Gamma-ray spectra and absorbed doses measured at EuXFEL undulator system.

Characterization of the stray radiation field present in the undulator systems at the European XFEL GmbH (EuXFEL) is of great importance, as the potential damage to undulator permanent magnets, electronics and diagnostic equipment depends both on the type of particles and the energy. This work presents the energy profile of the stray radiation measured in the upstream and downstream part of the undulator system near the beam pipe. The influence of machine operation settings on the radiation field intensity was also investigated. A comparison between gamma-ray energy measurements (30 keV to 1.5 MeV) and Geant4 Monte Carlo simulations shows that stray radiation in the upstream part of the undulator system originates from high-energy electrons intersecting the beam pipe wall. The intensity of the measured signal is approximately linear to the charge passing through the undulator system and to the electron energy but is inversely proportional to the square root of the undulator gap. Measurements in the upstream part of the undulator system show that the stray radiation level can be mitigated with careful choice of accelerator settings. The intensity of the radiation with energies below approximately 400 keV is higher in the downstream part of the undulator system compared with the upstream part, during regular operation. A CZT spectrometer and RADFET measurements confirm that the radiation field in the downstream part of the undulator system is dominated by low-energy synchrotron radiation (below 200 keV), which constitutes approximately 99% of the stray radiation field.