A soft X-ray, beam-splitting, multilayer optic has been developed for theBragg Reflection Polarimeter (BRP) on the NASA Gravity and Extreme MagnetismSmall Explorer Mission (GEMS). The optic is designed to reflect 0.5 keV X-raysthrough a 90 degree angle to the BRP detector, and transmit 2-10 keV X-rays tothe primary polarimeter. The transmission requirement prevents the use of athick substrate, so a 2 micron thick polyimide membrane was used. Atomic forcemicroscopy has shown the membrane to possess high spatial frequency roughnessless than 0.2 nm rms, permitting adequate X-ray reflectance. A multilayer thinfilm was especially developed and deposited via magnetron sputtering withreflectance and transmission properties that satisfy the BRP requirements andwith near-zero stress. Reflectance and transmission measurements of BRPprototype elements closely match theoretical predictions, both before and afterrigorous environmental testing.
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