X-Ray microcalorimeter detectors - Technology developments for high energy astrophysics space missions

Improvements in the design, fabrication, and performance of astronomical detectors has ushered in the so-called era of multi messenger astrophysics, in which several different signals (electromagnetic waves, gravitational waves, neutrinos, cosmic rays) are processed to obtain detailed descriptions of their sources. Soft x-ray instrumentation has been developed in the last decades and used on board numerous space missions. This has allowed a deep understanding of several physical phenomena taking place in astrophysical sources of different scales from normal stars to galaxy clusters and huge black holes. On the other hand, imaging and spectral capabilities in the the hard x-rays are still lagging behind with high potentials of discovery area. Modern cryogenic microcalorimeters have two orders of magnitude or more better energy resolution with respect to CCD detectors at the same energy in the whole X-ray band. This significant improvement will permit important progress in high energy astrophysics thanks to the data that will be provided by future missions adopting this detector technology such as the ESA L2 mission Athena, the JAXA/NASA mission XRISM, both under development, or the NASA LYNX mission presently under investigation. The JAXA/NASA mission Hitomi, launched in 2016 and failed before starting normal operation, has already given a hint of the high potential of such detectors. Due to their very high sensitivity, X-ray cryogenic microcalorimeters need to be shielded from out of band radiation by the use of efficient thin filters. These microcalorimeters work by measuring the temperature increase caused by a photon that hits an X-ray absorber. In neutron transmutation doped germanium (NTD Ge) devices the temperature increase in the absorber is measured by a semiconductor thermometer made of germanium doped by the neutron transmutation doping technique. They are characterized by relatively low specific heat and low sensitivity to external magnetic fields. These characteristics make them promising detectors for hard X-ray detectors for space and laboratory applications. Research groups of the the X-ray Astronomy Calibration and Testing (XACT) Laboratory of the Osservatorio Astronomico di Palermo – Istituto Nazionale di Astrofisica (INAF-OAPA), and of the Dipartimento di Fisica e Chimica “Emilio Segrè” (DiFC) of the Università di Palermo have already developed experience related to the design, fabrication and testing of NTD Ge microcalorimeters. Furthermore, the research group has participated for many years in the design and development of filters for x-ray detectors in different space missions. This thesis concerns the development of materials and technologies for high energy microcalorimeters. In particular its aim is to design and fabricate thick bismuth absorbers for NTD germanium microcalorimeter arrays to extend their detection band toward hard X-ray energies. Filters for shielding microcalorimeters from different background radiation arriving on the detectors were also studied. The design and fabrication of thick bismuth absorbers for hard x-rays detection (20 keV ≤ E ≤ 100 keV) is part of an ongoing effort to develop arrays of NTD Ge microcalorimeters by planar technologies for astrophysical applications. One potential application of such detectors is in the high spectral resolution (∆E ~ 50 eV) investigation of the hard X-ray emission from the solar corona, which is the goal of a stratospheric balloon borne experiment concept named MIcrocalorimeters STratospheric ExpeRiment for solar hard X rays (MISTERX) presently under study at INAF-OAPA. The characterization activity of filters for microcalorimeters in also related to the implementation of the European Space Agency high energy mission named Athena (Advanced Telescopes for High Energy Astrophysics). This thesis describes the design, fabrication, and characterization of the bismuth absorbers, as well as the characterization of filters for Athena. Chapter one summarizes the working principles of NTD Ge microcalorimeters and their applications. Chapter 2 describes the design of the bismuth absorber array on suitable substrates. Chapter 3 focuses on the electroplating process for the bismuth layer deposition, with details about the design and fabrication of the microlithographic mask for the array patterning, and about the development of the microlithographic process for the array fabrication on the chosen substrates. The fabrication of 4 x 4 absorber arrays is also described. Chapter 4 reports on the characterization activity of deposited bismuth layers by different techniques; their morphology was investigated by scanning electron microscopy. The electrochemical impedance spectroscopy technique was used to increase grown layer quality. Fabricated arrays were also characterized. Chapter 5 describes the characterization activity for different filter prototype samples developed for Athena. Mechanical robustness, radio frequency attenuation and radiation damage caused by protons were evaluated. Radiation damage effects at different doses were in particular investigated on silicon nitride filters by scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-Vis-IR spectroscopy and x-ray attenuation measurements. Details on both technical detector requirements and different sensor types are given in the Appendix

Deformation analysis of ATHENA test filters made of plastic thin films supported by a mesh under differential static pressure

Within ESA Cosmic Vision 2015-2025 Science Program, ATHENA was selected to be a Large-class high energy astrophysics space mission. The observatory will be equipped with two interchangeable focal plane detectors named X-Ray Integral Field Unit (X-IFU) and Wide Field Imager (WFI). In order to optimally exploit the detector sensitivity, X-ray transparent filters are required. Such filters need to be extremely thin to maximize the X-ray transparency, that is, no more than a few tens of nm, still they must be able to sustain the severe stresses experienced during launch. Partially representative test filters were made with a thin polypropylene film, coated with Ti, and supported by a thin highly transparent mesh either in stainless steel or niobium. Differential static pressure experiments were carried out on two filter samples. In addition, the roles of the mesh on the mechanical deformation is studied, adopting a finite element model (FEM). The numerical analysis is compared with experimental results and found in good agreement. The FEM is a promising tool that allows to characterize materials and thicknesses in order to optimize the design

A Temperature-Dependent X-Ray Absorption Characterization of Test Filters for the ATHENA Mission X-IFU Instrument

In order to work properly, the X-ray Integral Field Unit of the ATHENA mission requires a set of thermal filters that block the infrared radiation, preventing it to reach the detector. Each filter will be mounted and thermally anchored onto a shield of the multistage cryostat and will be kept at the specific temperature of the stage. On the other hand, the filters partially absorb X-rays, and their transmittance has to be carefully characterized. The effect of temperature on the absorption edges of the elements that make up the filters has not been investigated yet. Here, we report the results of a preliminary run on the optical transmission data around the edges of C, N, and O at different temperatures for a selected test sample with 500 nm of polyimide and 100 nm of aluminum

Thermal modelling of the ATHENA X-IFU filters

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

Electroplated bismuth absorbers for planar NTD-Ge sensor arrays applied to hard x-ray detection in astrophysics

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

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

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