Borexino and Solar Neutrinos
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NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Borexino and Solar Neutrinos
Emanuela MeroniUniversità di Milano & INFN
On behalf of the Borexino Collaboration
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Physics and detection principles
Borexino aims to measure low energy solar neutrinos in real time by elastic neutrino-electron scattering in a volume of highly purified liquid scintillator
Mono-energetic 0.862 MeV 7Be ν is the main target
Pep, CNO and possibly pp ν
Geoneutrinos
Supernova ν
Detection via scintillation light Very low energy threshold Good position recostruction Good energy resolution
Drawbacks: No direction measurements
ν induced events can’t be distinguished from β-decay due to natural radioactivity
Extreme radiopurity of the scintillator
Typical rate
(SSM+LMA+Borexino)
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Detector design and layout
Water Tank: and n shield water Č
detector208 PMTs in
water2100 m3
20 legs
Carbon steel plates
Scintillator:270 t PC+PPO in a 150 m thick nylon vessel
Stainless Steel Sphere:
2212 photomultipliers 1350 m3
Nylon vessels:Inner: 4.25 mOuter: 5.50 m
Design based on the principle of graded shielding
Borexino detector at LNGS
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Background suppression strategies= 15 years of work
• ’s from rocks, PMT, tank, nylon vessel– Detector design: concentric shells to shield the inner scintillator – Material selection and surface treatment– Clean construction and handling
• Internal background (238U, 232Th, 40K, 39Ar, 85Kr, 222Rn)– Scintillator purification:
• Distillation (6 stages distillation, 80 mbar, 90 °C)• Vacuum Stripping by LAK N2 (222Rn: 8 Bq/m3, Ar: 0.01 ppm, Kr: 0.03
ppt) • Humidified with water vapor 30%
– Master solution (PPO) purification: • Water extraction ( 5 cycles)• Filtration• Single step distillation• N2 stripping with LAKN
– Leak requirements for all systems and plants < 10-8 atm/cc/s• Critical regions (pumps, valves, big flanges, small failures) were
protected with additional nitrogen blanketing
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
First result in August 2007 During the water filling During the PC filling Finally, May 15th, 2007
We have measured the scattering rate of 7Be solar s on electrons
7Be Rate: 47 ± 7STAT ±
12SYS c/d/100 tAugust 16(2007): PLB 658, August 16(2007): PLB 658, 101(2008)101(2008)
From Aug 2006
From Jan 2007
Hight purity water
Liquid scintillatorLow Ar and Kr N2
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
47 ± 7stat cpd/100tons for 862 keV 7Be solar Using LMA with:
m122=7.92·10-5 eV2
sin212=0.314
and BPS07(GS98)
Syst. Error: 25%
Expected rate (cpd/100 t)
No oscillation 75 ± 4
BPS07(GS98) HighZ
49 ± 4
BPS07(AGS05)LowZ
44 ± 4
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Background: 232Th content Assuming secular equilibrium, 232Th is measured with the delayed
coincidence:
212Bi 212Po 208Pbβ
= 432.8 ns
2.25 MeV ~800 KeV eq.
From 212Bi-212Po correlated events in the scintillator: 232Th: < 6 ×10-18 g(Th)/g (90% C.L.)
Specs: 232Th: 1. 10-16 g/g 0.035 cpd/ton
2 2 2R x y z= + + 2 2cR x y= +
Only fewbulk candidates
212Bi-212Po
Time (ns)
=423±42 ns
Events are mainly in the south vessel surface (probably particulate)
z (m
)
(m) (m)
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Background: 238U content
Assuming secular equilibrium, 238U is
measured with the delayed coincidence:214Bi 214Po 210Pb
β = 236 s
3.2 MeV ~700 KeV eq.
214Bi-214Po
=240±8s
Time s
214Bi-214Po
2 2cR x y= +
z (m
)
Setp - Oct 2007
Specs: 238U: 1. 10-16 g/g
< 2 cpd/100 tons
238U: = 6.6 ± 1.7×10-18 g(U)/g
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Background: 210PoNOTES• The bulk 238U and 232Th
contamination is negligible• The 210Po background is
NOT related neither to 238U contamination NOR to 210Pb contamination
210Po decay time: 204.6 days
• 210Bi no direct evidence----> free parameter in the total fit
cannot be disentangled, in the 7Be energy range, from the CNO
60 cpd/1ton
• Not in equilibrium with 210Pb !• 210Po decays as expected
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Only 4 events (βare selected in the IV in ~120 d.1.4 events were expected from 14C-210Po random coincidences
the 85Kr contamination upper limits <35 counts/day/100 ton (at 90% C.L.)
More statistics is needed---> Taken as free parameter in the total fit
Background: 85Kr
85Kr is studied through :
85Kr β decay :(βdecay has an energy spectrum similar
to the 7Be recoil electron )
85Kr β
85Rb
687 keV
= 10.76 y - BR: 99.56%
85Rb85Kr 85mRb
= 1.46 s - BR: 0.43%
514 keV
β
173 keV
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Cosmic are identified by the OD and by the ID• OD eff: ~ 99%• ID analysis based on pulse shape
variables– Pulse mean time, peak position in
time• Estimated overall rejection factor:
– > 104 (still preliminary)
ID efficiency
A muon in OD
Muon flux:(1.21±0.05)h-
1m-2
Muon angular distributions
After cuts, are not a relevant background for 7Be analysis– Residual background: < 1 c/d/100 t
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Position reconstruction• Position reconstruction algorithms
– Base on time of flight fit to hit time distribution– developed with MC, tested and validated in CTF– cross checked and tuned in Borexino on selected events (14C,
214Bi-214Po, 11C)
The fit is compatible with the expected r2-like shape with R=4.25m.
The time and the total charge are measured, and the position is reconstructed for each event . Absolute time is also provided (GPS)
14C
Radius (m)
Spatial resolution: 35 cm at 200 keV16 cm at 500 keV (scaling as )
€
N p.e.−1/ 2
214Bi-214Po
β distance(m)
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Fiducial volume
Radial distribution
R2
gauss
2 2 2R x y z= + + 2 2cR x y= +
z vs Rc scatter plot
FV
the nominal Inner Vessel radius: 4.25m (278 tons of scintillator)the effective I.V. radius has been reconstructed using: # 14C events # Thoron (=80s) on the I.V. surface (emitted by the nylon) # External background gamma # Teflon diffusers on the IV surface maximum uncertainty : ~ +-12%
FM: by rescaling background components known to be uniformly distributed within the LS and using the known LS mass (278.3 t)
from PMTs that penetrate the buffer
z < 1.8 m, was done to remove gammas from IV endcaps
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Light Yield
The 11C sample is selected through the triple coincidence with muon and neutron. We limited the sample to the first 30 min of 11C time profile, which reduces the random coincidence to a factor 1/14.
The Light Yield has been evaluated fitting the 14C spectrum,
(Borex. Coll. NIM A440, 2000)
and the 11C spectrum
14C spectrum (β decay-156 keV, end point)
11C spectrum(β+ decay-960 keV)
Light Yield = 500 +- 12 p.e./MeV
The energy equivalent to the sum of the two quenched 511 keV gammas: E2511) = 0.83 +- 0.03 MeV. Energy resolution: 10% at 200
keV 8% at 400 keV 6% at 1 MeV
The light yield has been evaluated also by taking it as free parameter in a global fit on the total spectrum (14C,210Po, 210Po ,7Be Compton edge)
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Final spectrum Final spectrum after all cutsafter all cuts
Understanding the final spectrum:
main components
14C No cutsKr+Be
11C After cut
AfterFV cuts
10C+ ext. bkg
210Po (only, not in eq. with 210Pb!)
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Spectral fit to determine the signal
• Stat. error at present includes lack of knowledge of 85Kr
• Syst. uncertainty comes from Fiducial Mass estimation (max error)
47 days of live time (August 2007)• Strategy:
– Fit the shoulder region only– Use between 14C end point and 210Po
peak to limit 85Kr content– pep and 8B neutrinos fixed at SSM-
LMA value– Other backgrounds (U, Th)
negligible with the present radiopurity
– 210Po peak not included in this fit
• Fit components– 7Be, 85Kr– CNO+210Bi combined
• very similar in this limited energy region
– Light yield left free
These bins used to limit 85Kr content in fit
7Be Rate: 47 ± 7STAT ± 12SYS c/d/100 t
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Spectral fit in ~200 days
• Improvements:– Better definition of FV (use internal source and diffuser balls deployed on the
IV surface)– PMT charge equalization– LY (but still free parameter in a global fit on the total spectrum)– Better background measurements– Detector stability– Fit in the range 150-2000 keV
• Background issue:– 85Kr– 210Bi - 40K no signature– 11C : reduction by
tagging -induced neutrons identification is in progress
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Comments on errors
• Statistical:– Right now, it includes combined the effect of statistics itself, the
lack of knowledge of 85Kr content, and the lack of a precise energy calibration
– These components are left free in the final fit, and contribute to the statistical error
• Systematic (the evaluation is still in progress):– Fiducial volume determination: it is improved due to a
better understanding of the detector response. – Max. range of 7Be flux due to poor knowledge of the
background 210Bi/40K which are in competition with CNO ’s:– Events selection (background subtraction: muons, Rn.. ), energy
scale
7Be Rate: 47 ± 7STAT ±
12SYS c/d/100 t
August (2007)August (2007)
15%
6%
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
• s may produce 11C by spallation on 12C – n are also produced ~
90% of the times– Untill now, only the
first neutron after a muon can be currently detected
– Events that occur within 2 ms after a are rejected
CCn+
+e++en capture MeV)
Cylindrical cut Around muon-track
Spherical cut around2.2 gamma to reject 11C event
Neutron production
Muon track
Main problem: 11C
pep and CNO fluxespep and CNO fluxes
CNO11C
pep
Simulated
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
11C and neutrons after muons
• electronics improvement to detect all the neutrons produced by a muon– Implementation of the main
electronics– FADC in parallel to the main
electronics
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
What next What next
@ possibly p-p neutrinos
@ seasonal variations of the solar flux due to the eccentricity of the Earth orbit
250-800 keVEn. window
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
@ search for antineutrinos (from Sun, Earth,reactors) Borex. Coll. Eur. Phys.J. C47,2006
good tagging: +p n+e+ signal > 1 MeV ≈200s neutron capture: signal 2.2 MeV--->> geoneutrinos
Main bckg: from reactorsIn 300 tons: 7- 17 ev/y (BSE)- S/N=1Antineutrinos from Reactors; long base line: ≈1000 kmRate: ≈ 20 ev/y
€
€
What next (cont.)What next (cont.)
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
CONCLUSIONSCONCLUSIONS
>> Borexino just started the study of the various solarneutrino sources below 2 MeV, with a real time detection ( pp,7Be, pep, CNO)
>> Future goal (in a few months): – try to tag 11C – CNO study
>> The program includes also the study of the antineutrinosfrom Sun, Earth, Reactors)
>> Borexino in also a useful observatory for the Supernova
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
Borexino collaboration
Kurchatov Institute(Russia)
Dubna JINR(Russia)
Heidelberg(Germany)
Munich(Germany)
Jagiellonian U.Cracow(Poland)
Perugia
Genova
APC Paris
MilanoPrinceton University
Virginia Tech. University
NO-VE April 15-18, 2008 Emanuela Meroni Univ. & INFN Milano
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