Afterpulse prediction for SUBMET experiment

The SUB-Millicharge ExperimenT (SUBMET) investigates an unexplored parameter space of millicharged particles with mass $m_{\chi }<$ 1.6 GeV/c${}^2$ and charge $Q_{\chi }<{10}^{-3}e$. The detector consists of an Eljen-200 plastic scintillator coupled to a Hamamatsu Photonics R7725 photomultiplier tube (PMT). PMT afterpulses, delayed pulses produced after an energetic pulse, have been observed in the SUBMET readout system, especially following primary pulses with a large area. We present a prediction method for afterpulse rates based on measurable parameters, which reproduces the observed rate with approximately 20% precision. This approach enables a better understanding of afterpulse contributions and, consequently, improves the reliability of background predictions.