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A Stacked Prism Lens Concept for Next-Generation Hard X-Ray Telescopes

Time: Fri 2019-09-27 10.00

Location: E3, Osquars backe 14, KTH Campus, Stockholm (English)

Subject area: Physics

Doctoral student: Wujun Mi , Medicinsk bildfysik, Medical Imaging

Opponent: Doctor Ralf K. Heilmann, Massachusetts Institute of Technology

Supervisor: Professor Mats Danielsson, Medicinsk bildfysik; Doctor Peter Nillius, Medicinsk bildfysik

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Over the past half century, the focusing X-ray telescope has played a very prominent role in X-ray astronomy at the frontier of fundamental physics. The finer angular resolution and increased effective area have enabled more and more exciting discoveries and detailed studies of the high-energy universe, including the cosmic X-ray background (CXB) radiation, black holes in active galactic nuclei (AGN), galaxy clusters, supernova remnants, and so on. At present, nearly all the state-of-the-art focusing X-ray telescopes are based on Wolter-I optics or its variations, for which the throughput is severely restricted by the mirror’s surface roughness, figure error, alignment error, and so on.

Within the course of this work, we have developed a novel point-focusing refractive lens, the stacked prism lens (SPL), which is built by stacking disks embedded with various number of prismatic rings. As a Fresnel-like X-ray lens, it could provide a significantly higher efficiency and larger effective aperture than the conventional compound refractive lenses (CRLs). The aim of this thesis is to demonstrate the feasibility of the stacked prism lens and investigate the application to a next-generation hard X-ray telescope.

First, SU-8 prototype lenses are fabricated by focused ultraviolet (UV) lithography, for which a UV lens is used as a photomask to form 3D patterns in the photoresist. The UV lens is homemade by grayscale electron beam lithography (EBL), and a proximity effect correction (PEC) method based on multivariate adaptive regression splines (MARS) ensures accurate control of the desired UV lens profile. The details of the whole fabrication process are described, and the fabrication results are discussed. Following that, the completed stacked prism lenses are characterized in the synchrotron radiation facility, and the results show the expected performance.

Finally, a hard X-ray focusing telescope concept based on the proposed stacked prism lens array is presented. The performance, in terms of angular resolution, effective collecting area, field of view (FOV), mass and so on, is investigated by self-developed simulation software based on ray-tracing method and compared with the current Wolter telescopes. The results suggest that the proposed stacked prism lens is a promising candidate for next-generation hard X-ray telescope with high angular resolution and large effective collecting area.