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.
The LAD (large area detector) instrument, onboard the Sino-European mission eXTP (enhanced x-ray timing and polarimetry), will perform single-photon, high-resolution timing and energy measurements, in the energy range 2 to 30 keV, with a large collecting area. Its silicon drift detectors need shielding from NIR/Vis/UV light by astrophysical sources and the bright Earth, to avoid performance degradation. Filters made of an Al coated thin polyimide (PI) membrane will guarantee the needed out-of-band rejection while offering high x-ray transparency. They will be placed between the detectors and the capillary plate plate collimators, open to the external environment. The mission is now in phase B2 and a baseline design for the filters was produced. We describe the filter design and modeling activity, and report the characterization performed so far on x-ray transmission, pinhole and defects, thermo-vacuum cycling endurance, and bright Earth optical load shielding properties.
In this paper, we present the first results from an investigation performed on nanometric thin pellicles based on carbon nanotubes (CNT) of potential interest for manufacturing large area optical blocking filters to protect soft x-ray detectors in astrophysics space missions. In order to evaluate the effective capability of such materials to block UV/VIS/IR radiation, while being highly transparent in the soft x-rays and strong enough to withstand the severe launch stresses, we have performed a suite of characterization measurements. These include: UV/VIS/IR and x-ray absorption spectroscopy, x-ray photoelectron spectroscopy and scanning electron microscopy on bare and Al coated small self-standing pellicles; static mechanical tests on small freestanding samples.
Advanced Telescope for High-Energy Astrophysics is a large-class astrophysics space mission selected by the European Space Agency to study the theme “Hot and Energetic Universe.” The mission essentially consists of a large effective area x-ray telescope and two detectors: the X-ray Integral Field Unit (X-IFU) and the Wide Field Imager (WFI). Both instruments require filters to shield from out-of-band radiation while providing high transparency to x-rays. The mission is presently in phase B; thus, to consolidate the preliminary design, investigated filter materials need to be properly characterized by experimental test campaigns. We report results from high-resolution x-ray transmission measurements performed using different synchrotron radiation beamlines to assess the filter calibration accuracy and mitigate the risk related to selecting a unique calibration facility. The main goals of these test campaigns are (i) to verify the compliance of the investigated filter design to the scientific requirements, (ii) to develop an accurate x-ray transmission model, and (iii) to start identifying suitable measurement facilities and achievable accuracy for the flight filters calibration program. In particular, the x-ray transmission model of the X-IFU and WFI filters has been refined within the edges of Al, C, N, and O by deriving the optical constants from two reference samples measured by synchrotron light. The achievable filter calibration accuracy has been estimated by evaluating the agreement between the best-fit according to the developed transmission model and the experimental data.
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-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.
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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