Double multilayer monochromator for harmonic rejection in the 5-to60-keV range

A W/Si double multilayer monochromator has been installed on the ESRF BM29 (multipurpose XAFS) beamline with the specific task of suppressing higher order harmonics from the synchrotron x-rays of a bending magnet. This novel technique uses an identical pari of multilayers which were deposited by a method based on distributed electron cyclotron resonance plasma sputtering. In situ growth monitoring enabled the clear identification of a WSi chemical interface with an approximate width of 1nm: the relative size of the interface was found to severely limit the ability of the multilayer to completely reject specific harmonics. In total, the monochromator enabled suppression of all higher order harmonics to less than six orders of magnitude with an approximately 30 percent throughput in the first Bragg peak (at 8 keV). The use of such a multilayer monochromator for harmonic rejection in tandem with a crystal monochromator replaces the traditional method of deploying large grazing incidence mirrors; particular advantages for the multilayers are the significant reduction in size and cost.