Geoneutrinos detection in Borexino

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Geoneutrinos detection in Borexino
ISAPP 2004 International School on AstroParticle
Physics LNGS Italy June 28th July 9th 2004
Lino Miramonti
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Earth emits a tiny heat flux with an average
value of FH 60-80 mW/m2 Integrating over
the Earth surface HE 30-40 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.
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238U 232Th 40K
Radioelements
The 235U chain contribution can be neglected
(e is the present natural isotopic abundance)
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Principle of anti-neutrino detection
The best method to detect electron antineutrino
is the classic Cowan Reines reaction of capture
by proton in a liquid scintillator
The electron antineutrino tag is made possible by
a delayed coincidence of the e and by a 2.2 MeV
?-ray emitted by capture of the neutron on a
proton after a delay of 200 µs
Threshold
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238U and 232Th chains have 4 ß with E gt 1.8 MeV

end.point
Th-chain 228Ac lt 2.08 MeV
Th-chain 212Bi lt 2.25 MeV
U-chain 234Pa lt 2.29 MeV
U-chain 214Bi lt 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 and low
energy component coming with contributions from U
Th chains This signature allows individual
assay of U and Th abundance in the Earth
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Borexino is an unsegmented detector featuring 300
tons of ultra-pure liquid scintillator (C9H12)
viewed by 2200 PMTs
?M is the neutron-proton mass difference and f?n
values come from n ß decay
PC PPO (1,5 g/l) r 0.88 g cm-3 n 1.505
background
The most problematic background for this reaction
is due to fast neutrons (especially those
produced by muon interactions) At LNGS µ
reducing factor 106 ( 1 µ m-2 h-1) Borexino µ
veto 1/5000 ( 0.07 µ m-2 y-1)
Threshold 250 keV (due to 14C) Energy
Resolution FWHM ? 12 _at_ 1 MeV Spatial
Resolution ? 10 cm _at_ 1 MeV
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Geo-neutrinos can probe the Earths interior
Geochemical analysis Only the crust and the very
upper mantle are directly accessible to
geochemical analysis Seismology By seismology
analysis is possible to reconstruct the density
profile but not the chemical composition of the
earth.
Geoneutrinos Geoneutrinos can provide the
chemical composition (in terms of U, Th and K) of
the Earth interior

• Thank to Geoneutrinos it will be possible
• To measure the long lived radioisotopes inside
  the Earth (Earths radioactivity)
• To test the origins of the Earth The Bulk
  Silicate Earth

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Equation for Heat (H) and Neutrinos Luminosity
(L)
H
L
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The starting point for determining the
distribution of U, Th and K in the present CRUST
and MANTLE is understanding the composition of
the Bulk Silicate Earth (BSE), which is the
model representing the primordial mantle prior to
crust formation consistent with observation and
geochemistry (equivalent in composition to the
modern mantle plus crust).
Primitive Mantle
BSE concentrations of have been suggested
H
M Mantle 68 M Earth M(U) 20 ppb 0.68
61027g 8.51019g

• In the BSE model
• The radiogenic heat production H rate is 20 TW
• ( 8 TW from U, 8.6 TW from Th, 3 TW
  from K)
• The antineutrino production L is dominated by K.

L
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During the formation of the Earths crust the
primitive mantle was depleted (in U, Th and K)
while the crust was enriched.
Continental Crust average thickness 40 km
Oceanic Crust average thickness 6 km CC is
about 10 times richer in U and Th than OC
Samples measurements of the crust provide
isotopic abundance information
238U 232Th
Primitive Mantle (BSE) 20 ppb (20 ppb)3.8
Continental Crust 910 ppb 3500 ppb
Oceanic Crust 100 ppb 360 ppb
Present depleted Mantle 15 ppb 60 ppb
It is possible to deduce the average U and Th
concentrations in the present depleted mantle.
Crust type and thickness data in the form of a
global crust map A Global Crustal Model at 2 x
2 (http//quake.wr.usgs.gov/study/CrustalStructur
e/)
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Borexino is located in the Gran Sasso
underground laboratory (LNGS) in the center of
Italy 42N 14E
Calculated anti-?e flux at the Gran Sasso Laboratory (106 cm-2 s-1) Calculated anti-?e flux at the Gran Sasso Laboratory (106 cm-2 s-1) Calculated anti-?e flux at the Gran Sasso Laboratory (106 cm-2 s-1) Calculated anti-?e flux at the Gran Sasso Laboratory (106 cm-2 s-1) Calculated anti-?e flux at the Gran Sasso Laboratory (106 cm-2 s-1) Calculated anti-?e flux at the Gran Sasso Laboratory (106 cm-2 s-1)
U U Th Th Total (UTh) Reactor BKG
Crust Mantle Crust Mantle
1.8 1.4 1.5 1.2 5.9 0.65
Data from the International Nuclear Safety Center
(http//www.insc.anl.gov)
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Positron energy spectrum from antineutrino events
in Borexino
The number expected events in Borexino
are The background will be The reactor
anti-neutrino background has a well-known shape
it can be easily subtracted allowing (8 of them
in the same spectral region as the terrestrial
anti-?)
UTh
European Reactors