Abstract: Described herein are devices for converting a broadband x-ray beam to at least one substantially monochromatic x-ray beam. The devices may be an adaptor for existing x-ray machines or for use with a standalone machine. The device 20 includes a shielded housing 22 having an inner cavity and a fluorescent target 26, 26? disposed in the inner cavity 24 wherein the fluorescent target 26, 26? emits at least one substantially monochromatic x-ray beam when exposed to a broadband x-ray beam. The housing 22 includes a first opening 30 in the housing 22 configured to allow the broadband x-ray beam from an x-ray source to enter the inner cavity 24 and irradiate the fluorescent target 26, 26? and a second opening 34 in the housing configured to allow the at least one substantially monochromatic x-ray beam emitted by the fluorescent target to exit the housing 22. Also described herein are sources of monochromatic x-rays 60, 84, 112, as well as diagnostic and therapeutic methods of using of monochromatic x-ray beams.