A Soft X-ray Beam-splitting Multilayer Optic for the NASA GEMS Bragg Reflection Polarimeter

TL;DR

This paper presents a novel soft X-ray beam-splitting multilayer optic designed for NASA's GEMS mission, enabling simultaneous reflection and transmission of X-rays for polarimetry with high precision.

Contribution

The development and characterization of a specialized multilayer optic with high reflectance and transmission, optimized for X-ray polarimetry in space applications, using a thin polyimide membrane and magnetron sputtering.

Findings

High spatial frequency roughness less than 0.2 nm rms.

Reflectance and transmission measurements match theoretical models.

Optic maintains performance after environmental testing.

Abstract

A soft X-ray, beam-splitting, multilayer optic has been developed for the Bragg Reflection Polarimeter (BRP) on the NASA Gravity and Extreme Magnetism Small Explorer Mission (GEMS). The optic is designed to reflect 0.5 keV X-rays through a 90 degree angle to the BRP detector, and transmit 2-10 keV X-rays to the primary polarimeter. The transmission requirement prevents the use of a thick substrate, so a 2 micron thick polyimide membrane was used. Atomic force microscopy has shown the membrane to possess high spatial frequency roughness less than 0.2 nm rms, permitting adequate X-ray reflectance. A multilayer thin film was especially developed and deposited via magnetron sputtering with reflectance and transmission properties that satisfy the BRP requirements and with near-zero stress. Reflectance and transmission measurements of BRP prototype elements closely match theoretical predictions, both before and after rigorous environmental testing.