kB = (15%) cm/MeV
Ph.Y. ~ 9000 photons/MeV
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

45. New results with 192 days of statistics
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

46. New results with 192 days of statistics
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

47. Systematic and Final Result
Estimated 1σ Systematic Uncertainties* [%]
*Prior to Calibration
Expected interaction rate in absence of oscillations:
75±4 cpd/100 tons
for LMA-MSW oscillations:
48±4 cpd/100 tons, which means:
Total Scintillator Mass
0.2
Fiducial Mass Ratio
6.0
Live Time
0.1
Detector Resp. Function
Cuts Efficiency
0.3
Total
8.5
Signal from 7Be neutrinos was extracted through a fit in the region keV.
We determined that the neutrino signal is 47±7±12 cpd/100 tons. The large systematic error quoted in the paper is due to the lack of calibrations at the time of publication of the results - 2 1/2 months after the detector was full! Let me remark that the Borexino calibration program is designed to reach an accuracy of ±2%. The systematic error will be strongly suppressed in a short time.
The signal expected in absence of oscillations is 75±4 cpd/100 tons. The signal expected for LMA-MSW oscillations is 49±4 cpd/100 tons.
Our result is consistent - within errors - with the LMA-MSW expectation.
7Be Rate: 49±3stat±4syst cpd/100 tons , which means
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

48. Before Borexino
Here is what was known about the survival probability of electron neutrinos before Borexino.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

49. After Borexino
This is the improved situation after the initial release of the Borexino results, offering a clearer picture, and more consistent with the LMA-MSW paradigm.
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

50. Constraints on pp and CNO fluxes
Combining Borexino 7Be results with other experiments,the expected rate in Clorine and Gallium experiments is
Survival Probability
measured over
predicted flux ratio
where
Ri,k and Pi,k are calculated in the hypothesis of high-Z SSM and MSW LMA
Rk are the rates actually measured by Clorine and Gallium experiments
f8B is measured by SNO and SuperK to be 0.87 ±0.07
f7Be =1.02 ±0.10 is given by Borexino results
Plus luminosity constraint:
best determination of pp flux!
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

51. Neutrino Magnetic Moment
Neutrino-electron scattering is the most sensitive test for mn search
EM current affects cross section: spectral shape sensitive to μν sensitivity enhanced at low energies (c.s.≈ 1/T)
Estimate
Method
10-11 μB
SuperK 8B
<11
Montanino et al.
7Be
<8.4
GEMMA
Reactor
<5.8
Borexino
<5.4
A fit is performed to the energy spectrum including contributions from 14C, leaving μn as free parameter of the fit
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

52. What next? Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

53. BOREXino
CERN: 100 and 190 GeV muon beams on a 12C target: 11C represents 80% of all the muon-induced contaminants and more than 99% in the CNO pep-n energy window
Hagner et al., Astropart. Phys. 14, 33 (2000)
KamLAND
(from the Mitzui presentation at Neutrino06)
11C Rate
(cts / day / 100 tons)
All energy
0.8 – 1.4 MeV
KamLAND
107 55
BOREXino 15
7.4
SNO+
0.15
0.074
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

54. 11C production and decay
m (+ secondaries) + 12C → m (+ secondaries) + 11C + n
Coincidence among:
cosmic muon:
rate at LNGS (3700 mwe): 1.16 hr-1 m-2
average energy: 320 GeV
gamma from neutron capture:
energy: 2.2 MeV
capture time: 250 ms
positron from 11C decay:
deposited energy between and MeV
mean life: 30 min
n + p → d + g
11C → 11B + e+ + ne
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

55. Large scintillator detector potentialm
PC+PPO
11C n g
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

56. Muon track reconstruction
pep and CNO n fluxes
software algorithm based on a three-fold coincidence analysis to subtract efficiently cosmogenic 11C background
Muon track reconstruction
8B at low energy region (3-5 MeV)
pp seasonal variations (?)
High precision measurements
systematic reduction
calibrations
geoneutrinos
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008

57. Conclusion
Borexino opened the study of the solar neutrinos in real time below the barrier of natural radioactivity (4 MeV)
Two measurements reported for 7Be neutrinos
Best limits for pp and CNO neutrinos, combining information from SNO and radiochemical experiments
Opportunities to tackle pep and CNO neutrinos in direct measurement
Borexino will run comprehensive program for study of antineutrinos (from Earth, Sun, and Reactors)
Borexino is a powerful observatory for neutrinos from Supernovae explosions within few tens of kpc
Best limit on neutrino magnetic moment. Improve by dedicated measurement with 51Cr neutrino source
Davide Franco – Università di Milano & INFN
Lisbon – July 9, 2008