X-ray detectors for space astrophysics missions are susceptible to noise caused by photons with energies outside the operating energy range; for this reason, efficient external optical blocking filters are required to shield the detector from the out-of-band radiation. These filters play a crucial role in meeting the scientific requirements of the X-ray detectors, and their proper operation over the life of the mission is essential for the success of the experimental activity. We studied thin sandwich membranes made of silicon nitride and aluminum as optical blocking filters for high-energy detectors in space missions. Here, we report the results of a multi-technique characterization of SiN membranes with thicknesses in the range from 40 nm to 145 nm coated with few tens of nanometers of aluminum on both sides. In particular, we have measured the X-ray transmission at synchrotron radiation beamlines, the rejection of ultraviolet, visible, and near-infrared radiation, the amount of native oxide on the aluminum surfaces by X-ray photoelectron spectroscopy, the morphology of the sample surfaces by atomic force microscopy, and the aging effects under proton irradiation.
Single sensors or small arrays of manually assembled neutron transmutation doped germanium (NTD-Ge) based microcalorimeters have been widely used as high energy-resolution detectors from infrared to hard X-rays. Several planar technological processes were developed in the last years aimed at the fabrication of NTD-Ge arrays, specifically designed to produce soft X-ray detectors. One of these processes consists in the fabrication of the absorbers. In order to absorb efficiently hard X-ray photons, the absorber has to be properly designed and a suitable material has to be employed. Bismuth offers interesting properties in terms of absorbing capability, of low heat capacity (needed to obtain high energy resolution) and deposition technical feasibility, moreover, it has already been used as absorber for other types of microcalorimeters. Here we present the electroplating process we adopted to grow bismuth absorbers for fabricating planar microcalorimeter arrays for hard X-rays detection. The process was specifically tuned to grow uniform Bi films with thickness up to ~ 70 μm. This work is part of a feasibility study for a stratospheric balloon borne experiment that would observe hard X-rays (20-100 keV) from solar corona.
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.
The X-IFU instrument of the ATHENA mission requires a set of thermal filters to reduce the photon shot noise onto its cryogenic detector and to protect it from molecular contamination. A set of five filters, operating at different nominal temperatures corresponding to the cryostat shield temperatures, is currently baselined. The knowledge of the actual filter temperature profiles is crucial to have a good estimation of the radiative load on the detector. Furthermore, a few filters may need to be warmed-up to remove contaminants and it is necessary to ensure that a threshold temperature is reached throughout the filters surface. For these reasons, it is fundamental to develop a thermal modeling of the full set of filters in a representative configuration. The baseline filter is a polyimide membrane 45 nm thick coated with 30 nm of highpurity aluminum, mechanically supported by a metallic honeycomb mesh. In this paper, we describe the implemented thermal modeling and report the results obtained in different studies: (i) a trade-off analysis on how to reach a minimum target temperature throughout the outer filter, (ii) a thermal analysis when varying the emissivity of the filter surfaces, and (iii) the effect of removing one of the filters.
ATHENA is a Large high energy astrophysics space mission selected by ESA in the Cosmic Vision 2015-2025 Science Program. It will be equipped with two interchangeable focal plane detectors: the X-Ray Integral Field Unit (X-IFU) and the Wide Field Imager (WFI). Both detectors require x-ray transparent filters to fully exploit their sensitivity. In order to maximize the X-ray transparency, filters must be very thin, from a few tens to few hundreds of nm, on the other hand, they must be strong enough to survive the severe launch stresses. In particular, the WFI OBF, being launched in atmospheric pressure, shall also survive acoustic loads. In this paper, we present a review of the structural modeling performed to assist the ATHENA filters design, the preliminary results from vibration and acoustic tests, and we discuss future activities necessary to consolidate the filters design, before the preliminary requirement review of the ATHENA instruments, scheduled before the end of 2018.
The X-ray Integral Field Unit (X-IFU) is one of the two detectors 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 sensors (TES) micro-calorimeters covering a field of view of ~5’ diameter, sensitive in the energy range 0.2-12 keV, and providing a spectral resolution of 2.5 eV at 7 keV. Both the TES and superconducting quantum interference devices (SQUID) based read-out electronics are very sensitive to electromagnetic interferences (EMI), and a proper shielding of the focal plane assembly (FPA) is required to prevent a deterioration of the energy resolution. A set of thin filters, highly transparent to X-rays, will be mounted on the FPA and on the cryostat thermal shields in order to attenuate the infrared radiative load, and to protect the detector from contamination. Some of these filters are also aimed at providing proper radio frequency (RF) shielding in the frequency range of the satellite telemetry downlink antenna. In addition, filters should also be effective in shielding any RF interference generated by other on-board electronics. In this paper, we present results from RF measurements performed on thin plastic foils coated with an aluminum layer, with and without metal meshes, and identify the filter characteristics matching the RF shielding requirements.
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