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The Borexino
Solar Neutrino Experiment
Frank Calaprice
for the
Borexino Collaboration
1
June 14, 2013
RENO Workshop Seoul Korea
Borexino Collaboration
Princeton University
Genova
Milano
APC Paris
Virginia Tech. University
Perugia
Univ. Massachusetts
Dubna JINR
Kurchatov
Institute
Jagiellonian U.
Cracow
MPI Heidelberg
June 14, 2013
RENO Workshop Seoul Korea
Tech. Univ. Munich
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The Borexino Detector
(Mostly Active Shielding)
• Shielding Against Ext. Backgnd.
– Water:
2.25m
– Buffer zones:
2.5 m
– Outer scintillator zone: 1.25 m
• Main backgrounds: in Liq. Scint.
–
14C/12C
• 10-18 g/g. cf. 10-11 g/g in air CO2
– U, Th impurities
• Dirt
• Needed:
• Obtained:
10-6 g/g
10-16 g/g
10-18 g/g
– Radon daughters (210Pb, 210Bi, 210Po)
• Light yield (2200 PMT’s)
– Emitted:
– Detected:
11,000 photons/Mev
500 pe/MeV (~4%)
• Pulse shape discrimination.
– Alpha-beta particle separation
June 14, 2013
RENO Workshop Seoul Korea
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Solar Nuclear Fusion Cycles
The pp cycle
June 14, 2013
The CNO cycle
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Historical Note
• Chlorine experiment:
– First solar neutrino detector was the chlorine radiological experiment.
– Technique avoids the intense source of radiological backgrounds by
producing 37Ar by the reaction 37Cl(n,e)37Ar.
• Gallium radiochemical experiment
– Used simliar technique to measure pp neutrinos
• Kamiokande, Super-K, and SNO
– Detected high enegy 8B neutrinos (> 5 MeV ) to avoid radiological
backgrounds
• Borexino
– First experiment to directly detect neutrinos in the midst of soup of
radiological background @ E < 3 MeV.
– Made possible by development of new low-background methods.
– I like to call it a major breakthrough in experimental physics.
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RENO Workshop Seoul Korea
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Neutrino Detection
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Solar Neutrino Spectra
Neutrino Energy Spectrum
p.e.)
10
x
tons
100
x
(day
/
Events
103
102
10
1
10-1
10-2
10-3
Total spectrum
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n( B) = 0.46 cpd/100 tons
862
n(7 Be) = 47.6 cpd/100 tons
n(CNO) = 5.36 cpd/100 tons
n(pep) = 2.8 cpd/100 tons
n(pp) = 133 cpd/100 tons
100 200 300 400 500 600 700 800 900 1000
Photoelectrons
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RENO Workshop Seoul Korea
June 14, 2013
Neutrino-Electron Elastic Scattering
Energy Spectrum
Borexino Measurements 2007-2012
Solar Neutrinos
✓7Be
46.0
✓ 8B (> 3 MeV)
0.22
✓ Pep
3.1
✓ CNO limit
< 7.9
✓7Be day/night asy.
✓7Be annual mod.
cpd/100t
± 5%.
cpd/100t
± 19%
cpd/100t
± 22%
cpd/100t
A = 0.001 ± 0.014
PRL
PRD
PRL
PRL
PLB
PLB
2011
2010
2012
2012
2012
2012
PLB
2013
PRC
PRD
2010
2012
Geo-neutrinos
 Geo-neutrinos
14.3 ± 3.4 eV/(613 t-yr)
Rare Processes
 Test of Pauli Exclusion Principle in Nuclei
 Solar axion upper limit
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RENO Workshop Seoul Korea
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General Comments
Backgrounds
• Long-lived Cosmogenic:
–
–
•
in hydrocarbon liq. Scint.
Use material from deep site
•
•
•
Active shielding
Detector materials
•
•
Self shielding
Scintillation Pulse shape Discrimination
rejects a’s in scintillator
Radon daughters
background.
210Bi, 210Po
are serious
June 14, 2013
~ 7% @ 1 MeV
Event position determination
–
–
•
•
11,000 photons/ MeV
500 pe/MeV with 28% QE PMTs
Energy resolution
–
•
2200 8“ PMTs with concentrators.
Coverage: ~ 34%
Light yield:
–
–
•
Pseudocumene + 2.5 g/l DMP
Scintillation light is quenched.
Photomultipliers:
–
–
Rock (room background)
•
–
222Rn, 210Pb. 210Bi,
Pseudicumene + 1.5 g/l PPO
Buffer zones
–
–
Need deep site & active shielding.
Radiogenic (U, Th, K,
210Po)
–
•
–
14C
Short-lived Cosmogenic
–
•
Specifications.
• Liquid scintillator
photon time-of-flight.
Resolution: ~12 cm @ 1 MeV
Muon flux: 1.1 mu/m2/hr.
Alphs/beta separation: pulse shape
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2011 spectrum 7Be with 210Po a’s
210Po
210Bi
85Kr
CNO
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RENO Workshop Seoul Korea
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7Be:
fit of the energy spectrum
R  46 ± 1.5 ( stat ) 11..65 ( syst ) cpd / 100t
Rno oscillation  74± 5.2 cpd / 100t
5 s evidence of oscillation
ne flux reduction 0.62 +- 0.05
electron neutrino survival probability 0.51 +- 0.07
•Search for a day night effect:
•not expected for 7Be in the LMA-MSW model
•Large effect expected in the “LOW” solution (excluded by solar exp+Kamland)
ADN 
N D
 0.001± 0.012 ( stat) ± 0.007( sys)
( N  D) / 2
G. Bellini et al., Borexino Collaboration, Phys. Lett. B707 (2012) 22.
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The first pep n measurement :
multivariate analysis and background subtraction
Expected pep interaction rate: 2-3 cpd/100t
Background: 11C 210Bi external g
210Bi and CNO spectra: very similar
11C
210Bi
G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012) 051302..
pep
CNO
Three Fold Coincidence: 11C reduction
Novel pulse shape discrimination: e+ from 11C decay form Positronium
live time before annihilation in liquid: few ns
delayed scintillation signal
(Phys. Rev. C 015522 (2011))
Multivariate analysis:
fit of the energy spectra
 fit the radial distribution of the events ( external background is not uniform)
June 14, 2013
RENO Workshop Seoul Korea
 fit the pulse shape parameter
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Physics implication of the solar n Borexino results:
the Neutrino Survival Probability Pee(E)
Confirms MSW Vacuum to Matter Enhanced Oscillations
Before the Borexino results
G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012) 051302..
First solar pep neutrino detection
G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 107 (2011) 141362.
High precision 7Be solar neutrino measurement
Combined analysis
Borexino&solar
G. Bellini et al., Borexino Collaboration, Phys. Rev. D82 (2010) 033006.
8B flux with a threshold of 3MeV (e- recoil)
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RENO Workshop Seoul Korea
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Terrestrial and Reactor Neutrinos
• Terrestrial neutrinos are produced by long-lived radioactive
elements, U, Th, K.
– Energy is confined to < 3 MeV
– Radioactive decay accounts for significant part of known
heat produced inside earth
• Reactor neutrinos are produced by the decay of fission
fragments in nuclear reactors.
– Energies of reactor neutrinos are higher than geoneutrinos, but they can be an important background.
– No nuclear power reactors in Italy; background is small.
• Both neutrinos are seen together at low comparable rate.
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RENO Workshop Seoul Korea
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geon results: evidence of the signal
Others
back.
Ngeo
Nreactor
Ngeo
Nreactor
measured
measured
measured
measured
Events
events
events
events
TNU
TNU
60.4±2.4
0.70±0.18 14.3±4.4 31.2-6.1+7 38.8±12.0 84.5+19.3-16.9
Nreactor
Nreactor
Expected
with osc.
Expected
no osc.
events
33.3±2.4
Unbinned likelihood fit
No geon signal: rejected at 4.5 s C.L.
geon
reactor
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geon results: U and Th separation
Chondritic U-Th ratio
Fit with weight of 238U and 232Th spectra free
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RENO Workshop Seoul Korea
Best fit
S(238 U)= 26.5 ± 19.5 TNU
S(232 T) = 10.6 ± 12.7 TNU
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Borexino Phase 2
Solar Neutrino Program
• Technical goals:
– Reduce scintillator backgrounds with loop
purification
•
•
210Bi (210Pb)
85Kr
by nitrogen stripping
• Measurement goals
– pp neutrino observation
– CNO neutrinos detection or lower limit
– Improve pep, 7Be, 8B measurement
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RENO Workshop Seoul Korea
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Phase-2 Borexino Program
Scientific Goals
• The Metallicity Problem:
– Measurement of CNO neutrinos will shed light on
the controversial abundance of heavy elements.
• Sterile Neutrinos:
– The “SOX” Source Experiment will place a 10 MCi
51Cr source under Borexino to search for short
baseline beutrino oscillations.
• Motivated by reactor, gallium, and Miniboone neutrino
anamolies
June 14, 2013
RENO Workshop Seoul Korea
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The Solar Metallicity Problem
• In 1998 the metallicity (abundance of elements heavier
than 4He) determined from line spectra in Sun’s
atmosphere agreed well with other data.
– Standard solar model based on uniform composition.
– Helioseismology data
– Solar neutrino data (8B by SNO)
• Improvements were made in the analysis of solar
atmospheric spectra over next 10 years (3D model,etc.)
– A 2009 assessment of data resulted in a lower metallicity.
• Z /X = metal/hydrogen ratio = 0.024 (GS98)  0.018 (AGSS09).
– The new resukts are in conflict with helioseismic data that probe
the composition at greater depths in the sun.
• This is a serious problem for stellar models because it
implies that the chemical composition is not uniform.
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RENO Workshop Seoul Korea
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Re-Purification of the Liquid
Scintillator for Lower Background
• Reducing backgrounds is essential for Phase 2 solar program.
– 210Bi obscures CNO and pep neutrinos.
– 85Kr interferes with 7Be neutrinos
• Purification of the scintillator by “water extraction” and
“nitrogen stripping” was carried out recently.
– Backgrounds were reduced significantly.
– Lower background is still necessary.
• Refinements in water extraction are being developed.
• Discussion of purification in my next talk.
June 14, 2013
RENO Workshop Seoul Korea
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Lower Backgrounds after Recent
Scintillator Purification by
Water Extraction and N2 Stripping
Before Re-purification of L.S.
85Kr
June 14, 2013
210Bi
= 38 ± 2.9 cpd/100t
After Re-purification:
= 28 ± 5 cpd/100t
RENO Workshop Seoul Korea
210Bi
= 21 ± 4 cpd/100t
85Kr < 5 cpd/100t
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Short distance ne Oscillations
with Borexino (SOX)
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RENO Workshop Seoul Korea
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SOX Expected Sensitivity (51Cr)
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RENO Workshop Seoul Korea
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Conclusions
• Borexino was started in the early 90’s to
determine if the low energy 7Be solar neutrinos
exhibit neutrino oscillations.
• Twenty years later, the evidence for oscillations
with the peculiar energy dependence in matter
predicted in MSW theory is convincing.
• The new data were made possible with
innovations in low background methods that are
relevant for new rare event challenges:
– Direct detection of dark matter WIMPS
– Neutrinoless double beta decay
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RENO Workshop Seoul Korea
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