Continuing Development of Bragg Reflector Optics and Gratings for Polarimetry

We propose to continue laboratory work to test multilayer mirrors and reflection gratings, as well as to develop and test novel twisted crystal Bragg reflectors as a higher energy and much more efficient alternative to multilayer mirrors. With previous NASA/APRA funding, we demonstrated the performance of transmission gratings and laterally graded multilayer mirrors (LGMLs) with polarized and unpolarized X-rays up to 0.4 keV. One goal of this proposal is to advance the state of laterally graded multilayer coated mirrors to higher X-ray energies. We also aim to test a new concept for polarizing reflection by using Bragg reflection off of crystals, which will then be twisted to satisfy the Bragg condition at different energies across their surface. Finally, we plan to test off-plane, blazed reflection gratings for polarization sensitivity. The astrophysical prospects of X-ray polarimetry are currently limited by instrumentation. Above 2-3 keV, photoelectron tracking and Thomson scattering methods can be used, the basis of the Imaging X-ray Polarization Explorer and X-Calibur. Below 0.4 keV, the Rocket Experiment Demonstration of a Soft X-ray Polarimeter implements multilayer-based polarimetry, proposed elsewhere for development as a sounding rocket payload. Between 0.4 and 2 keV, no methods have demonstrated sufficient efficiency. Here, we propose to develop methods to close this gap in order to support designs for more advanced polarimetry instrumentation such as an X-ray Polarization Probe. We have constructed a source of polarized X-rays in the lab that operates at a wide range of energies with a selectable polarization angle for testing prototype components of our proposed instrument. In 2013, we demonstrated that the polarimetry beam-line can provide 100% polarized X-rays at 0.525 keV with a single period multilayer. In 2014, we upgraded the source by installing a mirror with a laterally graded multilayer (LGML) coating, providing a wide energy range. In 2015 and 2017, we tested new LGMLs with two more material combinations (C/CrCo, La/B4C, and Cr/Sc) in order to obtain higher efficiencies in different soft X-ray bands than our early LGML made of W and B4C. In 2018 we tested the polarization sensitivity of critical angle transmission (CAT) gratings across a wide energy range. The existing LGMLs are highly efficient up to 0.40 keV and we are still working with APRA funding to extend the capability to 0.46 keV. Improving beyond the O-K edge at 0.54 keV requires different materials and better layer interfaces that we propose to develop under this program. We also are planning to test and develop the concept of using twisted crystals as Bragg reflectors in this energy range. Another approach to bridge the gap is to use blazed reflection gratings in the off-plane configuration. While Marlowe et al. showed that unblazed gratings show no significant polarization sensitivity, we would test blazed gratings.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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