Testing multilayer-coated polarizing mirrors for the LAMP soft X-ray telescope

TL;DR

This study evaluates multilayer-coated mirrors for the LAMP X-ray telescope, focusing on their reflectivity and scattering at large angles, to improve polarization measurements in the soft X-ray range.

Contribution

It investigates the reflectivity and scattering properties of multilayer coatings at 45 degrees, using various substrates and compositions, for potential use in X-ray telescope mirrors.

Findings

Multilayer coatings show promising reflectivity at 45 degrees.

Different substrate materials affect coating performance.

X-ray reflectivity tested successfully at the target energy range.

Abstract

The LAMP (Lightweight Asymmetry and Magnetism Probe) X-ray telescope is a mission concept to measure the polarization of X-ray astronomical sources at 250 eV via imaging mirrors that reflect at incidence angles near the polarization angle, i.e., 45 deg. Hence, it will require the adoption of multilayer coatings with a few nanometers d-spacing in order to enhance the reflectivity. The nickel electroforming technology has already been successfully used to fabricate the high angular resolution imaging mirrors of the X-ray telescopes SAX, XMM-Newton, and Swift/XRT. We are investigating this consolidated technology as a possible technique to manufacture focusing mirrors for LAMP. Although the very good reflectivity performances of this kind of mirrors were already demonstrated in grazing incidence, the reflectivity and the scattering properties have not been tested directly at the unusually large angle of 45 deg. Other possible substrates are represented by thin glass foils or silicon wafers. In this paper we present the results of the X-ray reflectivity campaign performed at the BEAR beamline of Elettra - Sincrotrone Trieste on multilayer coatings of various composition (Cr/C, Co/C), deposited with different sputtering parameters on nickel, silicon, and glass substrates, using polarized X-rays in the spectral range 240 - 290 eV.