Finding light in chaos Characterization of the stray radiation field at the European XFEL undulator systems

by Olga Ewa Falowska-Pietrzak (Stockholm University)

Wednesday 7 May 2025, 10:00 → 13:00 Europe/Stockholm

FB42 (AlbaNova Main Building)

Abstract
The European X-Ray Free Electron Laser (EuXFEL) is a free-electron laser that generates coherent X-ray pulses with
energies between 0.26 and 25 keV. Radiation is generated as electrons with energies up to 17.5 GeV are subjected to the
periodic magnetic field while traveling through long arrangements of permanent magnets called undulators. Measurements
show that stray radiation is present outside the beam pipe in the EuXFEL undulator systems. It can potentially damage
permanent magnets and diagnostic and correction equipment located along the beam pipe. This thesis gives an overview
of the stray radiation at EuXFEL undulator systems. Besides Radfet dosimeters and gamma and neutron monitors installed
near the undulators, additional detectors were used within the scope of this work. Gafchromic films placed in different
undulator cells give information on the spatial distribution of the absorbed dose near permanent magnets and other
components. The photon energy distribution near the beam pipe was measured with two gamma-ray CZT spectrometers.
Measurements are supported by Geant4 Monte Carlo simulations of electron interactions with the beam pipe wall.
Measurements and simulations show that radiation is distributed non-uniformly along individual undulator segments
and the whole undulator system. It means that some of the permanent magnets are exposed to higher doses. It can lead to
non-uniform magnetic field degradation and issues with phase matching between electron and photon fields. The presented
results indicate that the origin of stray radiation differs in the upstream and downstream parts of the undulator system. In the
upstream undulator part, it is created as a result of electron interactions with the beam pipe. In the downstream undulator
part, it is dominated by low-energy synchrotron radiation. Geant4 simulations show that neutrons with energies exceeding
1 MeV are created as a result of high-energy electrons interacting with the beam pipe. They can cause permanent damage
to the undulator permanent magnets. Radfet and spectrometer measurements show that the intensity of the stray radiation
depends on the charge passing through the undulators, the electron energy and the undulator gap. Careful choice of these
parameters can reduce the stray radiation level in the undulator systems.