  Understanding nano- and micro-scale crystal strain in CVD diamond is crucial
to the advancement of diamond quantum technologies. In particular, the presence
of such strain and its characterization present a challenge to diamond-based
quantum sensing and information applications -- as well as for future dark
matter detectors where directional information of incoming particles is encoded
in crystal strain. Here, we exploit nanofocused scanning X-ray diffraction
microscopy to quantitatively measure crystal deformation from growth defects in
CVD diamond with high spatial and strain resolution. Combining information from
multiple Bragg angles allows stereoscopic three-dimensional reconstruction of
strained volumes; the diffraction results are validated via comparison to
optical measurements of the strain tensor based on spin-state-dependent
spectroscopy of ensembles of nitrogen vacancy (NV) centers in the diamond. Our
results open a path towards directional detection of dark matter via X-ray
measurement of crystal strain, and provide a new tool for diamond growth
analysis and improvement of defect-based sensing.
