Geoneutrinos in Borexino

Honolulu 16/Dec/2005 Marco G. Giammarchi, Infn Milano

Geoneutrinos in Borexino

Marco G. Giammarchi & Lino Miramonti

Dip. di Fisica dell’Universita’ and Infn Milano

• Introduction to Borexino

• Radiopurity in Borexino

• Physics Test results

• Borexino and Geoneutrinos



Borexino•Borexino is located under the Gran sasso mountain which provides a shield against cosmic rays (residual flux = 1 /m2 hour);

•The core of the detector is shielded by successive layers of increasingly pure materials

Core of the detector: 300 tons of liquid scintillator contained in a nylon vessel of 4.25 m radius (PC+PPO);

1st shield: 1000 tons of ultra-pure buffer liquid (pure PC) contained in a stainless steel sphere of 7 m radius;

2nd shield: 2400 tons of ultra-pure water contained in a cylindrical dome;

2214 photomultiplier tubes pointing towards the center to view the light emitted by the scintillator;

200 PMTs mounted on the SSS pointing outwards to detect light emitted in the water by muons


Nylon vessels installation


Nylon vessels installed and inflated (May 2004)


Experimental Hall In Gran Sasso (Hall C) Stainless Steel Sphere (SSS)

PMTs ready to be mounted Optical fiber istallation


Final closure of the Inner detector (2004)


e Lie Be 77

Monocromatic ! E=862 keV

SSM=4.8x109 /sec/cm2

“ window” (0.25-0.8 MeV)

xx ee

=10-44 cm2

e

x

expected rate (LMA hypothesis) is 35 counts/day in the neutrino window


Radiopurity constraints• To lower the threshold down to 250 keV, it is mandatory to reach very

high radiopurity levels in the active part of the detector ;

• This translates into the following requirements on the most critical contaminants (238U , 232Th , 40K, 210Po, 210Pb, 39Ar, 85Kr) :

Intrinsic contamination of the scintillator for what concerns isotopes belonging to the U and Th chain < 10-16 g/g; Intrinsic contamination of the scintillator for what

concerns 40K < 10-14 g/g;

Contamination of the buffer liquid in U and Th chain < 10-14 g/g;

Contamination of the nylon vessel for what concerns the U and Th chain < 10-12 g/g;

Constraints on N2 used to sparge scintillator: <0.14 ppt of Kr in N2 (0.2 Bq 85Kr/m3 N2) Constraints on N2 used to sparge scintillator:

<0.36 ppm of Ar in N2 (0.5 Bq 39Ar/m3 N2)

Each of these points required careful selection and clean handling of materials, + implementation of purification techniques

Contamination of the external water in U and Th chain < 10-10 g/g;

14C /12C <10-18 in the scintillator


Counting Test Facility (CTF)

• 100 PMTs

• 4 tons of scintillator

• 4.5m thickness of water shield

• Muon-veto detector

CTF high mass and very low levels of background contamination make it a unique detector to search for rare or forbidden processes with high sensitivity

CTF campaigns

1. CTF1: 95-97

2. CTF2: 2000 (pxe)

3. CTF3: 2001 still ongoing

• CTF is a prototype of BX. Its main goal was to verify the capability to reach the very low-levels of contamination needed for Borexino


Physics results of the Counting Test Facility of Borexino (CTF)


Limits on electron stability

[Phys. Lett. B 525 (2002) 29]

- Non-conservation of electric charge would lead to electron decay via two processes:

e + , e + + - search for the decay

e+ (256keV line) - > 4.6 x 1026 y (90%

C.L.) - currently world best limit

(quoted on the PdG)

Neutrino magnetic moment[Physics Letters B 563 (2003) 35

- a non-zero would increase the e scattering cross-section by the term;

- this effect becomes dominant at low energy

- < 5.5 x 10-10 B (90% C.L.) - it is the best limit with low energy neutrino

νe

2υ

20νe

e

em

E

1

T

1μrπE,T

dT

dσ

1. e- scattering

2. 14C spectrum

3. Residual radioactive bkg


Limits on nucleon decay into invisible channels

[Physics Letters B 563 (2003) 23]

- different channels were considered in which a single nucleon or a pair of nucleons bounded in C or O nuclei decay with the emission of invisible particles (neutrinos, majorons…)

- the obtained limits are comparable or improve previous limits;

Limits on Heavy neutrino mixing in 8B decay

[JETP Lett. Vol. 78 No 5 (2003)261]

- If heavy neutrinos H with m > 2 me are emitted in 8B reaction in the sun then the decay H L + e+ + e- should be observed;

- CTF significantly improves limits on

(mH - ׀UeH 2׀ ) parameter space;

Limits on Pauli Esclusion Principle

[Europ. Physical Journal C37 (2004) 421]

- we look for non-Paulian transitions in 12C and 16O nuclei from 1P shell to a filled 1S1/2 shell;

- the obtained limits significantly improves (up to three order of magnitude) previous limits

Other papers are under preparation: “Constraints on the solar anti-neutrino flux obtained with the BX prototype” < 3x105 cm-2 s-1 (90% C.L.) first limit at low energy


Earth emits a tiny heat flux with an average value of

ΦH ~ 60 mW/m2

Integrating over the Earth surface:

HE ~ 30 TW

Giving constrain on the heat generation within the Earth.

Detecting antineutrino emitted by the

decay of radioactive isotopes

It is possible to study the radiochemical composition of the Earth


238U and 232Th chains have 4 β with E > 1.8 MeV :

end.point

[Th-chain] 228Ac < 2.08 MeV

[Th-chain] 212Bi < 2.25 MeV

[U-chain] 234Pa < 2.29 MeV

[U-chain] 214Bi < 3.27 MeV

Anti-neutrino from 40K are under threshold

The terrestrial antineutrino spectrum above 1.8 MeV has a “2-component” shape.

high energy component coming solely from U chain andlow energy component coming with contributions from U + Th chains

This signature allows individual assay of U and Th abundance in the Earth

enpe


Borexino is located in the

Gran Sasso underground laboratory (LNGS)

in the center of Italy: 42°N 14°E

Calculated anti-νe flux at the Gran Sasso Laboratory

(106 cm-2 s-1)

U Th Total (U+Th) Reactor BKG

Crust Mantle Crust Mantle

3.3 0.95 3.0 0.77 8.0 0.39

Data from the International Nuclear

Safety Center (http://www.insc.anl.gov)

Background from nuclear

Reactors

Earth data from F. Mantovani et al., Phys. Rev. D 69 (2004) 013001


Pb concentration measured in the Counting Test Facility

20 /Bq ton

Background from Po-210

Alpha particles reacting on C-13:13 16( , )C n O

• Pb-210 related background negligible

• Only significant source of background are nuclear reactors

• Accidental rate also negligible (< 10% of reactors background)


The number expected events in Borexino are:

The background will be:

yr

events6

yr

events19

Predicted accuracy of about 30%

in 5 years of data taking


•Following August 2002 accident, Borexino activity has suffered from severe restrictions especially for what concerns fluid handling operations;

•In spite of this, the detector installation has continued and was completed in 2004;

•Following this, it was possible to start the re-commissioning of all ancillary plants which had been stopped three years ago; the re-commissioning is currently taking place;

•We expect to start filling the detector with scintillator in June 2006;

•We expect to start data-taking with the filled detector in november 2006

Conclusion and outlook

Borexino is a low background high sensitivity underground detector which is located on continental crust and can give important information on geoneutrino fluxes.

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Geoneutrinos in Borexino