  A channel-cut Si(111) crystal with a channel width of 90 $\mu$m was developed
for achieving reflection self-seeding in hard X-ray free-electron lasers
(XFELs). With the crystal, a monochromatic seed pulse is produced from a
broadband XFEL pulse generated in the first-half undulators with an optical
delay of 119 fs at 10 keV. The small optical delay allows a temporal overlap
between the seed optical pulse and the electron bunch by using a small magnetic
chicane for the electron beam at the middle of the undulator section. A peak
reflectivity reached 67%, which is a reasonable value as compared with the
theoretical one of 81%. By using this monochromator, a monochromatic seed pulse
without broadband background in spectrum was obtained at SACLA with a
conversion efficiency from a broadband XFEL pulse of $\sim 2 \times 10^{-2}$,
which is $\sim 10$ times higher than that of transmission self-seeding using a
thin diamond (400) monochromator.
