slide1
Download
Skip this Video
Download Presentation
Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration

Loading in 2 Seconds...

play fullscreen
1 / 30

Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration - PowerPoint PPT Presentation


  • 86 Views
  • Uploaded on

Getting the first 7 Be n detection: scintillator purification, detector response and data analysis in Borexino. Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration. Contents. Physics goals, detector design, construction & filling Design guidelines

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration' - neka


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Getting the first 7Be n detection:scintillator purification, detector response and data analysis in Borexino

Marco Pallavicini

Università di Genova & INFN

On behalf of the Borexino Collaboration

contents
Contents
  • Physics goals, detector design, construction & filling
    • Design guidelines
    • Radiopurity issues
    • Plants and Filling
  • Detector response & Data analysis
    • Event selection
    • Detector response
    • Background content
    • Spectral fits

M. Pallavicini - Università di Genova & INFN

borexino collaboration
Milano

Perugia

Borexino Collaboration

Genova

Princeton University

APC Paris

Virginia Tech. University

Munich

(Germany)

Dubna JINR

(Russia)

Kurchatov

Institute

(Russia)

Jagiellonian U.

Cracow

(Poland)

Heidelberg

(Germany)

M. Pallavicini - Università di Genova & INFN

slide4
Abruzzo, Italy

120 Km from Rome

Laboratori

Nazionali del

Gran Sasso

Assergi (AQ)

Italy

~3500 m.w.e

External Labs

Borexino Detector and Plants

M. Pallavicini - Università di Genova & INFN

detection principles and n signature
Detection principles and n signature
  • Borexino detects solar n via their elastic scattering off electrons in a volume of highly purifiedliquid scintillator
    • Mono-energetic 0.862 MeV 7Ben 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)

M. Pallavicini - Università di Genova & INFN

detector layout and main features
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

M. Pallavicini - Università di Genova & INFN

15 years of work in three slides i
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

15 years of work in three slides (I)

M. Pallavicini - Università di Genova & INFN

picture gallery i
Picture gallery (I)

Pmt sealing: PC & Water proof

2000

Nylon vessels installation (2004)

PMT installation in SSS

2002

M. Pallavicini - Università di Genova & INFN

15 years of work in three slides ii
Water ( production rate 1.8 m3/h)

RO, CDI, filters, N2 stripping

U, Th: < 10-14 g/g

222Rn: ~ 1 mBq/m3

226Ra: <0.8 mBq/m3

18.2-18.3 MW/cm typical @ 20°C

Scintillator

IV: PC+PPO (1.5 g/l)

OV & Buffer: PC+DMP (5 g/l)

PC Distillation (all PC)

6 stages distillation

80 mbar, 90 °C

Vacuum stripping with low Ar-Kr N2

Humidified with water vapor 60-70%

PPO purification

PPO is solid.

A concentrated solution (120 g/l) in PC is done first (“master solution”)

Master solution was purified with:

Water extraction ( 4 cycles)

Filtration

Single step distillation

N2 stripping with LAKN

Filling operations

Purging 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

15 years of work in three slides (II)

M. Pallavicini - Università di Genova & INFN

picture gallery ii
Picture gallery (II)

Water Plant

Storage area and Plants

CTF and Plants

M. Pallavicini - Università di Genova & INFN

low argon krypton nitrogen
LTALow Argon Krypton Nitrogen

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)

LAKN developed for:

  • IV/OV inflating/flushing
  • scintillator purification
  • blanketing and cleaning

Production rate reaches 100 m3/h (STP)

Achieved results:

High Purity Nitrogen: 222Rn < 0.3 µBq/m3

222Rn: 8 Bq/m3

Ar: 0.01ppm

Kr: 0.02 ppt

Details discussed by G. Zuzel “Low-level techniques applied in the expe-

riments looking for rare events”, Wed. 12.09, Solar & Low BG Techniques.

1 ppb Ar in N2 ~1.4 nBq/m3 for 39Ar; 0.1 ppt Kr in N2 ~0.1 µBq/m3 for 85K

M. Pallavicini - Università di Genova & INFN

15 years of work in three slides iii
15 years of work in three slides (III)

M. Pallavicini - Università di Genova & INFN

what s important of previous table
What’s important of previous 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 as (no evidence of 210Pb or 210Bi at that level)
    • Some 85Kr contamination, probably due to a small air leak during filling

M. Pallavicini - Università di Genova & INFN

finally may 15 th 2007
Finally, May 15th, 2007

M. Pallavicini - Università di Genova & INFN

our first result astro ph 0708 2251v2
Our first result (astro-ph 0708.2251v2)
  • We have detected the scattering rate of 7Be solar ns on electrons

7Be n Rate:47 ± 7STAT ± 12SYS c/d/100 t

How did we get here ?

M. Pallavicini - Università di Genova & INFN

the starting point no cut spectrum
The starting point: no cut spectrum

14C dominates below 200 KeV

210Po NOT in eq. with 210Pb

Arbitrary units

Mainly external gs and ms

Photoelectrons

Statistics of this plot: ~ 1 day

M. Pallavicini - Università di Genova & INFN

m cuts
m are identified by the OD and by the ID

OD 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)

m cuts

Outer detector efficiency

Preliminary

m with

OD tag

No OD tag < 1%

A muon

in OD

m track

ID efficiency

M. Pallavicini - Università di Genova & INFN

spectrum after m cut above 14 c
Spectrum after m cut (above 14C)
  • After cuts, m are not a relevant background for 7Be analysis
    • Residual background: < 1 c/d/100 t

No cuts

After m cut

M. Pallavicini - Università di Genova & INFN

position reconstruction
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

M. Pallavicini - Università di Genova & INFN

fiducial volume cut
Fiducial volume cut
  • External background is large at the periphery of the IV
    • g 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

Preliminary

Radial distribution

z vs Rc scatter plot

R2

gauss

FV

M. Pallavicini - Università di Genova & INFN

spectrum after fv cut
Spectrum after FV cut
  • External background is the dominant background component in NW, except in the 210Po peak region

Clear 7Be shoulder

No cuts

After

FV cuts

11C

No ms

M. Pallavicini - Università di Genova & INFN

11 c and neutrons after muons
11C and neutrons after muons
  • ms 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 detected
      • Work in progress to try to improve this
    • Events that occur within 2 ms after a m are rejected

Preliminary

Neutron Capture Time

Neutron spatial distribution

t ~ 210 ms

M. Pallavicini - Università di Genova & INFN

final spectrum after all cuts
Final spectrum after all cuts

Understanding the final spectrum: main components

210Po (only, not in eq. with 210Pb!)

14C

85Kr+7Be n

11C

Last cut: 214Bi-214Po and Rn daughters removal

M. Pallavicini - Università di Genova & INFN

energy calibration and stability
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

M. Pallavicini - Università di Genova & INFN

238 u and 232 th content
t = 432.8 ns

t = 236 ms

b

b

a

a

214Bi

212Bi

212Po

214Po

210Pb

208Pb

~800 KeV eq.

~700 KeV eq.

3.2 MeV

2.25 MeV

238U and 232Th content

212Bi-212Po

Assuming secular equilibrium, 232Th and 238U

are measured with the delayed concidences:

232Th Events are mainly in the south vessel surface (probably particulate)

212Bi-212Po

214Bi-214Po

Only 3

bulk candidates

238U: < 2. 10-17 g/g

232Th: < 1. 10-17 g/g

M. Pallavicini - Università di Genova & INFN

a b discrimination
a/b discrimination

Full separation at high energy

Small deformation due to average

SSS light reflectivity

a particles

b particles

ns

250-260 pe; near the 210Po peak

200-210 pe; low energy side of the 210Po peak

2 gaussians fit

2 gaussians fit

a/b Gatti parameter

a/b Gatti parameter

M. Pallavicini - Università di Genova & INFN

7 be signal fit without a b subtraction
7Be signal: fit without a/b subtraction
  • 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 n
    • 85Kr
    • CNO+210Bi combined
      • very similar in this limited energy region
    • Light yield left free

210Po peak not included in this fit

7Be n

CNO + 210Bi

85Kr

These bins used to limit 85Kr content in fit

M. Pallavicini - Università di Genova & INFN

7 be signal fit a b subtraction of 210 po peak
The large 210Po background is subtracted in the following way:

For each energy bin, a fit to the a/b Gatti variable is done with two gaussians

From the fit result, the number of a 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

7Be signal: fit a/b subtraction of 210Po peak

2 gaussians fit

a

b

The two analysis yield fully compatible results

M. Pallavicini - Università di Genova & INFN

comments on errors
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:
    • 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

M. Pallavicini - Università di Genova & INFN

conclusions
Conclusions
  • Borexino has performed the first real time detection of sub/MeV solar neutrinos
    • Quite surprising even for us, after just two months of data
      • A 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

M. Pallavicini - Università di Genova & INFN

ad