Investigation of the stray radiation origin and composition at the European XFEL undulators with gafchromic films measurements and Geant4 simulations

Abstract

At EuXFEL, a stray field of ionizing radiation is present near undulator magnets which eventually can lead to their demagnetization and decreased efficiency of the Self-Amplified Spontaneous Emission (SASE) process. In this work we investigate both the origin and composition of the radiation field at the undulator magnets with gafchromic film measurements and Geant4 Monte Carlo simulations. Both measurements and simulations suggest that the radiation field in the upstream undulator cells comes from high-energy electrons striking the beam pipe. In addition, horizontal dose distributions at the entrance of the undulator segments depend on where the electrons hit the beam pipe. Geant4 simulations show that the radiation fluence near magnets is dominated by photons, while charged secondary electrons and positrons contribute almost entirely to the ionizing dose absorbed by magnets.