Monochromatization with crystal diffraction has been achieved to a resoluti
on (lambda/Delta lambda) beyond 10(8). The monchromator is specifically des
igned for 23.880 keV synchrotron radiation (lambda = 51.9 pm) for applicati
ons involving nuclear resonant scattering from Sn-119. The design uses asym
metrically cut silicon (12 12 12) crystal reflections from two single-cryst
alline monoliths oriented in a dispersive geometry. A transmitted energy ba
ndwidth of 140 +/- 20 mu eV was measured, corresponding to a resolution of
1.7 +/- 10(8). Methods of improving efficiency, wavelength stability and re
solution are discussed.