New Spallation Background Rejection Techniques to Greatly Improve the Solar Neutrino Sensitivity of JUNO

TL;DR

This work tackles the challenge of muon-induced spallation backgrounds in JUNO's solar-neutrino program, made more severe by JUNO's shallower depth and lack of directionality. Using FLUKA-based simulations, it characterizes isotope yields, decay spectra, and the crucial role of hadronic showers—whose associated neutrons provide a tagging handle—to develop shower-type–dependent cuts. The proposed strategy, including neutron tagging and region-focused cylindrical vetoes around the shower peak, reduces background above $2.3$ MeV by a factor of about 5 in deadtime efficiency, potentially halving the running time to reach target exposure. These improvements advance JUNO's capability to measure $^8$B and $hep$ solar neutrinos and can be adapted to other scintillator detectors facing spallation backgrounds.

Abstract

While the potential of the Jiangmen Underground Neutrino Observatory (JUNO) to measure solar neutrinos is known, realizing this potential requires new techniques to reduce detector backgrounds. One of the most serious backgrounds is due to the beta decays of unstable nuclei produced through muon breakup (spallation) of nuclei. This background is much more significant in JUNO compared to Super-Kamiokande due to JUNO's shallower depth and its lack of directional information. We present the first detailed theoretical calculations of spallation backgrounds in JUNO, showing the underlying physical processes and new ways to cut backgrounds while preserving signals. A key point is showing the importance of neutron tagging to identify hadronic showers, which are rare but produce almost all of the dangerous isotopes. With our new techniques, JUNO will be able to reduce deadtime (signal loss) by a factor of five and to reduce the running time needed to meet sensitivity goals by a factor of two. This will give JUNO greatly improved sensitivity to $^8$B and $hep$ solar neutrinos, as we will explore in a separate paper.

Figures (9)

• Figure 1: Cosmic-ray muon flux at JUNO as a function of energy. The dashed lines represent the energy loss of a minimum ionizing muon and the critical energy of muons in LAB.
• Figure 2: Probability distribution of the total energy loss for throughgoing muons in JUNO, normalized per muon. The red dotted line corresponds to the average muon energy loss.
• Figure 3: Spallation background energy spectra and time distributions, before and after our cuts (note the changes in the axes ranges). The expected energy spectrum of the $^8$B solar neutrino signal (taken from Ref. JUNO:2020hqc) is shown for comparison.
• Figure 4: The energy spectrum of secondary particles produced directly by the muons in JUNO, normalized per muon.
• Figure 5: Energy spectra of electromagnetic and hadronic showers induced by muons in JUNO. The top panels show the energy spectra for all showers, normalized per muon. The bottom panel shows the spectra of isotope-producing showers only, weighted by their isotope yields. In all panels, the different shades represent the associated neutron yields as shown in the top right panel.