An X-ray beam of 3 x 10(7) photons s(-1) with 2 x 10(-8) relative ener
gy resolution has been obtained at a third-generation synchrotron undu
lator X-ray source using the (13 13 13) Bragg reflection from a silico
n perfect crystal. The production of these 25.70 keV X-rays with 450 /- 50 mu eV bandpass opens up new possibilities in X-ray optics and sp
ectroscopies.