Beryllium windows are used on many X-ray synchrotron beamlines to separate and protect the ultra-high vacuum of the
storage ring from the experimental environment. Currently, such a window is typically made of a thin, high-purity,
beryllium foil, which may or may not have been polished. It is well known that these windows affect the transmitted
beam quality. The impact ranges from non-perceptible to profound, depending on the experiment.
The degradation of the X-ray beam is of increasing importance and concern, however, and in fact a number of beamlines
now are run windowless or with a very small and thin silicon nitride window. There remain many instances where a
large and robust window is desirable or necessary, and it is for this reason that developing windows that have little or no
impact on the transmitted X-ray beam quality is important.
This presentation reports on the progress in developing single-crystal beryllium X-ray windows. Due to its high purity
and homogeneity, relative structural perfection, and high polishiblity single-crystal beryllium is an attractive window
material candidate, particularly for beamlines conducting imaging or coherence-based experiments. Development of
thin and uniform windows with less than 1 nm rms surface roughness and their preliminary characterization results are