An innovative optical element for X-ray beam aperture, monitoring, and diagnostic at the CARNAÚBA beamline at SIRIUS/LNLS

Abstract

The CARNAÚBA beamline is the tender-to-hard X-ray nanoprobe under commissioning for the new source Sirius at the Brazilian Synchrotron Light Laboratory (LNLS). The all-achromatic optics relies on horizontal deflection mirrors, a horizontal secondary source aperture (SSA), and Kirkpatrick–Baez (KB) mirrors to reach beam size down to ~30x30 nm2 at the sample position. To handle the highly focused power density of the pink beam and reach the desired stability for the horizontal secondary source, essential for the desired focusing and coherent properties at the sample position, we present here an SSA device based on a cryogenically cooled Si crystal in a channel-cut geometry. The crystal slabs define the channel aperture, which is controlled by a fine rotation. Simultaneously to the aperture function, the device plays the key role of beam position monitor and beam diagnostic. The scattered signal from the slabs is used to monitor the horizontal beam position during experiments and serves as a feedback input for the pitch control system of the upstream focusing mirror to stabilize the beam intensity and wavefront. The Bragg magnifier geometry of the device spreads the beam over the slabs, allowing the diffracted image of the beam to be collected by an area detector. The beam spread on the slabs also reduces the power density reaching the device, keeping it stable and under controlled temperatures.