Simulation and Optimization of a Soft Gamma-Ray Concentrator Using Thin-Film Multilayer Structures
Abstract
Gamma-ray astrophysics in the energy range of ∼ 80 keV to several hundred keV has an extraordinary potential for understanding physical processes in the Universe. Currently, astronomical observations in this energy band are only possible with coded-aperture telescopes (such as INTEGRAL) or background-limited collimated instruments (like the Soft Gamma-ray Detector (SGD) on ASTRO-H/Hitomi). In such telescopes, the ratio of the collecting to detector area is < 1 and they must be large and heavy in order to achieve sufficient collecting area. In spite of the important science accomplished by these instruments, a large sensitivity improvement is needed to take full advantage of the scientific potential in the soft gamma-ray energy band. Using focusing methods through multilayer coatings in Laue lens or X-ray mirrors would be capable of concentrating the signal from a large collector onto a small spot on the detector and decreasing the instrumental background. This has now been achieved at energies below 80 keV by the NuSTAR mission. However, applying similar technique on energies above 100 keV needs long focal lengths (20-100 m) which in turn require either new extendable mast technology or double-spacecraft missions. In this study, we have investigated the use of multilayer thin-film structures for channeling and concentrating soft gamma rays with energies greater than 100 keV , beyond the reach of current grazing-incidence hard X-ray mirrors. A suitable arrangement of bent multilayer structures of alternating low- and high-density materials will channel soft gamma-ray photons via total external reflection and then concentrate the incident radiation to a point. Building on initial investigations at Los Alamos National Laboratory (LANL), we fabricated a 150 bilayers AuPd/PMMA structure using Magnetron Sputtering (MS) and spin-coating techniques. Although the desired smoothness and layer thicknesses were achieved, the production of this multilayer was very time-consuming. The Pulsed Laser Deposition (PLD) technique was also studied as an alternative method to grow PMMA, Bis-DMA and C thin films. In the next phase, we investigated the production of Ir/Si and W/Si multilayer structures with the required thicknesses, smoothness and uniformity, coated entirely using MS system which has a reproducible deposition control and good film qualify over long deposition times. We present the results of appropriate MS parameters for minimizing roughness/stress and increasing deposition rate. We also developed a flexible set of computer modeling tools to compute the optical properties of multilayer structures, predict the channeling efficiency for a given multilayer configuration and aid in the optimization of potential gamma-ray concentrator-based telescope designs. This modeling includes multilayer optical properties calculated by the IMD software, IDL gamma-ray tracing code and a focal plane detector simulation by Medium-Energy Gamma-ray Astronomy library (MEGAlib). The final result after the complete data analysis chain including the generation of individual photon interactions and event selection contains the simulated effective area, instrument sensitivity, and polarization performance. This technology offers the potential for soft gamma-ray telescopes with focal lengths of less than 10 m, removing the need of formation flying spacecraft and providing greatly increased sensitivity for modest cost and complexity and opening the field up to balloon-borne instruments.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2018
- Bibcode:
- 2018PhDT.......215S
- Keywords:
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- Physics;Astrophysics;High energy physics