Transcript Document

ISAPP 2004
International School on AstroParticle Physics
LNGS Italy – June 28th – July 9th 2004
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Lino Miramonti
1
Earth emits a tiny heat flux with an
average value of
ΦH ~ 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.
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Lino Miramonti
2
238U 232Th 40K
The 235U chain contribution can be neglected
NA
A
238
 12300
U

ln 2
1
Bq
g
T h  4000 Bq g
40
Bq
K
 30
g
40
( K  0.0118 % of natK)
238
U206 P b  8α  6e-  6 ν  51.7 MeV
e
232
T h208 P b  6α  4e-  4 ν  42.8 MeV
e
2
232
12300
Heat

9.5 10
ε(T h) 
2.7  10
ε(U)
ε(K)
-8
-8
12
 3.6 10
W
g
W
g
W
g
 40 K40 Ca  e   ν  1.32 MeV (89%)
e
 40

40
 K  e  Ar  νe  1.51MeV (11%)
Bq
MeV 

8 W
 51.7 MeV  1.6 10-13
  9.8 10
g
J 
g

 (U )
e
Neutrinos
 7.4  104
 (Th)  1.6  104
e
 ( K )

27
 ( K )

3 .3
e
e
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
12300
e
Bq

 6 e  7.4  104 e
g
gs
gs
e
gs
e
gs
(ε is the present
natural isotopic
abundance)
e
gs
Lino Miramonti
3
The best method to detect electron
antineutrino is the classic Cowan Reines
reaction of capture by proton in a liquid
scintillator:
e  p  n  e

Threshold
T hee signal energy:
E(e  )  E( ν e )  Q  2mec 2 (Q
1.8 MeV)
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
The electron antineutrino tag
is made possible by a delayed
coincidence of the e+ and by a
2.2 MeV γ-ray emitted by
capture of the neutron on a
proton after a delay of ~ 200 µs
Lino Miramonti
4
and 232Th chains have 4 β
with E > 1.8 MeV :
238U
Anti-neutrino from
40K
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
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
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Lino Miramonti
5
Borexino is an unsegmented detector featuring 300 tons of ultra-pure liquid scintillator (C9H12) viewed by 2200 PMTs
no of protons(in 300T)  1.81031

2 2h3
E  Mc2 E  Mc2 2  mec2 2
 ( E e )  5 8
me c f n
PC + PPO (1,5 g/l)
r = 0.88 g cm-3 n = 1.505

1
2
ΔM is the neutron-proton mass difference and fn values
come from n β decay
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% @ 1 MeV
Spatial Resolution:  10 cm @ 1 MeV
Sensitivity : 
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
1 ν e event
(in 300 tons)
yr
Lino Miramonti
6
Geo-neutrinos can probe the Earth’s 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 (Earth’s radioactivity)
•To test the origins of the Earth: The Bulk Silicate Earth
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Lino Miramonti
7
Equation for
Heat (H)
and
Neutrinos Luminosity (L)
Each element has a fixed ratio :
Heat

H  9.510-8 · M(U)  2.710-8 · M(T h) 3.6·10-12 M(K) [W]

 
4
4
 L  7.4·10 · M(U)  1.6·10 · M(T h) 27 · M(K)  

s
Luminosity
 L  3.3· M(K)  

 s 


Heat
Everything is fixed in term of 3 numbers : M(U),
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Th
,
U
K
U
Lino Miramonti
8
Primitive Mantle
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).
BSE concentrations of:
U  20 ppb (20%)
have been suggested
Th
 3.8
U
K
 10000
U
M Mantle= 68% M Earth
M(U) = 20 ppb · 0.68 · 6·1027g = 8.5·1019g
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.
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Lino Miramonti
9
During the formation of the Earth’s 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
Primitive Mantle (BSE)
It is possible to deduce the average U and Th
concentrations in the present depleted mantle.
232Th
20 ppb
(20 ppb)·3.8
Continental Crust
910 ppb
3500 ppb
Oceanic Crust
100 ppb
360 ppb
15 ppb
60 ppb
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/CrustalStructure/)
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Lino Miramonti
10
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
Crust
Th
Mantle
1.8
1.4
Crust
Total (U+Th)
Reactor BKG
5.9
0.65
Mantle
1.5
1.2
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Data from the
International Nuclear
Safety Center
(http://www.insc.anl.gov)
Lino Miramonti
11
The number expected events in
Borexino are:
events
8
yr
The background will be:
 30
events
yr
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-ν)
ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004
Lino Miramonti
12