Developing a Gamma Ray Reflectometer for 511 keV Multilayer Performance Characterization

The soft-gamma-ray emission line at 511 keV is an important tracer of electron-positron annihilation in our Galaxy. Current 511 keV emission maps show an unusual concentration in the Galactic center (GC) region and along the disk, a spatial distribution that does not match that seen in other wavelengths. Due to the challenges of imaging the annihilation emission, the source of Galactic positrons remains an open question. 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. An imaging telescope with arcminute-level angular resolution will be necessary to study the detailed distribution and nature of positron point sources. One of the most promising technologies for such an imaging telescope is grazing-incidence optics coated with multilayers (MLs) to boost reflectivity in the soft gamma-ray regime (>100 keV). ML coatings for photon energies <80 keV are a flight-proven technology, and laboratory tests demonstrated reflectivity for soft gamma rays up to 650 keV. Developing ML mirror coatings for grazing-incidence focusing optics to enable a future 511 keV telescope is the subject of the RTF-associated APRA effort. Testing and characterizing the reflectivity performance of the fabricated MLs at the target photon energy is necessary to advance the TRL of this technology for a 511 keV telescope. Testing MLs at 511 keV requires high-energy synchrotron facilities outside the US, such as the European Synchrotron Radiation Facility (ESRF). Relying on awarded beam time at a state-of-the-art synchrotron facility introduces a bottleneck in the design/fabricate/test feedback loop and potential barrier to advancing the ML TRL from 2 to 4. We propose to design and construct a laboratory-based soft gamma ray reflectometer (GRR) for use in 511 keV reflectivity testing of ML coatings fabricated in the associated APRA. A 511 keV GRR would provide a readily available testing platform to keep pace with the iterative fabrication schedule. Furthermore, it would provide critical infrastructure as an adaptable testbed for future hard X-ray and soft gamma ray ML optics development. This effort would launch Dr. Swarm's technology development research group as a faculty member in the Department of Physics & Astronomy at University of Iowa (UI).

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