Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration
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Getting the first Getting the first 77Be Be detection:detection:
scintillator purification, scintillator purification, detector response and data detector response and data
analysis in Borexinoanalysis in BorexinoMarco Pallavicini
Università di Genova & INFN
On behalf of the Borexino Collaboration
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
ContentsContents
Physics goals, detector design, construction & fillingDesign guidelines
Radiopurity issues
Plants and Filling
Detector response & Data analysisEvent selection
Detector response
Background content
Spectral fits
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Borexino CollaborationBorexino Collaboration
Kurchatov Institute(Russia)
Dubna JINR(Russia)
Heidelberg(Germany)
Munich(Germany)
Jagiellonian U.Cracow(Poland)
Perugia
Genova
APC Paris
MilanoPrinceton University
Virginia Tech. University
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Abruzzo, Italy120 Km from Rome
LaboratoriNazionali del Gran Sasso
Assergi (AQ)Italy~3500 m.w.e
Borexino Detector and Plants
External Labs
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Detection principles and Detection principles and signaturesignature
Borexino detects solar via their elastic scattering off electrons in a volume of highly purified liquid scintillator
Mono-energetic 0.862 MeV 7Be are the main target, and the only considered so farMono-energetic pep , CNO and possibly pp will be studied in the future
Detection via scintillation light:
Very low energy thresholdGood position reconstructionGood energy resolution
BUT…No direction measurement The induced events can’t be distinguished from other events due to natural radioactivity
Extreme radiopurity of the scintillator is a must!
Typical rate (SSM+LMA+Borexino)
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Detector layout and main Detector layout and main featuresfeatures
Water Tank: and n shield water Č detector208 PMTs in water2100 m3
20 legsCarbon steel plates
Scintillator:270 t PC+PPO in a 150 m thick nylon vessel
Stainless Steel Sphere:2212 PMTs 1350 m3
Nylon vessels:Inner: 4.25 mOuter: 5.50 m
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
15 years of work in three slides 15 years of work in three slides (I)(I)
Detector & PlantsAll 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 tightLeak 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 id
Nylon vesselsMaterial 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
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Picture gallery (I)Picture gallery (I)
2000
Pmt sealing: PC & Water proof
2002
PMT installation in SSS
Nylon vessels installation (2004)
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
15 years of work in three slides 15 years of work in three slides (II)(II)
Water ( production rate 1.8 m3/h)RO, CDI, filters, N2 stripping
U, Th: < 10-14 g/g222Rn: ~ 1 mBq/m3
226Ra: <0.8 mBq/m3
18.2-18.3 M/cm typical @ 20°C
ScintillatorIV: PC+PPO (1.5 g/l)OV & Buffer: PC+DMP (5 g/l)
PC Distillation (all PC) 6 stages distillation80 mbar, 90 °C
Vacuum stripping with low Ar-Kr N2
Humidified with water vapor 60-70%
PPO purificationPPO is solid.
A concentrated solution (120 g/l) in PC is done first (“master solution”)
Master solution was purified with:Water extraction ( 4 cycles)FiltrationSingle step distillation
N2 stripping with LAKN
Filling operationsPurging of the SSS volume with LAKN (early ‘06)Water filling (Aug. 06 Nov. 06)Replacement of water with PC+PPO or PC+DMP (Jan. 07 May. 07)
Mixing online
DATA TAKING from May 15, 2007
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Picture gallery (II)Picture gallery (II)
CTF and Plants
Water Plant
Storage area and Plants
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Low Argon Krypton NitrogenLow Argon Krypton Nitrogen
High Purity Nitrogen: 222Rn < 0.3 µBq/m3
LTA
1 ppb Ar in N2 ~1.4 nBq/m3 for 39Ar; 0.1 ppt Kr in N2 ~0.1 µBq/m3 for 85K
LAKN developed for:
- IV/OV inflating/flushing - scintillator purification - blanketing and cleaning
Production rate reaches 100 m3/h (STP)
Specification: 222Rn 7 µBq/m3
Ar 0.4 ppm Kr 0.2 ppt
Expected signal from 39Ar, 85Kr and 222Rn in the Borexino FV 1 cpd (for each isotope)
Achieved results:
Details discussed by G. Zuzel “Low-level techniques applied in the expe-riments looking for rare events”, Wed. 12.09, Solar & Low BG Techniques.
222Rn: 8 Bq/m3
Ar: 0.01 ppm Kr: 0.02 ppt
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
15 years of work in three slides (III)15 years of work in three slides (III)RadioIsotope Concentration or Flux Strategy for Reduction
Name Source Typical Required Hardware Software Achieved
cosmic ~200 s-1 m-2 ~ 10-10 Underground Cherenkov signal <10-10
at sea level Cherenkov detector PS analysis (overall)
Ext. rock Water Tank shielding Fiducial Volume negligible
Int. PMTs, SSS Material Selection Fiducial Volume negligible
Water, Vessels Clean constr. and handling
14C Intrinsic PC/PPO ~ 10-12 ~ 10-18 Old Oil, check in CTF Threshold cut ~ 10-18
238U Dust ~ 10-5-10-6 g/g < 10-16 g/g Distillation, Water Extraction < 10-17
232Th Organometallic (?) (dust) (in scintillator) Filtration, cleanliness < 10-17
7Be Cosmogenic (12C) ~ 3 10-2 Bq/t < 10-6 Bq/ton Fast procurement, distillation Not yet measurable ?
40K Dust, ~ 2 10-6 g/g < 10-14 g/g scin. Water Extraction Not yet measurable ?
PPO (dust) < 10-11 g/g PPO Distillation
210Pb Surface contam. Cleanliness, distillation Not yet measurable ?
from 222Rn decay (NOT in eq. with 210Po) 210Po Surface contam. Cleanliness, distillation Spectral analysis ~ 60
from 222Rn decay 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) < 1 c/d/100 t Select vendor, leak tightness Not yet measurable ?85Kr Air (nitrogen) ~ 1 Bq/m3 in air < 1 c/d/100 t Select vendor, leak tightness Spectral fit ~ 0.2
<0.01 ppt (learn how to measure it) fast coincidence <0.35
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
What’s important of previous What’s important of previous table…table…
238U and 232Th content in the scintillator and in the nylon vessels meet specifications or sometimes are even below specs
GOAL: < 10-16 g/g (< 10 c/d/FV) ACHIEVED: < 10-17 g/g
14C is ~ 10-18 g/g as expected (2.7 10-18 g/g measured)
Muon rejection is fine: < 10-4
Two main backgrounds are still above specs, although are managable:
Off equilibrium 210Po s (no evidence of 210Pb or 210Bi at that level)
Some 85Kr contamination, probably due to a small air leak during filling
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Finally, May 15Finally, May 15thth, 2007, 2007
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Our first result (astro-ph Our first result (astro-ph 0708.2251v2) 0708.2251v2)
We have detected the scattering rate of 7Be solar s on electrons
7Be Rate: 47 ± 7STAT ± 12SYS c/d/100 t
How did we get here ?
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
The starting point: no cut The starting point: no cut spectrumspectrum
14C dominates below 200 KeV
210Po NOT in eq. with 210Pb
Mainly external s and s
Photoelectrons
Statistics of this plot: ~ 1 day
Arb
itra
ry u
nit
s
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
cutscuts
are identified by the OD and by the IDOD eff: ~ 99%
ID analysis based on pulse shape variables
Deutsch variable: ratio between light in the concentrator and total light
Pulse mean time, peak position in time
Estimated overall rejection factor:> 104 (still preliminary)
withOD tag
No OD tag < 1%
Outer detector efficiency
ID efficiency
A muon in OD
track
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Spectrum after Spectrum after cut (above cut (above 1414C)C)
After cuts, are not a relevant background for 7Be analysisResidual background: < 1 c/d/100 t
No cuts
After cut
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Position reconstructionPosition 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
2 2cR x y
214Bi-214Po (~800 KeV) 14±2 cm
14C (~100 KeV): 41±4 cm
z vs Rc scatter plot
Resolution
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Fiducial volume cutFiducial volume cut
External background is large at the periphery of the IV from materials that penetrate the buffer
They are removed by a fiducial volume cut R < 3.276 m (100 t nominal mass)
Another volumetric cut, z < 1.8 m, was done to remove some Rn events caused by initial scintillator termal stabilization
Radial distribution
R2
gauss
2 2 2R x y z 2 2cR x y
z vs Rc scatter plot
FV
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Spectrum after FV cutSpectrum after FV cut
External background is the dominant background component in NW, except in the 210Po peak region
No cuts
No s
Clear 7Be shoulder
AfterFV cuts
11C
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
1111C and neutrons after muonsC and neutrons after muons
s may produce 11C by spallation on 12C n are also produced ~ 90% of the times
Only the first neutron after a muon can be currently detectedWork in progress to try to improve this
Events that occur within 2 ms after a are rejected
Neutron Capture Time
~ 210 s
Neutron spatial distribution
2 2cR x y
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Final spectrum after all cutsFinal spectrum after all cuts
Kr+Be 14C
210Po (only, not in eq. with 210Pb!)
11C
Understanding the final spectrum: main components
Last cut: 214Bi-214Po and Rn daughters removal
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Energy calibration and stabilityEnergy 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 spectrumEnergy stability and resolution monitored with 210Po peak
Difficult to obtain a very precise calibration because:14C intrinsic spectrum and electron quenching factor poorly known
Light yield monitored with 210Po peak position
Light yield determined from 14C fit
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
238238U and U and 232232Th content Th content
Assuming secular equilibrium, 232Th and 238U are measured with the delayed concidences:
212Bi 212Po 208Pb
= 432.8 ns
2.25 MeV ~800 KeV eq.
Only 3bulk candidates
232Th Events are mainly in the south vessel surface (probably particulate)
214Bi-214Po
212Bi-212Po
212Bi-212Po
214Bi 214Po 210Pb
= 236 s
3.2 MeV ~700 KeV eq.
238U: < 2. 10-17 g/g232Th: < 1. 10-17 g/g
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
// discrimination discrimination
particles
Small deformation due to averageSSS light reflectivity
particles
250-260 pe; near the 210Po peak 200-210 pe; low energy side of the 210Po peak
2 gaussians fit 2 gaussians fit
Full separation at high energy
ns
Gatti parameter Gatti parameter
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
77Be signal: fit without Be signal: fit without subtractionsubtraction
Strategy:Fit the shoulder region only
Use between 14C end point and 210Po peak to limit 85Kr content
pep neutrinos fixed at SSM-LMA value
Fit components:7Be 85Kr
CNO+210Bi combinedvery similar in this limited energy region
Light yield left free
7Be
85KrCNO + 210Bi
210Po peak not included in this fit
These bins used to limit 85Kr content in fit
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
77Be signal: fit Be signal: fit subtraction of subtraction of 210210Po peakPo peak
The large 210Po background is subtracted in the following way:
For each energy bin, a fit to the Gatti variable is done with two gaussians
From the fit result, the number of particles in that bin is determined
This number is subtracted
The resulting spectrum is fitted in the energy range between 270 and 800 KeV
A small 210Po residual background is allowed in the fit
Results are totally consistent with those obtained without the subtraction
2 gaussians fit
The two analysis yield fully compatible results
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
Comments on errorsComments 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:Mostly due to fiducial volume determination
With 45 days of data taking, and without an internal source calibration, we estimate an upper limit of 25% for this error
Can be much improved even without internal calibration with more statistics and better understanding of the detector response
TAUP 2007 - Sendai, September 11-15, 2007 M. Pallavicini - Università di Genova & INFN
ConclusionsConclusions
Borexino has performed the first real time detection of sub/MeV solar neutrinos
Quite surprising even for us, after just two months of dataA clear 7Be neutrino signal is visible after a few cuts
We made no attempt to under-estimate the errors. Better results to come in the near future
The central value is well in agreement with MSW/LMA.
Significant improvements are expected shortly
In memory of: Cristina Arpesella, Martin Deutsch, Burkhard Freudiger,
Andrei Martemianov and Sandro Vitale
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