Geoneutrino:state of the art and prospects

Fabio Mantovani* – mantovani@fe.infn.it – www.fe.infn.it/radioactivity/University of Ferrara – INFN Ferrara*In collaboration with G. Bellini, K. Inoue, A. Serafini, V. Strati, H. WatanabeOutline•Terrestrial heat power Earth and geoneutrinos•KamLAND and Borexino results•Mantle geoneutrino signals from multi-sitedetection•Understanding the Earth’s heat budget withgeoneutrinos•Perspectives for future detectors

Open questions about natural radioactivity in the Earth• What is the radiogenic contribution toterrestrial heat production?
• At which thermal conditions the Earthinitially is formed?
• How much U and Th are in the crust andin the mantle?
• What is hidden in the Earth’s core?
(geo-reactor,40K, …)• Is the standard geochemical model (BSE)consistent with geo-neutrino data?

Heat power of the Earth• Heat power of the Earth Q [30-49 TW] is the equivalent of ~ 104 nuclearmW / m2••
•power plants.
The conduction is not the only way of Earth’s cooling: convective motions areresponsible for significant fraction of surface heat loss.
The quantitative assessment of heat transport by hydrothermal circulationremains a difficult task.
Heat flow observations are sparse, non-uniformly distributed and not reliablein the oceans.
ContinentsOceansTotalREFERENCEqCT [mW m-2]

qOCS[mW m-2]

Q (TW)

Williams et al., 1974

61

92

43 ± 6

Davies, 198055

95 ± 1041 ± 4

Sclater et al., 1980

579942Pollack et al., 1993

65 ± 2101 ± 244 ± 1

Hofmeister and Criss, 2005

6165

31 ± 1

Jaupart et al., 2015

65

107

46 ± 2

Davies and Davies, 2010

71

105

47 ± 2

Davies, 20136596

45Lucazeau, 201966.789.044

Earth’s heat budget• HCC = radiogenic powerof the continental crust• HCC = radiogenic powerof the continental crust• HCLM = radiogenicpower of the continentallithospheric mantle• The mass of the lithosphere (~ 2% of the Earth’s mass) contains••~ 40% of the total estimated HPEs and it produces HLS ~ 8 TW.
Radiogenic power of the mantle HM and the contributions to Cfrom mantle (CM) and core (CC) are model dependent.
UR is defined as convective Urey ratio.

Neglecting tidal dissipation andgravitation contraction (<0.5 TW),the two contributions to the total heatloss (Q) are:• Secular Cooling (C): cooling downcaused by the initial hotenvironment of early formation’sstages• Radiogenic Heat (H) due tonaturally occurring decays of HeatProducing Elements, HPEs (U, Thand K) inside our planet.

?

Geo-neutrinos: anti-neutrinos from the EarthU, Th and 40K in the Earth release heat together with anti-neutrinos, in a well-fixed ratio:• Earth emits (mainly) antineutrinoswhereas Sun shines in neutrinos• A fraction of geo-neutrinos from U and Th (not from 40K)are above threshold for inverse b on protons:n + p → e + + n − 1.8 MeV• Different components can be distinguished due to differentenergy spectra: e. g. anti-n with highest energy are from U• Signal unit: 1 TNU = one event per 1032 free protons/yearBorexino and KamLAND geoneutrino results• KamLAND is 1 kTon liquid•scintillator detectorsurrounded by 1325 17”PMTs and 554 20” PMTs.
The ratio Srea / Sgeochanged drastically duringthe data taking.

Data taking

2002-2019

Reactors events

629.0 ± 34.4Tot bkg events

337.9 ± 23.7Geo-n events (U+Th)

168.8+26.3-26.5S(U+Th) [TNU]

32.1 ± 5.0Data taking

2007-2019

Reactors events

39.5 ± 0.7Tot bkg events

8.3 ± 1.0Geo-n events (U+Th)

52.6+9.6-9.0S(U+Th) [TNU]

47.0+8.6-8.1• Borexino is 0.3 kTon liquid•scintillator detectorsurrounded by ~2200 8”PMTs.
The ratio Srea / Sgeo fluctuatedregularly in a range of ± 25%during the data taking.

Timeline of KamLAND and Borexino geoneutrino results• In 2005 KamLAND collaboration published the first geoneutrino observation.
• Horizontal bars traces the expected signal at 1σ C.L.
• In the second decade of the 21st century the results published with greater statisticalsignificance highlighted the necessity of geophysical and geological models forunderstanding geoneutrino signal.

Mantle geoneutrino signals from multi-site detectionThe Far Field Lithosphere (FFL)is the superficial portion of theEarth including the Far Field Crust(FFC) and the ContinentalLithospheric Mantle (CLM).

Detectorand NFC

U and Th distributed in the NearField Crust (NFC) gives asignificant contribution to the signal(~ 50% of the total).
𝑖𝑺𝒊𝑴 𝑼 + 𝑻𝒉 = 𝑆𝐸𝑥𝑝𝑈 + 𝑇ℎ − 𝑺𝒊𝑭𝑭𝑪 𝑼 + 𝑻𝒉 − 𝑺𝒊𝑪𝑳𝑴 𝑼 + 𝑻𝒉 − 𝑺𝒊𝑵𝑭𝑪 𝑼 + 𝑻𝒉

The geological models need to comply with the following constraints:• FFC model needs to be unique for i detectors for avoiding systematic biases.
• NFC should be built with geochemical and/or geophysical information typical of the local regions.
• NFC must be geometrically complementary to the FFC.
• All geoneutrino signal contributions should be separately reported together with their uncertainties.

Mantle geoneutrinos from KamLAND and Borexino• The FFC and the CLM signals of KL and BX are fully correlated,••since they are derived from the same geophysical and geochemicalmodel,.
SNFCBX(U+Th) and SNFCKL(U+Th) are considered uncorrelated.
Using only the experimental signals published by BX and KLcollaborations without any spectral information, the PDFs ofexperimental KL and BX signals are reconstructed.

• The joint distribution SMKL+BX(U+Th) can be inferred from the PDFsby requiring that SMKL(U+Th) = SMBX(U+Th), obtaining the combinedmantle geoneutrino signal:SMKL+BX(U+Th) = 8.9 +5.1 -5.5 TNU

Bulk Silicate Earth Models• The BSE describes the•primordial, non-metallicEarth condition thatfollowed planetaryaccretion and coreseparation, prior to itsdifferentiation into amantle and lithosphere.
Different authors*proposed a range of BSEmodels based on differentconstraints (carbonaceouschondrites, enstatitechondrites, undepletedmantle, etc.)* The codes reported in the plot are explicitlyindicated in the back slide.

H(U+Th+K) [TW]Poor

MediumRich

12.4 ± 1.919.7 ± 3.131.7 ± 3.4Mantle radiogenic power from U and ThSince HLS (U+Th)= 6.9−1.2+1.6TW is independent from theBSE model, the discriminationcapability of the combinedgeoneutrino measurementamong the different BSEmodels can be studied in thespace SM(U+Th) vs HM(U+Th):

b High Scenario = 0.98 TNU/TW

b High Scenario = 0.75 TNU/TW

SM(U+Th) = b · HM(U+Th)

HM(U+Th) [TW]Poor

MediumRichKL+BX

3.2+2.0-2.19.3 ± 2.920.2+3.2-3.310.3+5.9-6.4Collection of the geoneutrino mantle signals

Understanding the Earth’s heat budget with geoneutrinos

AdoptedThe combined geoneutrino analysis of KL andBX results constraint:• the ratio of heat production over heat loss
• the Urey ratio URKL+BX = 0.35+0.19-0.20

Combined KL + BX

Q [TW]

47 ± 2

HLS(U+Th+K) [TW]8.1+1.9-1.4HM(U+Th+K) [TW]11.3+3.3-3.412.5+7.1-7.7H [TW]19.3 ± 2.920.8+7.3-7.9C [TW]28 ± 426 ± 8

Georeactor investigation• A possible existence of GeoReactor (GR), i.e., natural nuclear fission

Borexinodetector

reactor in the Earth interior, was first suggested by Herndon in 1993.

• Different models suggest the existence of natural nuclear reactors atdifferent depths: at the center of the core, at the inner core boundary,and the core-mantle boundary.

• Borexino collaboration tested the GR hypothesis by performing thespectral fit after constraining the expected number of reactorantineutrino events.
GR1

GR2

GR3

Distance from BX

2900 km

6371 km

9842 km

GR power excluded at 95% CL

0.5 TW

2.4 TW

5.7 TW

Expected geoneutrino signal at SNO+• Deepest underground detector (~ 5800 mwe)• 780 tons of LS detector with ~ 9300 PMTs
• Expected react-n in [1.8-3.3 MeV] = 48.5+1.8-1.5 TNU (Srea / Sgeo ~ 1.2)S(U+Th) [TNU]

Wipperfurth et al., 2020(using global crustal models)

50.2+9.7-8.146.2+9.3-7.746.8+9.3-7.8Strati et al., 2017(combining global crustal model and local geological data)41.8+9.6-6.2Expected geoneutrino signal at JUNO• JUNO is a 20 kton LS detectorsurrounded by ~18.000 20” PMT

N° ofcores

Thermalpower/core

Yangjiang

62.9 GW

Taishan

24.6 GW• Expected geo-n ~ 400events/year (~ 40 TNU)

• Expected react-n in [1.8-3.3 MeV]~ 260 TNU (Srea / Sgeo ~ 7)

U [ppm]S(U+Th)[TNU]Strati et al., 2015(using global crustal model)Wipperfurth et al., 2020(using global crustal models)

39.7+6.5-5.241.3+7.5-6.341.2+7.6-6.440.05+7.4-6.2Gao et al., 2020 (*)(combining global crustal model and local geological data)

49.1+5.6-5.0(*)

Take-away messages• To deeply understand the experimental geoneutrinoresults, the use of refined geological models is essential• The mantle radiogenic power HM (U+Th)= 10.3+5.9-6.4 TWfrom combined KL+BX geoneutrino analysis agrees withmedium-H BSE models.

• The presence of georeactor PGR = 2.4 TW in the centerof the Earth is excluded at 95% C.L.

• Very soon we will investigate the deepEarth with KL, BX, SNO+ and JUNOresults: the era of "multi-site detection"of geoneutrinos is definitely open

Thank youBack up slideReference

CodeJackson and Jellinek, 2013

JJ13O’Neill and Palme, 2008

OP08

Javoy and Kaminski, 2014

JK14

Javoy et al., 2010

J10

McDonough and Sun, 1995

MS95

Lyubetskaya and Korenaga,2007

LK07

Palme and O’Neill, 2007

PO07

Arevalo, 2010

A10

Wang et al., 2018

W18

Palme and O’Neill, 2014

PO14

Turcotte, 2002*

T02

Turcotte, 2014

T14

H(U+Th+K) [TW]

PoorMediumRich

12.4 ± 1.919.7 ± 3.131.7 ± 3.4