Expected geoneutrino signal at JUNO

Constraints on the Earth's composition and on its radiogenic energy budget come from the detection of geoneutrinos. The KamLAND and Borexino experiments recently reported the geoneutrino flux, which reflects the amount and distribution of U and Th inside the Earth. The KamLAND and Borexino experiments recently reported the geoneutrino flux, which reflects the amount and distribution of U and Th inside the Earth. The JUNO neutrino experiment, designed as a 20 kton liquid scintillator detector, will be built in an underground laboratory in South China about 53 km from the Yangjiang and Taishan nuclear power plants. Given the large detector mass and the intense reactor antineutrino flux, JUNO aims to collect high statistics antineutrino signals from reactors but also to address the challenge of discriminating the geoneutrino signal from the reactor background.The predicted geoneutrino signal at JUNO is 39.7 $^{+6.5}_{-5.2}$ TNU, based on the existing reference Earth model, with the dominant source of uncertainty coming from the modeling of the compositional variability in the local upper crust that surrounds (out to $\sim$ 500 km) the detector. A special focus is dedicated to the 6{\deg} x 4{\deg} Local Crust surrounding the detector which is estimated to contribute for the 44% of the signal. On the base of a worldwide reference model for reactor antineutrinos, the ratio between reactor antineutrino and geoneutrino signals in the geoneutrino energy window is estimated to be 0.7 considering reactors operating in year 2013 and reaches a value of 8.9 by adding the contribution of the future nuclear power plants. In order to extract useful information about the mantle's composition, a refinement of the abundance and distribution of U and Th in the Local Crust is required, with particular attention to the geochemical characterization of the accessible upper crust.

1 which is estimated to contribute for the 44% of the signal. On the base of a worldwide reference model for reactor antineutrinos, the ratio between reactor antineutrino and geoneutrino signals in the geoneutrino energy window is estimated to be 0.7 considering reactors operating in year 2013 and reaches a value of 8.9 by adding the contribution of the future nuclear power plants.
In order to extract useful information about the mantle's composition, a refinement of the abundance and distribution of U and Th in the Local Crust is required, with particular attention to the geochemical characterization of the accessible upper crust where 47% of the expected geoneutrino signal originates and this region contributes the major source of uncertainty.

Background
The first experimental evidence of geoneutrinos, i.e. electron antineutrinos produced in beta decays along the 238 U and 232 Th decay chains, was claimed by the KamLAND Collaboration in 2005(KamLAND Collaboration 2005, which ushered in a new method for exploring the Earth's interior and provided constraints on the planet's composition and specifically its radiogenic element budget (Fiorentini et al. 2007). The geoneutrino energy spectrum contains in it distinctive contributions from U and Th, each one resulting from different rates and shapes of their decays (see Figure 3 and Figure 5 of (Fiorentini et al. 2007)) and from concentrations and spatial distributions of these elements inside the Earth. The most significant source of background for geoneutrino measurements is due to reactor antineutrinos, i.e. electron antineutrinos emitted during the beta decays of fission products from 235 U, 238 U, 239 Pu and 241 Pu burning. Approximately 30% of the reactor antineutrino events are recorded in the geoneutrino energy window extending from the IBD threshold up to the endpoint of the 214 Bi beta decay spectrum (3.272 MeV) (Fiorentini et al. 2010). The Terrestrial Neutrino Unit (TNU), which is the signal that corresponds to one IBD event per 10 32 free protons per year at 100% efficiency, is used to compare the different integrated spectral components (i.e. antineutrinos from U, Th and reactors) measured by the detectors or just beneath the Earth's surface.
Massive (>10kton) detectors such as the JUNO (Li 2014) and Reno-50 (Kim 2013) experiments are being constructed at medium baseline distances (a few tens of km) away from bright reactor antineutrino fluxes in order to assess significant physics goals regarding the neutrino properties, in first place the mass hierarchy. These experiments intend also to obtain sub-percent precision measurements of neutrino oscillation parameters and along the way make observations of events of astrophysical and terrestrial origin. . This amount of overburden will attenuate the cosmic muon flux, which contributes to the overall detector background signal, but this overburden is significantly less than that at the KamLAND and Borexino experiments. The detector energy response and the spatial distribution of the reactor cores are the most critical features affecting the experimental sensitivity (Li et al. 2013) required to achieve the intended physics goals. The goal of this present study is to predict the geoneutrino signal at JUNO on the basis of an existing reference Earth model (Huang et al. 2013), together with an estimate of the expected reactor antineutrino signal. Since a significant contribution to the expected geoneutrino signal comes from U and Th in the continental crust surrounding the site, we follow past approaches to study the local contribution (Coltorti et al. 2011;Fiorentini et al. 2012;Huang et al. 2013;Huang et al. 2014), with a particular interest in focusing in on the 5 closest 6° × 4° grid surrounding the detector, we define this latter region as the LOcal Crust (LOC) (Figure 1).

Methods
The geoneutrino signal expected at JUNO is calculated adopting the same methodology and the same inputs of the reference Earth model developed in (Huang et al. 2013)  The HPEs abundances in the Sed, OC and UC layers are assumed to be relatively 7 homogenous and correspond to the values reported in Table 3 of (Huang et al. 2013 In the reference model of (Huang et al. 2013 where m BSE = 8.1 ·10 16 kg is the U mass in the Bulk Silicate Earth (BSE) (McDonough and Sun 1995) and m C = 3.1 ·10 16 kg is the total U mass in the crust (Huang et al. 2013). The mantle geoneutrino signals reported in Table 1 are calculated with U DM = 8 ng/g and U EM = 34 ng/g together with (Th/U) DM = 2.8 and (Th/U) EM = 4.8.

Results and discussion
In Table 1  predicted mantle contribution at JUNO is assumed to be S M ≈ 9 TNU known (Huang et al. 2013). The expected geoneutrino signal from the mantle is essentially model dependent and it is estimated according to a mass balance argument. Uncertainty in the assumed mantle model is much less than that predicted for the lithosphere (e.g. δG ≈ ±6 TNU). An extensive discussion of different mantle's structure is described in (Šrámek et al. 2013), which considers a range of geoneutrino signals for different mantle's models.
Thus, a future refinement of the abundances and distribution of HPEs in the UC surrounding the JUNO detector is strongly recommended, as this region provides ~ 47% of G and is a significant contributor to the total uncertainty.
Plotting the cumulative geoneutrino signal as a function of the distance from JUNO for the different Earth reservoirs (Figure 3), we observe that half of the total signal comes from U and Th in the regional crust that lies within 550 km of the detector. Since the modeling of the geoneutrino flux is based on 1° × 1° cells, we study the signal produced in LOC subdivided in six 2°× 2° tiles (Figure 1). The geoneutrino signals from U and Th in the lithosphere of each tile are reported in Table 2 with their uncertainties. The main contribution (27% of G) comes from tile T2 in which the JUNO experiment is located (Figure 1). The thick UC in this tile, which is covered by a very shallow layer of Sed (Figure 2), is predicted to give a signal of Therefore a refined study of the U and Th content of the UC in tile T2 is a high-value target for improving the accuracy and precision of the predicted geoneutrino signal at JUNO.
Evaluating the antineutrino signal requires knowledge of several ingredients necessary for modeling the three antineutrino life stages: production, propagation to the detector site and detection in liquid scintillation detectors via the IBD reaction. The propagation and detection processes are independent by the source of the particles and we modeled these two stages using the oscillation parameters from (Ge et al. 2013) and the IBD cross section from (Strumia and Vissani 2003). Spectral parameters for U and Th geoneutrinos are from (Fiorentini et al. 2007) and modulation of these fluxes are based on (Huang et al. 2013).
Reactor antineutrino production is calculated adopting data from a worldwide reference model from (Baldoncini et al. 2014). Reported in Figure

Conclusions
Designed as a 20kton liquid scintillator detector, the JUNO experiment will collect high statistics for antineutrino signals from reactors and form the Earth. In this study we focused on predicting the geoneutrino signal using the Earth reference model of (Huang et al. 2013). The contribution originating from naturally occurring U and Th in the 6 ° × 4° LOcal Crust (LOC) surrounding the JUNO detector ( Figure 1) was determined. The main results of this study are summarized as follows.

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The HPEs in the regional crust extending out to 550 km from the detector produce half of the total expected geoneutrino signal ( Figure 3). The U and Th in the 2° × 2° tile that hosts JUNO produces There is a potential to achieve up to 10% accuracy on geoneutrinos after 105 days of data accumulation, under conditions of Yangjiang and Taishan nuclear power plants being off.
The JUNO experiment has the potential to reach a milestone in geoneutrino science, although some technical challenges must be addressed to minimize background (e.g. production of cosmic-muons spallation, accidental coincidences, radioactive contaminants in the detector). Assuming S OFF /G = 0.7, JUNO can collect hundreds of low background geoneutrino events in less than a year under optimal conditions. A future refinement of the U and Th distribution and abundance in the LOC is strongly recommended. Such data will lead to insights on the radiogenic heat production in the Earth, the composition of the mantle and 16 constraints on the chondritic building blocks that made the planet.