  X-ray free electron lasers (X-FELs) present new opportunities to study
ultrafast lattice dynamics in complex materials. While the unprecedented source
brilliance enables high fidelity measurement of structural dynamics, it also
raises experimental challenges related to the understanding and control of
beam-induced irreversible structural changes in samples that can ultimately
impact the interpretation of experimental results. This is also important for
designing reliable high performance X-ray optical components. In this work, we
investigate X-FEL beam-induced lattice alterations by measuring the
shot-to-shot evolution of near-Bragg coherent scattering from a single
crystalline germanium sample. We show that X-ray photon correlation analysis of
sequential speckle patterns measurements can be used to monitor the nature and
extent of lattice rearrangements. Abrupt, irreversible changes are observed
following intermittent high-fluence monochromatic X-ray pulses, thus revealing
the existence of a threshold response to X-FEL pulse intensity.
