Geoneutrinos detection in Borexino

ISAPP 2004 International School on AstroParticle Physics - Laboratori Nazionali del Gran Sasso - 28 June 9 July 2004 Lino Miramonti 1

ISAPP 2004 International School on AstroParticle Physics

LNGS Italy – June 28th – July 9th 2004

http://www.fisica.unimi.it/dipartimento/html/index.php


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

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


K)of% 0.0118K( nat40

gBq40

gBq232

gBq238

30K

4000Th

12300U

K)of% 0.0118K( nat40

gBq40

gBq232

gBq238

30K

4000Th

12300U

(ε is the present natural isotopic abundance)

(11%) MeV1.51e

νAreK

(89%) MeV1.32e

νeCaK

MeV42.8e

ν44e6αPbTh

MeV51.7e

ν66e8αPbU

4040

4040

-208232

-206238

238U 232Th 40K

g

W103.6ε(K)

g

W102.7ε(Th)

g

W109.5ε(U)

21

8-

8-

Heat

sgK

sgK

sgTh

sgU

Neutrinos

e

e

e

e

e

e

e

e

3.3)(

27)(

106.1)(

104.7)(

4

4

The 235U chain contribution can be neglected

2ln2

1A

N A

g

W109.8

J

MeV101.6MeV51.7

g

Bq12300 813-

sge

e

4104.76g

Bq12300


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 µsMeV) 1.8(Q

2ee c2mQ)νE()E(e

:energy signal e TheThreshold

The best method to detect electron antineutrino is the classic Cowan Reines reaction of capture by proton in a liquid scintillator:

enpe


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 MeVAnti-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


Threshold: 250 keV (due to 14C)Energy Resolution: FWHM 12% @ 1 MeVSpatial Resolution: 10 cm @ 1 MeV

PC + PPO (1,5 g/l) = 0.88 g cm-3 n = 1.505

Borexino is an unsegmented detector featuring 300 tons of ultra-pure liquid scintillator (C9H12) viewed by 2200 PMTs

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)

tons)300(inyr

eventν1:ySensitivit e

31101.8T) 300 (in protons of on

21

22222

85

322)( cmMcEMcE

fcm

hE e

nee

ΔM is the neutron-proton mass difference and fn values come from n β decay


Geo-neutrinos can probe the Earth’s interiorGeo-neutrinos can probe the Earth’s interior

Geochemical analysisOnly the crust and the very upper mantle are directly accessible to geochemical analysis

SeismologyBy seismology analysis is possible to reconstruct the density profile but not the chemical composition of the earth.

GeoneutrinosGeoneutrinos 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


Heat

:ratio fixeda haselement Each

U

K,

U

ThM(U),:numbers 3 of termin fixed is Everything

Equation for Heat (H) and Neutrinos Luminosity (L)

sL

L

H

M(K)· 3.3

s M(K)· 27 M(Th)· 1.6·10 M(U)· 7.4·10

Luminosity

[W] M(K)10 · 3.6 M(Th)· 10 2.7 M(U)· 10 9.5Heat

44

-12-8-8


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).

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.

BSE concentrations of:

have been suggested

3.8U

Th 00001

U

K

M Mantle= 68% M Earth

M(U) = 20 ppb · 0.68 · 6·1027g = 8.5·1019g

Primitive Mantle

20%)(ppb20U


During the formation of the Earth’s crust the primitive mantle was depleted (in U, Th and K) while the crust was enriched.

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 ModelGlobal Crustal Model at 2° x 2°(http://quake.wr.usgs.gov/study/CrustalStructure/)

Continental Crust: average thickness ~ 40 km Oceanic Crust: average thickness ~ 6 km CC is about 10 times richer in U and Th than OC


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

1.8 1.4 1.5 1.2 5.9 0.65

Data from the International Nuclear

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


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-ν)

yr

events8

yr

events30

Documents

Geoneutrinos detection in Borexino