Reconstruction of the Effective Energy-deposition Vertex of Muon Showers using PMT Waveform in a Large-scale Liquid Scintillator Detector

TL;DR

Cosmogenic muon–induced isotopes present a significant background in deep underground liquid scintillator detectors, with shower muons contributing the majority of isotopes. The authors develop a waveform-based method that subtracts the muon-track waveform and reconstructs the shower vertex by minimizing the $\chi^{2}$ between observed peak times $T^{obs}$ and predicted times $T^{pre}= t^{incident}+ t^{muon\_tof}+ t^{photon\_tof}+ t^{delay}+ t^{offset}$. They report that, for single showers, the 68% vertex resolutions are approximately $0.16$ m in X, $0.15$ m in Y, and $0.26$ m in Z, with a distance resolution around $0.49$ m, and a reconstruction efficiency exceeding $96\%$ for shower energies above ~3 GeV; most isotopes lie within $3$ m of the reconstructed vertex. The method enables localized spherical vetoes that can substantially suppress muon-induced backgrounds while preserving signal acceptance, and the approach is readily adaptable to JUNO-scale and other large LS detectors.

Abstract

Cosmogenic muon-induced radioactive isotopes pose a significant background source for deep-underground low-background experiments. Although rock overburdens at underground sites substantially attenuate the cosmogenic muon flux, residual muon-induced backgrounds still require active suppression. For future multi-kiloton liquid scintillator (LS) detectors, such as the Jiangmen Underground Neutrino Observatory (JUNO), shower muons contribute to more than 88\% of all muon-induced isotopes. Consequently, precise reconstruction of shower vertices is essential for implementing localized spatial vetoes. We propose a novel waveform-based method to reconstruct the shower vertex, defined as the energy-deposition centroid. By subtracting the track contributions from non-shower muons in the recorded waveforms, the isolated shower component is extracted. Subsequently, combined with a photon propagation model and an iterative optimization algorithm, the shower vertex positions are reconstructed. Simulations show that for 68\% of events, the single shower vertex resolution is better than 0.16~m, 0.15~m, and 0.26~m along X, Y, and Z respectively. Furthermore, the reconstruction efficiency exceeds 96\% when requiring the distance between the reconstructed and true vertices to be less than 3.0 m. This method provides a critical technical foundation for muon-induced background suppression in JUNO and other large-scale LS detectors.

Figures (17)

• Figure 1: Schematic of a spherical target detector and different kinds of muon events labelled with shower muon, through-going muon, and stopping muon
• Figure 2: A schematic view of the detector in the simulation
• Figure 3: An example of muon energy deposition ($\sim$ 220 GeV) extension in three dimension space. For clarity, the two dimension plots, XY, XZ, YZ also are shown
• Figure 4: The shower muon energy deposit profiles. (a) The distance ($d_{V}$) distribution of the energy deposited voxels relative to the shower vertex. (b) The longitudinal distance ($d_{L}$) distribution of the energy deposited voxels relative to the muon track. The positive (negative) values are the voxels project onto the forward (backward) place of the shower vertex in the muon track. (c) The transverse distance ($d_{T}$) distribution from energy deposited voxels to muon track
• Figure 5: The shower energy spectrum of each shower in LS