What did the Scientists Discover?
Even though the concept of x-ray transmission mirrors (XTM) for high-pass x-ray filtering was initially described and demonstrated over 30 years ago, very few working devices have been achieved and none are in routine use. XTM are the complement to reflecting X-ray optics: They are long, thin reflecting membranes that function as efficient high-pass x-ray filters with superior selectivity compared to common absorbers such as metal foils. The long development cycle in realizing practical XTM is largely due to the difficulty in combining a sub-micron radiation-hard membrane with a robust support that is itself thin enough to not block the transmitted beam. A simple, high-yield solution for fabrication of x-ray transmission mirrors was developed here at CHESS. The solution incorporates an inorganic Si3N4 membrane with a specially-fabricated monolithic substrate support made from 150 mm silicon wafers. The critical, enabling step for this advance is a process, conceived of by CHESS post-doc David Agymen-Budu, to selectively control the etch depth on different portions of the support substrate. Partial etching the downstream end of the support (see figure) creates an unobscured exit path for the transmitted beam.
Why is this important?
These characteristics makes XTM optics attractive as potential X-ray optics for high-pass harmonic selectors and tunable wide bandpass monochromators. With further instrumentation development, they can be potentially applied to high heat load filtering applications at synchrotrons.
What are the broader impacts of this work?
The XTM optic leverages the high efficiency of total external reflection of the top surface of the suspended membrane to remove the low energy x-ray photons at a critical energy. This critical energy cutoff is tunable by a simple adjustment of the angle at which the beam intercepts the optic. As such, high-pass filtering is tunable, which is not possible with absorption filters. The range of critical energies can be further extended by incorporating a metal coating on top of the membrane. In summary these novel optics can add to the repertoire of optical elements that can implemented at a beamline.
Why did this research need CHESS?
The nature of this research and developmental effort required an organization like CHESS where novel initiatives are promoted. Most of the fabrication work also required the use of Cornell NanoScale Science & Technology Facility (CNF), which is also on Cornell’s campus and close by, making the iterative process of design refinement, fabrication and testing faster.
- David N. Agyeman-Budu , Cornell High Energy Synchrotron Source, Cornell University
- Arthur R. Woll, Cornell High Energy Synchrotron Source, Cornell University
How was the work funded?
This work was performed in part at Cornell NanoScale Facility, an NNCI member supported by NSF Grant NNCI-1542081 and CHESS is supported by the NSF under Award DMR-1332208.