To promote the incident fluxes, we recently added a high-throughput double multilayer monochromator (DMM) to the beamline that allows dual-mode operation together with the double crystal monochromator (DCM). The two monochromators share a same Si 111 single crystal substrate; a unique geometric shape has been designed to accommodate the largely different incident angles and the beam footprint dynamics for the two monochromators.

The bilayer of the DMM is made of Ni-B4C with the unit spacing d = 29.76 Å and the asymmetric ratio 1/3. About 300 layers are deposited on a super-polished flat Si 111 single crystal substrate using a modular atomic-layer deposition system at the APS. The corresponding bandwidth is 1.3%. The bandwidth of DMM and the intrinsic multilayer reflectivity (R2=0.7-0.9 as the function of energy) delivers 70-90 times DCM beam flux (Table. 1).

The DMM gain in the flux improves the measurement efficiency by nearly two orders of magnitude compared to that of DCM but at the cost of energy resolution. As an example, a direct comparison between powder diffraction patterns of CeO2 powders measured with DCM and DMM realistically shows the gain and the cost (Fig. 2). The DMM beam has been successfully utilized for multiple user experiments including strength measurement using a radial diffraction, DAC-based high-pressure tomographic imaging, and a fast-2D scanning diffraction microscopy.   

Table 1. Gain by DMM bandwidth relative to DCM.