ATHENA X-IFU thermal filters development status toward the end of the instrument phase-A

Abstract

The X-ray Integral Field Unit (X-IFU) is one of the two instruments of the Athena astrophysics space mission approved by ESA in the Cosmic Vision 2015-2025 Science Programme. The X-IFU consists of a large array of transition edge sensor micro-calorimeters that will operate at ~100 mK inside a sophisticated cryostat. A set of thin filters, highly transparent to X-rays, will be mounted on the opening windows of the cryostat thermal shields in order to attenuate the IR radiative load, to attenuate radio frequency electromagnetic interferences, and to protect the detector from contamination. Thermal filters are critical items in the proper operation of the X-IFU detector in space. They need to be strong enough to survive the launch stresses but very thin to be highly transparent to X-rays. They essentially define the detector quantum efficiency at low energies and are fundamental to make the photon shot noise a negligible contribution to the energy resolution budget. In this paper, we review the main results of modeling and characterization tests of the thermal filters performed during the phase A study to identify the suitable materials, optimize the design, and demonstrate that the chosen technology can reach the proper readiness before mission adoption.

A Review of X-ray Microcalorimeters Based on Superconducting Transition Edge Sensors for Astrophysics and Particle Physics

“…The focal plane assembly should also be designed to minimize the infra-red and optical loading on the TES. This should be achieved by thermal filters in the aperture [163] and Au shielding in the proximity of the array. On top of that, the energy resolution of the detectors can be affected by thermal bath fluctuations from the energy deposited on the substrate by cosmic rays from astrophysical sources [164,165] and by micro-vibrations induced by the mechanical coolers [162,166].…”

“…One way to directly probe the neutrino mass scale in the laboratory is by performing calorimetric measurement of the energy released in electron capture decay of holmium( 163 Ho). The HOLMES and ECHO projects [8,176,177] employ, respectively, arrays of low temperature TES X-ray calorimeters and metallic magnetic calorimeters to detect the spectrum of 163 Ho between 0 and 3 keV, with an energy resolution better than 3 eV. What makes these experiments unique is the fact that the radioactive source is embedded into the absorber.…”

Design of optical/IR blocking filters for the Lynx X-ray Microcalorimeter

“…4 and 5. In this calculation, we assume the Athena support meshes are thick metal meshes with 2% blocking factor 13 and Lynx support meshes are identical to the mesh of the Astro-H SXS dewar main shell filter, with 97.5% open-area coarse Si mesh (208-μm Si) plus 97.5% open-area fine mesh (8-μm Si). We assume that the waveguide-cutoff filters for Lynx have the same geometry as the Astro-H meshes, including a thick coarse mesh and a thinner fine mesh but are made of Au.…”

“…The smallest LXM filter is similar to the size of the Astro-H SXS mesh filters, and the larger LXM filters are in-family with the size of the Athena X-IFU filters, which are currently under development. 12,13 While we have baselined Si meshes similar to For LXM two of the six filters are assumed to be waveguide-cutoff filters: they are counted as a filter with a mesh but they do not have thin films, and thus do not contribute to the tabulated Al or polyimide thicknesses. For Lynx, we include a curve showing the baseline configuration (4 films/6 meshes) as well as a more conservative option where seven filters are in use (5 films/7 meshes).…”

“…While the basic design will be similar to those developed for previous microcalorimeter missions, the X-IFU filters team is exploring several options, including meshes of different material than silicon and replacing the thin film to use SiN instead of polyimide. 12,13 One interesting option currently under consideration for the X-IFU is to coat the microcalorimeter pixels with a thin layer of Au to increase the reflectivity in the optical and IR. The pixels under development for the X-IFU use Au/Bi absorbers, where the material facing the telescope is Bi, which has lower reflectivity in the optical and IR compared to the Au/Bi/Au stack under consideration.…”