  Young double-slit experiments using monochromatic hard X-rays with the energy
of 25 keV are presented. The experiments were performed at a synchrotron source
with a distance of 110 m between the interferometer and the detector to produce
an interference pattern with a sufficiently broad period that could be
adequately sampled by a photon-counting detector with 75 micrometre pixels. In
the single-particle version of the experiment, over one million image frames
with a single registered photon in each one were collected. The sum of these
frames showed a clear presence of the interference pattern with the expected
period. Subsequent analysis provided an objective estimation of the minimal
number of detected photons required to determine, in accordance with the Rose
criterion, the presence of the photon interference. Apart from a general
theoretical interest, these investigations were aimed at exploring the
possibility of medical X-ray phase-contrast imaging in photon-counting mode at
minimal radiation doses.
