Evidence for Anomalies in Muon-Induced Neutron Emissions from Pb

This paper examines neutron multiplicity spectra emitted from massive targets at depths of 3, 40, 210, 583, 1166, and 4000 m.w.e. The measurements, carried out between 2001 and 2024, used three experimental setups with either 14 or 60 He-3 neutron detectors and lead (Pb) targets weighing 306, 565, or 1134 kg. The total acquisition time exceeded six years. When available, the spectra obtained were compared with Monte Carlo simulations. Our data indicate potential anomalies that might limit the accuracy of modelling the muon-induced neutron multiplicity spectra with a single power-law function. Even at shallow depths, the single-power-law approach fails to account for a small but statistically significant excess of events at the highest multiplicities. This excess resembles a second power-law component. Since the anomaly varies only slightly with depth, it is unlikely to be directly linked to the muon flux. Our highest-quality data, acquired at 583 m.w.e., suggest a possible structure similar to the emission of approximately 74, 106, 143, and 214 neutrons from the target. We propose new underground measurements using low-cost, large-area, position-sensitive neutron arrays surrounding multi-ton Pb targets to verify and investigate these suspected anomalies and to determine their origin.