Designing and Fabricating Multilayer Optics to Enable a Future 511 keV Focusing Imager

The soft-gamma ray emission line at 511 keV is an important tracer of electron-positron annihilation in our galaxy. The 511 keV emission mapped by INTEGRAL/SPI shows an unusual concentration of emission in the the Galactic center region and and along the disk, a distribution that does not match the emission distributions seen in other wavelengths. Due to the challenges of imaging the annihilation emission, the source of Galactic positrons remains an open question. While the Compton Spectrometer and Imager (COSI) SMEX mission will provide an all-sky 511 keV survey with angular resolution around 3 degrees, an imaging telescope with much better angular resolution will be necessary to study the detailed distribution and nature of positron point sources. Identifying and spatially resolving an individual source of positrons shining at 511 keV would be a groundbreaking result that would significantly increase our understanding for this decades-long puzzle. One of the most promising technologies for such an imaging telescope will be grazing-incidence optics with sufficient effective area in the soft gamma-ray regime. We propose to develop multilayer mirror coatings for grazing-incidence optics that will maximize the effective area for collection of photons from 450-520 keV. Multilayer mirror coatings utilize Bragg diffraction within periodic material layers to achieve high reflectivity at a preferred graze angle. Multilayer coatings enable NuSTAR's wide energy bandpass for hard X-ray imaging, and regular-period multilayer coatings have demonstrated reflectivity for soft gamma-rays up to 650 keV. Further reflectivity enhancements can be achieved through techniques like depth-graded multilayers or aperiodic multilayers. In the first year of our proposed work we will conduct an optimization study for the optical design and multilayer coating formulation as supporting technology for a future 511 keV focusing imager. In the second and third year of the proposed effort we will apply multilayer coatings to representative substrates and optical elements and measure the reflectivity performance of the fabricated coatings. Our team consists of early-career researchers and experts in gamma-ray astrophysics, optics design, grazing-incidence mirror fabrication, and multilayer coating design and deposition. Development of multilayer mirror coatings optimized for our proposed energy bandpass is an enabling technology for future construction of a focusing telescope tailored for imaging the emission from positron annihilation. Furthermore, soft-gamma-ray imaging telescopes addresses Astro2020 objectives such as exploring the makeup and launch mechanism of jets or observing the by-products of dark matter annihilation.

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