SynradG4: A Geant4-Based Extension for Synchrotron Radiation Background Studies in the ePIC Detector at the Electron-Ion Collider

Abstract

The Electron-Ion Collider (EIC) will operate at high luminosity with multi-GeV, high-current electron beams, resulting in substantial synchrotron radiation (SR) emission in the electron storage ring (ESR). A detailed understanding of SR photon transport in the complex three-dimensional interaction region (IR) geometry is critical for estimating backgrounds in the ePIC detector and for developing effective shielding and masking strategies. This paper presents SynradG4, an EIC-specific extension of Geant4 for fast photon tracking in vacuum based on established models of SR reflection and rough-surface scattering. SynradG4 integrates the specular and diffuse photon reflection models of Synrad+ into the Geant4 geometry and field framework, while disabling all bulk matter interactions to achieve high-statistics transport through the 50-m-long IR vacuum system. The absorbed photons are then passed to a second-stage DD4hep simulation, in which electromagnetic processes such as photoabsorption, Compton scattering, Rayleigh scattering, and fluorescence are enabled for their propagation through the beam pipe and detector materials. SynradG4 is not intended to replace general SR simulation codes; rather, it complements them by providing the workflow and geometry integration capabilities needed for EIC-specific background studies. Benchmark tests show that SynradG4 reproduces Synrad+ to within about 1% for both specular and diffuse reflection, and that it agrees well with Synrad3D and with the native Geant4 X-ray reflection model for smooth surfaces. Using the full IR geometry and machine lattice, we present the first SR background estimates for the ePIC detector and evaluate the impact of potential SR masks.

arXiv comment13 pages, 9 figures, 3 tables. v4: revised reflection model (intensity Debye-Waller factor, local-frame diffuse sampling, current Synrad+ parameters), benchmark rerun, new Sec. 3.2 on the Geant4 roughness-factor convention