Rekonstrukcja oddziaływań neutrin w detektorze BOREXINO

M. Misiaszek (Instytut Fizyki UJ, Kraków)

Rekonstrukcja oddziaływań neutrin w detektorze BOREXINO

Kurchatov Inst.

(Russia) Dubna JINR

(Russia) Heidelberg

(Germany)

Munich (Germany) Jagiellonian U.

Cracow (Poland) Perugia

Genova

APC Paris Milano

Princeton University

Virginia Tech.University

Seminarium neutrinowe IFT, Wrocław 16 listopada 2009

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

Be ν is the main target Pep, CNO and possibly pp ν

Geoneutrinos Supernova ν

Detection via scintillation light

Very low energy threshold Good position reconstruction 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)

The physics goals and detection principles of Borexino

Detector design and layout

Water Tank:

 and n shield

 water Ch detector 208 PMTs in water 2100 m

20 legs Carbon steel plates

Scintillator:

270 t PC+PPO in a 125 m thick nylon vessel

Stainless Steel Sphere:

2212 photomultipliers 1350 m

Nylon vessels:

Inner: 4.25 m Outer: 5.50 m

Design based on the

principle of graded

shielding

CNO

7Be

11

C

10C

14

C

pp+pep+⁸B

238U + ²³²Th

The expected signal and the irreducible background

Borexino is continuously taking data since 13/05/2007

• Algorytmy do rekonstrukcji pozycji zdarzeń oparte są o metodę największej wiarygodności, którą poszukuje się najbardziej

prawdopodobnego miejsca emisji fotonów.

x

t

t

t

t

t

t

t

= const + tof

+ t

tof

= n/c * d

(x

,y

,z

)

• Zakładamy próbną pozycję zdarzenia x₀

• Obliczamy tof (czas przelotu) dla każdego fotonu

• Odejmujemy tof od każdego t_i

• Porównujemy otrzymany rozkład t'_i z oczekiwanym rozkładem fotonów emitowanych ze scyntylatora

• Algorytm przeszukuje inne pozycje x₀ dopóki nie znajdzie pozycji dla której dopasowanie jest najlepsze

t

t

(x

,y

,z

)

Rozkład przestrzenny zdarzeń

Odrzucenie zdarzeń tła (głównie pr. gamma) R < 3.3 m (100 t masy scyntylatora)

Rozkład radialny

R

gauss

2 2 2

R  x  y z

R

 x

 y

z vs R

scatter plot

FV

238 U and ²³² Th

212

Bi 

Po 

Pb

 = 432.8 ns

2.25 MeV ~800 KeV eq.

Only 3

bulk candidates (47.4d)

214

Bi-

Po

212

Bi-

Po

212

Bi-

Po

214

Bi 

Po 

Pb

 = 236 s

3.2 MeV ~700 KeV eq.

238

U: < 2. 10

g/g

Th: < 1. 10

g/g

Kształt impulsu w detektorze BOREXINO

[ns]

/ discrimination

 particles

Small deformation due to average SSS 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

Final spectrum after all cuts



Kr+

Be shoulder

14

C

210

Po (only, not in eq. with

Pb!)

11

C

Understanding the final spectrum: main components

Last cut:

Bi-

Po and Rn daughters removal

No s

After

fiducial volume cut

(“100 tons”)

Konwersja liczby zmierzonych

fotoelektronów do energii zdarzenia

Fit parametrów do kształtu elektronów z

C

~ 500 pe /MeV

Monitoring stabilności detektora

Liczba fotoelektronów Date

N

100 Hz

C+

Rn source diluted in PC:

115 points inside the sphere

b :

C,

Rn diluted in scintillator a :

Rn diluted in scintillator g :

Mn,

Sr,

Rn in air N : AmBe

Source localization within 2 cm

through red laser light and CCD camera

Accurate handling and manipulation of the source and of the materials inserted in the scintillator

The Borexino calibration

A first calibration campaign with on axis and off axis radioactive sources has been performed (Oct 08 on axis, Jan-Feb09 off axis)

 accurate position reconstruction

 precise energy calibration

 detector response vs scintillation position

The measured energy spectrum: May07 - Oct08

Records in the radiopurity achieved by Borexino

Material Typical conc. Borexino level

in the scintillator

14

C scintillator

C/

C<10

238

U,

Th equiv. - Hall C dust - stainless. steel - nylon

~1 ppm

~1ppb

~1ppt

K

Hall C dust ~1 ppm

222

Rn - external air.

- air underground

~20 Bq/m

~40-100 Bq/m

85

Kr

39

Ar

in N

for stripping ~1.1 Bq/m

~13 mBq/m

-

Rn

-

U,

Th equiv.

LNGS - Hall C water ~50 Bq/m

~10

g/g



14

C/

C  2 10



10

10

g /g



10

g /g



1  Bq /m



~ 0.16 mBq / m

~ 0.5 mBq / m



~ 30 

~ 10

g / g

•Fit between 100-800 p.e.;

•Light yield: a free fit parameter;

•Ionization quenching included (Birks’

parametrization);

•

Bi,

C and

Kr free fit parameters;

•Others v fixed

•Fit to the spectrum without and with subtraction is performed giving consistent results

R

= 49 ± 3

± 4

cpd/100 tons

The measurement of the ⁷ Be flux (192 days of live time)

Borexino Collaboration Phys. Lett. B 658 (2008) : after 2 months of data taking Borexino Collaboration PRL 101 (2008) : 192 days of live time

Expected rate

(cpd/100 t) No oscillation 75 ± 4 BPS07(GS98) HighZ 48 ± 4 BPS07(AGS05) LowZ 44 ± 4

No-oscillation hypothesis rejected at 4 level

7 Be: (49 ± 3 _stat ±4 _sys ) cpd/100 tons (192 days)

The analysis of the calibration data is in progress

The measurement of the ⁷ Be flux (192 days of live time)

Survival probability of the  _e

Before Borexino

After Borexino

Survival probability of the  _e

First measurement of the ratio between the

survival probabilities in vacuum and in matter

Limits obtained by Borexino after 200 days of data taking - the best in the literature

1- Limits on pp e CNO solar fluxes;

with the Luminosity constraint:

2- Limit on the neutrino magnetic moment:



5.4 10



(90%C.L.)



_pp(Borexino data) /_pp(SSM) 1.00_0.020^0.008

_CNO(Borexino data) /_CNO(SSM) 3.8 (90%C.D.)

The low threshold measurement of the 8B solar neutrinos

2.6 MeV ’s from ²⁰⁸Tl on PMT’s

and in the buffer

Borexino threshold: 2.8 MeV

Expected (MSW-LMA) count rate due to 8B neutrinos above 2.8 MeV:

0.26±0.03 c/d/100 tons

Borexino energy spectrum after muon subtraction: 246 days of live time

The low threshold measurement of the 8B solar neutrinos

1) Muons;

2) Gammas from neutron capture;

3) Radon emanation from the nylon vessel;

4) Short lived (t < 2 s) cosmogenic isotopes;

5) Long lived (t > 2 s) cosmogenic isotopes (

C);

6) Bulk

Th contamination (

Tl);

The Borexino

B spectrum

tons /100

counts/day )

0.02 0.04

0.26

(

8 .

2

  

 MeV

Rate

7Be and 8B flux measured with the same detector

Borexino 8B flux above 5 MeV agrees with existing data

Neutrino oscillation is confirmed by the 8B of Borexino

at 4.2 sigma

Results already achieved in Borexino

1. First direct experimental evidence of the vacuum regime and of the transition region in the neutrino oscillation at very low energy: measurement of the

Be flux (0.2-0.8 MeV) and strong limit on the pp

 flux.

2. First determination of the ratio between the 

survival probability in vacuum and in matter: 1.6 ± 0.33 (from the

Be flux and the

B flux, measured with a threshold down to 2.8 MeV).

3. Measurements of the day/night effect for  at very low energy:

4. First validation of the MSW-LMA model in the vacuum regime and in the transition region within the error (10% for the

Be flux measurement: stat.+ syst.).

5. Best limits for CNO flux,  magnetic moment, Pauli principle violation._

ADN  N  D

N  D  0.02  0.04

What next

A. Measurement of the

Be flux with a total error  final validation of the MSW-LMA model;

important insight for the Standard Solar Model metallicity puzzle and stronger limits on the pp flux.

B. Determination of the survival probability ratio, day/night effect, etc. with strongly reduced errors.

C. Study of the pep and CNO region (energy spectrum in the range 0.9-1.5 MeV) with the suppression of the

C muon produced.

D. Measurements of the geoneutrinos (the Gran Sasso region is especially favoured due to the absence of the main background: reactor ).





Observatory

• Borexino is a Supernova observatory in the network SNEWS .

Literatura

First real time detection of 7Be solar neutrinos by Borexino (Phys. Lett. B 658 (2008) 101-108)

New results on solar neutrino fluxes from 192 days of Borexino data (Phys. Rev. Lett. 101 (2008) 091302 )

Measurement of the solar

B neutrino flux with 246 live days of Borexino and observation of the MSW vacuum-matter transition

(arXiv:0808.2868v1)