Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration

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


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


Abruzzo, Italy120 Km from Rome

LaboratoriNazionali del Gran Sasso

Assergi (AQ)Italy~3500 m.w.e

Borexino Detector and Plants

External Labs


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)


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


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


Picture gallery (I)Picture gallery (I)

2000

Pmt sealing: PC & Water proof

2002

PMT installation in SSS

Nylon vessels installation (2004)


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


Picture gallery (II)Picture gallery (II)

CTF and Plants

Water Plant

Storage area and Plants


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


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


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


Finally, May 15Finally, May 15thth, 2007, 2007


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 ?


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


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


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


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


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


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


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


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


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


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


// 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


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


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


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


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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Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration

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