Brian Shortt

Silicon Drift Detectors for Readout of Scintillators in Gamma-Ray Spectroscopy

In this work, we report on a new development of Silicon Drift Detectors (SDDs) for gamma-ray spectroscopy with space science applications. The research is supported by the European Space Agency through the Technology Research Programme (TRP). The final goal of the development is the realization of monolithic arrays of SDDs which will be assembled to readout large (2” and 3”) LaBr3(Ce) scintillators. We present here the results of the development of a single SDD prototype, with 8 × 8 mm2 active area, produced at Fondazione Bruno Kessler (FBK) semiconductor laboratories. We discuss the design issues related to the specific use of this device as a photo-detector for scintillators. Then, we focus on the read-out electronics. Since this SDD does not include a front-end transistor on the silicon chip, we have adopted a CMOS charge preamplifier (CUBE) recently developed at Politecnico di Milano. This preamplifier has allowed the achievement of state-of-the-art noise performance using a SDD technology process without the integration of the FET (Field Effect Transistor) on the detector chip. A quantum efficiency of about 80% has been measured for the SDD at the emission wavelength band of LaBr3 (360-380 nm). First experimental measurements consisting of direct 55 Fe irradiation of the SDD without scintillator, have demonstrated energy resolution of 140 eV and 129 eV at -20°C and -43°C respectively. By coupling the SDD with a LaBr3(Ce) scintillator (9 mm diameter), we have measured energy resolution of 5.6% FWHM and 2.6% FWHM at 122 keV and 662 keV respectively.

X-ray optics developments at ESA

Future high energy astrophysics missions will require high performance novel X-ray optics to explore the Universe beyond the limits of the currently operating Chandra and Newton observatories. Innovative optics technologies are therefore being developed and matured by the European Space Agency (ESA) in collaboration with research institutions and industry, enabling leading-edge future science missions. Silicon Pore Optics (SPO) [1 to 21] and Slumped Glass Optics (SGO) [22 to 29] are lightweight high performance X-ray optics technologies being developed in Europe, driven by applications in observatory class high energy astrophysics missions, aiming at angular resolutions of 5” and providing effective areas of one or more square meters at a few keV. This paper reports on the development activities led by ESA, and the status of the SPO and SGO technologies, including progress on high performance multilayer reflective coatings [30 to 35]. In addition, the progress with the X-ray test facilities and associated beam-lines is discussed [36].

Performance and Properties of Ultra-Thin Silicon Nitride X-ray Windows

The spectral transmittance of a new generation of SiN based X-ray windows is characterized. The windows are strengthened by low aspect-ratio support grid. As expected for this unprecedented thin window material, the transmittance in the soft X-ray spectral region outperforms the present technologies. A detailed study of the various performance properties of the fabricated SiN X-ray windows is presented. Besides their high transmittance, the windows also have high uniformity, high mechanical strength and good leak tightness. The windows can withstand temperatures from cryogenic range to approximately 250°C. SiN foils are the first real nanotechnology-based choice for the practical realization of X-ray windows and bring the performance to a level that only nanotechnology can offer.

Ultra-Thin Silicon Nitride X-Ray Windows

We have demonstrated the fabrication of ultra-thin Si fine grid supported silicon nitride X-ray windows. These X-ray windows exhibit unequaled transmission of soft X-rays, high strength and excellent thermal stability. Measured soft X-ray transmission performance is significantly enhanced compared to typical polymer or beryllium based X-ray window structures. A double sided grid structure is used to demonstrate the scaling of the technology to larger areas.

Silicon pore optics for the ATHENA telescope

Silicon Pore Optics is a high-energy optics technology, invented to enable the next generation of high-resolution, large area X-ray telescopes such as the ATHENA observatory, a European large (L) class mission with a launch date of 2028. The technology development is carried out by a consortium of industrial and academic partners and focuses on building an optics with a focal length of 12 m that shall achieve an angular resolution better than 5”. So far we have built optics with a focal length of 50 m and 20 m. This paper presents details of the work carried out to build silicon stacks for a 12 m optics and to integrate them into mirror modules. It will also present results of x-ray tests taking place at PTB’s XPBF with synchrotron radiation and the PANTER test facility.

Development and characterization of coatings on Silicon Pore Optics substrates for the ATHENA mission

We present description and results of the test campaign performed on Silicon Pore Optics (SPO) samples to be used on the ATHENA mission. We perform a pre-coating characterization of the substrates using Atomic Force Microscopy (AFM), X-ray Re ectometry (XRR) and scatter measurements. X-ray tests at DTU Space and correlation between measured roughness and pre-coating characterization are reported. For coating development, a layer of Cr was applied underneath the Ir/B4C bi-layer with the goal of reducing stress, and the use of N2 during the coating process was tested in order to reduce the surface roughness in the coatings. Both processes show promising results. Measurements of the coatings were carried out at the 8 keV X-ray facility at DTU Space and with synchrotron radiation in the laboratory of PTB at BESSY II to determine re ectivity at the grazing incidence angles and energies of ATHENA. Coating development also included a W/Si multilayer coating. We present preliminary results on X-ray Re ectometry and Cross-sectional Transmission Electron Microscopy (TEM) of the W/Si multilayer.

ESA-led ATHENA/IXO optics development status

The International X-ray Observatory (IXO) is a candidate mission in the ESA Space Science Programme Cosmic Vision 1525, and was studied as a joint mission with NASA and JAXA. Considering the programmatic evolution of the international context, the mission is being reformulated as an ESA-led mission, under the name of ATHENA (Advanced Telescope for High Energy Astrophysics), with possible participation of NASA and JAXA. The mission is building on the novel Silicon Pore Optics (SPO) technology to achieve the required performance for this demanding astrophysics observatory. This technology is being developed by an industrial consortium, and involves also several research institutes [1-12]. A second optics technology, slumped glass optics (SGO), which is being developed in Europe and the USA, was the backup technology for IXO, and additionally work is progressing on improved reflective coatings and X-ray test facilities [13-17].

Coating optimization for the ATHENA+ mission

The ATHENA mission concept, now called ATHENA+, continues to be refined to address important questions in modern astrophysics. Previous studies have established that the requirement for effective area can be achieved using a combination of bi-layer coatings and/or simple graded multilayers. We find that further coating developments can improve on the baseline specifications and present here preliminary results on the optimization of coating design based on the new specifications of the ATHENA+ mission. The performances of several material combinations are investigated with the goal of maximizing the telescope effective area within the energy envelope of the mission and simulation of mirror performance is carried out.

ATHENA optimized coating design

The optimization of coating design for the ATHENA mission si described and the possibility of increasing the telescope effective area in the range between 0.1 and 10 keV is investigated. An independent computation of the on-axis effective area based on the mirror design of ATHENA is performed in order to review the current coating baseline. The performance of several material combinations, considering a simple bi-layer, simple multilayer and linear graded multilayer coatings are tested and simulation of the mirror performance considering both the optimized coating design and the coating baseline including on- and off-axis effective area curves are presented. We find that the use of linear graded multilayers can increas by 37% the integraed effective area of ATHENA in the energy range between 0.1 keV and 15keV.

Silicon pore optics developments and status

Silicon Pore Optics (SPO) is a lightweight high performance X-ray optics technology being developed in Europe, driven by applications in observatory class high energy astrophysics missions. An example of such application is the former ESA science mission candidate ATHENA (Advanced Telescope for High Energy Astrophysics), which uses the SPO technology for its two telescopes, in order to provide an effective area exceeding 1 m2 at 1 keV, and 0.5 m2 at 6 keV, featuring an angular resolution of 10” or better [1 to 24]. This paper reports on the development activities led by ESA, and the status of the SPO technology. The technology development programme has succeeded in maturing the SPO further and achieving important milestones, in each of the main activity streams: environmental compatibility, industrial production and optical performance. In order to accurately characterise the increasing performance of this innovative optical technology, the associated X-ray test facilities and beam-lines have been refined and upgraded.