A Hybrid PIPS/CR-39/ZnS(Ag) Detector for Alpha Discrimination in Relativistic Radiation Fields: Theory, Simulation Strategy, and Calibration Design

CR-39 nuclear-track detectors and ZnS(Ag) scintillators are well-established technologies for alpha-particle detection, but their combined response to a controlled GeV-scale mixed charged-particle background has not, to our knowledge, been systematically characterized in a single co-located instrument. We present a theory and methods design for a hybrid detector intended to (i) detect radon-222 alpha radiation under mixed minimum-ionizing-particle (MIP) exposure and (ii) quantify false-positive events produced by nuclear interactions of the background. Following feedback from several experimentalists, the design now uses a thin, commercial passivated-implanted-planar-silicon (PIPS) detector as the primary alpha channel: a 5.49 MeV alpha stops completely within a ≲300 µm depletion layer via direct charge collection, while a GeV-scale MIP deposits only a small fraction of its energy there, yielding a much larger and better-calibrated separation than a scintillation-only approach. The ZnS(Ag)/SiPM and CR-39 channels are retained, but their role shifts from primary alpha detection to characterizing the residual background and the nuclear-interaction "alpha-mimic" events that can survive the primary layer. We re-evaluate the energy deposited by GeV-scale background particles using the Bethe–Bloch formalism, specify and execute an initial Geant4 validation campaign, and define a statistical framework based on ROC analysis and pre-specified equivalence testing. A Geant4 validation campaign for the revised geometry is reported in Section 4.6; no detector-performance claim is made from beam data in this manuscript, since beam measurements have not yet been obtained. The present work otherwise defines the physical model, validation protocol, and calibration methodology required for that later measurement campaign. Simulation code, logs, and summary statistics: https://github.com/toshovjahongirmirzo/RadonDetectorSim-Geant4