![Implications on radiogenic heat: expected geo-neutrino signal in Borexino (TNU units), due to U and Th radioactive decays, as a function of radiogenic heat released. The three blue, green and pink regions represent predictions, from the left to the right, of the cosmochemical, geochemical and geodynamical BSE models [8]. We report the best values from Borexino, along with systematical and statistical errors combined.](https://www.researchgate.net/profile/A-Ludovico-2/publication/329562159/figure/fig4/AS:702636009455624@1544532887845/mplications-on-radiogenic-heat-expected-geo-neutrino-signal-in-Borexino-TNU-units-due_Q320.jpg)
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PoS(HQL2018)052
Results on geo-neutrinos at Borexino
Davide Basilico∗on behalf of the Borexino collaboration
Dipartimento di Fisica, Università degli Studi e INFN, Milano 20133, Italy
E-mail: davide.basilico@mi.infn.it
Borexino collaboration: D. Basilico, M. Agostini, K. Altenmüller, S. Appel, V. Atroshchenko,
Z. Bagdasarian, G. Bellini, J. Benziger, D. Bick, I. Bolognino, G. Bonfini, D. Bravo,
B. Caccianiga, F. Calaprice, A. Caminata, S. Caprioli, M. Carlini, P. Cavalcante, F. Cavanna,
A. Chepurnov, K. Choi, L. Collica, S. Davini, A. Derbin, X.F. Ding, A. Di Ludovico, L. Di Noto,
I. Drachnev, K. Fomenko, A. Formozov, D. Franco, F. Gabriele, C. Galbiati, M. Gschwender,
C. Ghiano, M. Giammarchi, A. Goretti, M. Gromov, D. Guffanti, C. Hagner, T. Houdy,
E. Hungerford, Aldo Ianni, Andrea Ianni, A. Jany, D. Jeschke, V. Kobychev, D. Korablev,
G. Korga, T.Lachenmaier, M. Laubenstein, E. Litvinovich, F. Lombardi, P. Lombardi,
L. Ludhova, G. Lukyanchenko, L. Lukyanchenko, I. Machulin, G. Manuzio, S. Marcocci,
J. Maricic, J. Martyn, E. Meroni, M. Meyer, L. Miramonti, M. Misiaszek, V. Muratova,
B. Neumair, L. Oberauer, D. Opitz, V. Orekhov, F. Ortica, M. Pallavicini, L. Papp, Ö. Penek,
L. Pietrofaccia, N. Pilipenko, A. Pocar, A. Porcelli, G. Raikov, G. Ranucci, A. Razeto, A. Re,
M. Redchuk, A. Romani, N. Rossi, S. Rottenanger, S. Schönert, D. Semenov, M. Skorokhvatov,
O. Smirnov, A. Sotnikov, L.F.F. Stokes, Y. Suvorov, R. Tartaglia, G. Testera, J. Thurn,
M. Toropova, E. Unzhakov, A. Vishneva, R.B. Vogelaar, F. von Feilitzsch, S. Weinz, M. Wojcik,
M. Wurm, Z. Yokley, O. Zaimidoroga, S. Zavatarelli, K. Zuber, G. Zuzel.
The latest geo-neutrinos Borexino results, published in Ref. [1], are briefly presented and dis-
cussed. Borexino [2] is a liquid scintillator detector located at the Gran Sasso National Laboratory
in Italy, whose primary purpose is the real-time spectroscopy of low energy solar neutrinos. It is
the only experiment so far to have provided an evidence of geo-neutrinos existence beyond a 5σ
significance level. The geo-neutrinos measurement and analysis, along with implications from
the geological point of view, are shortly discussed.
XIV International Conference on Heavy Quarks and Leptons (HQL2018)
May 27- June 1, 2018
Yamagata Terrsa, Yamagata,Japan
∗Speaker.
c
Copyright owned by the author(s) under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/
PoS(HQL2018)052
Results on geo-neutrinos at Borexino Davide Basilico
1. Introduction
Geo-neutrinos [3] are electron antineutrinos (νe) produced in the radioactive decays occurring
in the Earth interior layers. Their sources are the natural radioactive chains of nuclides, through
β−decays. The three natural chains of interest, starting respectively with 238U, 232Th or 40 K, can
be globally summarized as follows:
238U→206Pb +8α+6e−+6νe+51.7MeV (1.1)
232Th →208Pb +6α+4e−+4νe+42.7MeV (1.2)
40K→40Ca +e−+νe+1.31MeV (1.3)
It is important to underline the two-fold geo-neutrinos scientific interest. First of all, they
can be considered as unique messengers of information coming from the innermost Earth layers.
Their flux and the radiogenic heat, released in radioactive decays, are found in a well-known ratio
thanks to our knowledge of natural radioactive chains. Thus it is possible to measure the total
geo-neutrino flux, and connect it to the contribution of radiogenic heat released in radioactive
decays, and eventually to the total Earth heat flux. Secondly, their flux is critically related to to
the abundance and distribution of U and Th in the Earth: these are fundamental inputs for classes
of models describing the geological, geophysical, and geochemical processes occurring inside the
Earth. Eventually, the geo-neutrino signal provides information about the radiogenic power of the
deep mantle, which is completely inaccessible by means of direct sampling.
2. Borexino detector
Geo-neutrinos travel almost undisturbed through the Earth with a very small probability to
interact in the detectors; this is due to the very low cross section. This property implies severe
detection requirements in order to measure the related flux: large sizes and a very low radioactive
background are needed. To overcome successfully these detection constraints, advanced technolo-
gies and considerable efforts are needed. Only two experiments so far have the requirements needed
to measure and distinguish the geo-neutrino flux [4,5].
Borexino is a large volume liquid scintillator detector whose primary purpose is the real-time
measurement of low energy solar neutrinos [2]. It is schematically drawn in Fig. 1. It is located
deep underground (approximately 3800 meters of water equivalent) in the Hall C of the Gran Sasso
National Laboratory, in Italy. The Gran Sasso mountain natural shielding, combined with the
detector design, allows an extremely high muon flux suppression. Borexino has been data-taking
from 2007, achieving important results [6]: it detected and then precisely measured the flux of
the 7Be solar neutrinos, ruled out the day-night asymmetry of their interaction rate, made the first
direct observation of the pep neutrinos, and set the tightest upper limit so far on the flux of CNO
solar neutrinos.
The Borexino design is driven by the principle of graded shielding: a inner scintillating ultra-
pure core is found at the center of shielding concentric shells, with decreasing radio-purity from
inside to outside. The scintillator is a solution of PPO (2,5-diphenyloxazole) in pseudocumene
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PoS(HQL2018)052
Results on geo-neutrinos at Borexino Davide Basilico
(PC, 1,2,4-trimethylbenzene) at a concentration of 2.5g/l. The main scintillator mass is about 278
ton and is contained in a 125µm thick spherical nylon Inner Vessel (IV) of approximately 4.25m
radius. 2212 internal photo-multipliers (PMTs) are mounted on a Stainless Steel Surface (SSS) to
collect scintillation light and allowing the measurement of the position and of the energy of the
detected events.
The extremely low intrinsic radioactivity achieved in Borexino, the strong cosmic ray shield-
ing, the high photon yield have made possible a sensitive search for νein the MeV energy range.
While the solar neutrinos are measured through the elastic scattering with scintillator electrons,
Borexino is also able to measure νethrough the Inverse Beta Decay reaction νe+p→n+e−
(IBD). The typical cross section values at the MeV energy range are found around 10−44 cm2:
because of this extremely low cross section the geo-neutrinos detection is very challenging, in
spite of the 106cm−2s−1expected flux on the Earth surface. The IBD kinematic threshold is
Ethr =1.806MeV; the antineutrinos produced in the 40K chain cannot be detected because their
end-point energy spectrum of 1.31MeV is found below Ethr. Thus, only the U and Th chains re-
lated νecan be measured in Borexino [4,7]. The produced positron immediately comes to rest in
the liquid scintillator, and it annihilates with an emission of two 511keV γ, with a visible energy
of Eprompt =Eνe−0.782MeV (prompt event). It is noticeable to underline that the scintillation
light of the proton recoil is highly quenched and almost negligible. Instead, the emitted neutron
is captured on protons in a mean time of 256µs, with the emission of a 2.22 MeV de-excitation γ
(delayed event). The combination of these two events, through the characteristic time, spatial and
energetic coincidences, allows Borexino to detect geo-neutrinos in an extremely low-background
channel.
Figure 1: Schematic drawing of the Borexino detector.
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PoS(HQL2018)052
Results on geo-neutrinos at Borexino Davide Basilico
3. Data analysis and results
The data reported in the latest geo-neutrino paper (Ref. [1]) from Borexino were collected
between December 15, 2007 and March 8, 2015 for a total of 2055.9 days before any selection cut.
The antineutrinos coming from nuclear power plant are the main background for the geo-
neutrino data taking. Since no nuclear power plants are present in Italy, the LNGS is an ex-
cellent location for geo-neutrinos detection. In order to estimate the nuclear plant contributes,
one has to combine a large amount of information, including the reactor power as a function
of time, the component fractions, the reactors-detector distances and the νesurvival oscillation
probability. The number of reactor νeis extracted through Monte Carlo simulations and is 5.7±
0.3events / (100 ton yr). The extreme radiopurity of the Borexino detector and the coincidences
for IBD events candidates guarantee that the non-νebackground is almost negligible. The main
contributions are due to the internal radioactivity of the detector structural components (PMTs,
IV...), to the accidental coincidences, and to the cosmogenic-muon related decays. Neutrons and
long-lived cosmogenic radioactivity events have been discarded through time cuts respectively of
2 ms for every muon crossing the outer detector and within 2 s of muons crossing the inner detec-
tor. Many additional software cuts have been applied in order to select as possible the νeevents;
the most important ones select the prompt and delayed energy through the amount of scintillation
light detected, the correlation distance and time between prompt and delayed signals, the pulse
shape discrimination for delayed signals, the dynamical fiducial volume cut. More details about
the data selection can be found at Ref. [1]. The combined efficiency of the selection cuts is esti-
mated through Monte Carlo simulations and it is 84.2±1.5%, and a total of 77 candidates have
been selected in the considered time period.
Prompt νeevent candidates spectrum in 2056 days of Borexino data-taking [1], expressed in
units of photoelectrons (p.e.), is shown in Fig. 2. An unbinned maximal likelihood fit of the energy
spectrum has been performed. The best-fit blue dotted line shows the geo-neutrino and reactor
νespectra assuming the Th/U mass ratio fixed to the chondritic model value of 3.9. Blue and
light blue areas show respectively the result of the fit with U and Th contributions set as free and
independent parameters. The reactor signal (orange area) has been calculated adopting the reactor
data from IAEA. Two different fit approaches are shown. Non-νebackground has been considered
in the fit but it is constrained to independent estimations and it is completely negligible for our
purposes.
The numbers of detected antineutrinos can be easily converted into fluxes, commonly ex-
pressed in Terrestrial Neutrino Units (TNU). This corresponds to the number of antineutrino events
detected during one year on a target of 1032 protons (i.e. approximately 103ton of liquid scin-
tillator) and 100% detection efficiency. According to the fit choice (Th/U abundance fixed or
free parameter) different information can be extracted. The left panel of Fig. 3shows the best-fit
contours for 1σ, 3σand 5σregions in the Sgeo −Sreact parameters space, extracted through the
unbinned likelihood fit with Th/U abundance fixed to che chondritic model value. The absence
of geo-neutrinos hypotesis, i.e. Sgeo =0, is excluded for the first time at more than 5σ(5.9σdis-
covery). The right panel of Fig. 3instead shows the best-fit contours for 1σ, 2σand 3σregions
in the STh −SUspace, for the unbinned likelihood fit with U and Th left as free parameters. The
dashed line represents the predictions for the mass ratio Th/U=3.9 of the chondritic model. The
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PoS(HQL2018)052
Results on geo-neutrinos at Borexino Davide Basilico
Figure 2: Prompt νeevent candidates spectrum in 2056 days of Borexino data-taking, expressed in units
of photoelectrons (p.e.). The two different fit approaches are shown. The best-fit blue dotted line shows
the geo-neutrino and reactor νespectra assuming a fixed chondritic ratio (Th/U=3.9). Blue and light blue
areas show respectively the result of fit with U and Th set as free and independent parameters. The orange
area represents the reactor νecontribution.
Figure 3: Left panel: best-fit contours for 1σ, 3σand 5σin the Sgeo −Sreact parameters space, extracted
through the unbinned likelihood fit with Th/U mass ratio fixed to che chondritic model value. Right panel:
best-fit contours for 1σ, 2σand 3σin the STh −SUfor the unbinned likelihood fit with U and Th contri-
butions left as free parameters. Dashed line represents the predictions for the mass ratio Th/U=3.9 of the
chondritic model.
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PoS(HQL2018)052
Results on geo-neutrinos at Borexino Davide Basilico
Figure 4: Implications on radiogenic heat: expected geo-neutrino signal in Borexino (TNU units), due to U
and Th radioactive decays, as a function of radiogenic heat released. The three blue, green and pink regions
represent predictions, from the left to the right, of the cosmochemical, geochemical and geodynamical BSE
models [8]. We report the best values from Borexino, along with systematical and statistical errors combined.
central value of the fit is largely compatible with the predictions of the chondritic model. This
result show also that Borexino is able to perform a real time spectroscopy of geo-neutrinos, being
able to separate the two components of the detectable natural chains.
From the geophysical point of view, the geo-neutrino results have strong implications on our
radiogenic heat knowledge: Fig. 4shows the expected geo-neutrino signal in Borexino, due to U
and Th natural radioactive chains, as a function of the radiogenic heat [3].
4. Conclusions
The more recent and complete Borexino results related to geo-neutrinos are reviewed and
briefly described. Borexino has been taking νedata for 2056 days, thanks to an extremely low
background level and to position and energy reconstruction high precision. The main result consists
in the geo-neutrinos measurement with 5.9σsignificance. We have seen also that Borexino is
able to fit separately the two U and Th natural chain contributions, performing a real time geo-
neutrino spectroscopy. The study of the Earth’s geo-neutrino flux creates a new interdisciplinary
field between Geology and Physics [3].
References
[1] M. Agostini et al., Spectroscopy of geo-neutrinos from 2056 days of Borexino data,Phys.Rev. D 92
(2015) 3 [hep-ex/1506.04610].
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PoS(HQL2018)052
Results on geo-neutrinos at Borexino Davide Basilico
[2] G. Alimonti et al., The Borexino detector at the Laboratori Nazionali del Gran Sasso,NIM A 03 (600)
568-593 [physics.ins-det/0806.2400].
[3] G. Bellini, A. Ianni, L. Ludhova, F. Mantovani, W.F. McDonough, Geo-neutrinos,
Prog.Part.Nucl.Phys. 73 (2013) 1-34 [physics.geo-ph/1310.3732].
[4] G. Bellini et al., Observation of Geo-Neutrinos,Phys.Lett. B 687 (2013) 299-304
[hep-ex/1003.0284].
[5] T. Araki et al., Experimental investigation of geologically produced antineutrinos with KamLAND,
Nature 436 (2005) 499-503 [hep-ex/1506.04610].
[6] G. Bellini et al., Final results of Borexino Phase-I on low energy solar neutrino spectroscopy,Phys.
Rev. D 89 (2014) 11, 112007 [hep-ex/1308.0443].
[7] G. Bellini et al., Measurement of geo-neutrinos from 1353 days of Borexino,Phys. Rev. D 92, 031101
(2015) Phys.Lett. B 722 (2013) 295-300 [hep-ex/1303.2571].
[8] O. Šrámek et al., Geophysical and geochemical constraints on geoneutrino fluxes from Earth’s
mantle,Earth Planet. Sci. Lett. 361 (2013) 356-366 [geo-ph/1207.0853].
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