Transcript ppt

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
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Borexino
•Borexino is located under the Gran sasso mountain which provides a shield
against cosmic rays (residual flux = 1 m /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;
2214 photomultiplier tubes pointing
towards the center to view the light
emitted by the scintillator;
2nd shield: 2400 tons of ultra-pure water
contained in a cylindrical dome;
200 PMTs mounted on the SSS pointing outwards to detect light emitted in the water by muons
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Nylon vessels installation
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Nylon vessels installed and inflated (May 2004)
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Experimental Hall In Gran Sasso (Hall C)
PMTs16/Dec/2005
ready to be
Honolulu
Stainless Steel Sphere (SSS)
Optical fiber istallation
mounted Marco G. Giammarchi, Infn Milano
Final closure of the Inner detector (2004)
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Monocromatic ! E=862 keV
FSSM=4.8x109 /sec/cm2
7
Be  e   7 Li   e
e
x
e   x  e   x
“ window” (0.25-0.8 MeV)
s=10-44 cm2
expected rate (LMA hypothesis) is 35 counts/day in the neutrino window
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
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) :
14C /12C <10-18 in the scintillator
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 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 mBq 85Kr/m3 N2)
Constraints on N2 used to sparge scintillator:
<0.36 ppm of Ar in N2 (0.5 mBq 39Ar/m3 N2)
Contamination of the buffer liquid
Contamination of the
in U and Th chain < 10-14 g/g;
external water in U and Th
chain < 10-10 g/g;
Each of these points required careful selection and clean handling of
materials,
+ implementationMarco
of purification
techniques
G. Giammarchi, Infn
Milano
Honolulu 16/Dec/2005
Counting Test Facility (CTF)
•
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
•
100 PMTs
•
4 tons of scintillator
•
4.5m thickness of
water shield
•
Muon-veto detector
CTF campaigns
1.
CTF1: 95-97
2.
CTF2: 2000
(pxe)
3.
CTF3: 2001
still ongoing
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
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Physics results of the Counting Test
Facility of Borexino (CTF)
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Neutrino magnetic moment
[Physics Letters B 563 (2003) 35
Limits on electron
stability
[Phys. Lett. B 525 (2002) 29]
- Non-conservation of
electric charge would
lead to electron decay
via two processes:
e g+,
e++
- search for the decay
eg+ (256keV line)
- t > 4.6 x 1026 y (90%
C.L.)
- currently world best limit
(quoted on the PdG)
Honolulu 16/Dec/2005
- a non-zero m would increase the e
scattering cross-section by the term;


dσ em
Te , E ν   π r02μ 2υ  1  1 
dTe
 Te E ν 
- this effect becomes dominant at low energy
- m < 5.5 x 10-10 mB (90% C.L.)
- it is the best limit with low energy neutrino
1. e- scattering
2.
14C
spectrum
3. Residual
radioactive bkg
Marco G. Giammarchi, Infn Milano
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 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
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
(mH - ‫׀‬UeH ‫׀‬2) parameter space;
Other papers are under preparation:
“Constraints on the solar anti-neutrino flux obtained with the BX prototype”
F < 3x105 cm-2 s-1 (90% C.L.) first limit at low energy
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Earth emits a tiny heat flux with an
average value of
ΦH ~ 60 mW/m2
Integrating over the Earth surface:
HE ~ 30 TW
Detecting antineutrino
emitted by the
decay of radioactive isotopes
It is possible to study the
radiochemical composition of the Earth
Giving constrain on the heat generation within the Earth.
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
e  p  n  e

and 232Th chains have 4 β
with E > 1.8 MeV :
238U
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
The terrestrial antineutrino spectrum above 1.8 MeV has a
“2-component” shape.
high energy component coming solely from U chain and
low energy component coming with contributions from U + Th chains
This signature allows individual assay of U and Th abundance in the Earth
Anti-neutrino from 40K are under threshold
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
Background from nuclear
Reactors
Borexino
is located in the
Gran Sasso underground
laboratory (LNGS)
in the center of Italy:
42°N 14°E
Earth data from F. Mantovani et al., Phys. Rev. D 69 (2004) 013001
Calculated anti-νe flux at the Gran Sasso Laboratory
(106 cm-2 s-1)
U
Crust
3.3
Th
Mantle
0.95
Honolulu 16/Dec/2005
Crust
Total (U+Th)
Reactor BKG
Mantle
3.0
0.77
8.0
Marco G. Giammarchi, Infn Milano
0.39
Data from the
International Nuclear
Safety Center
(http://www.insc.anl.gov)
Background from Po-210
Alpha particles reacting on C-13:
C ( , n) 16O
13
Pb concentration measured in the Counting Test Facility
 20
m Bq / ton
• Pb-210 related background negligible
• Only significant source of background are nuclear reactors
• Accidental rate also negligible (< 10% of reactors background)
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
The number expected events in Borexino
are:
events
6
yr
The background will be:
 19
events
yr
Predicted accuracy of about 30%
in 5 years of data taking
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano
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.
•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 recommissioning 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
Honolulu 16/Dec/2005
Marco G. Giammarchi, Infn Milano