A method and apparatus for producing X-rays having any selected one of a plurality of specific different wavelength spectra greatly facilitates X-ray fluorescence analysis of samples to detect constituent elements. In the X-ray source, an electron beam is directed to any selectable one of an array of...http://www.google.es/patents/US3983397?utm_source=gb-gplus-sharePatente US3983397 - Selectable wavelength X-ray source
