Decaying muon charge spectra as a robust long-term calibration tool for water Cherenkov detectors

Quispe D.; Sarmiento-Cano C.; Otiniano L.; Asorey H.; Castromonte C.; Yanyachi R.; Perca R. · Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (2026) · DOI: 10.1016/j.nima.2026.171415

Resumen

Water Cherenkov Detectors (WCDs) experience progressive degradation of detector response over extended monitoring periods due to water quality deterioration and coating aging, manifesting as compression of charge spectra. We present a new long-term calibration method that exploits the charge spectrum of decaying muons (e-muons) captured inside the detector. Since muon decay is an internal, well-characterized process independent of geographic location, altitude, and atmospheric conditions, the e-muon spectrum provides a stable calibration reference spanning the full detector dynamic range. The method determines a daily scaling factor through quadratic minimization of spectral differences, making it computationally inexpensive and straightforward to implement. We demonstrate the method's effectiveness using multi-month datasets (3–5 months) from two LAGO detectors at different altitudes: Characatito (Arequipa, Perú, 2400 m a.s.l.) and Nahuelito (Bariloche, Argentina, 850 m a.s.l.). Validation through atmospheric pressure correlations shows that calibrated event rates achieve correlation coefficients exceeding 0.85 for muonic and 0.75 for electromagnetic components, with measurements falling within 90% confidence bands predicted by ARTI simulations. This confirms successful removal of instrumental drifts while preserving genuine atmospheric modulation signals. The method is particularly valuable for high-altitude WCD networks (LAGO, SWGO, HAWC) where traditional Vertical Equivalent Muon based calibration faces challenges from electromagnetic contamination, and can be integrated at the DAQ level for autonomous, real-time operation.

Instituciones: Universidad Nacional de San Agustín de Arequipa; Universidad Nacional de Ingeniería