A. Rachevski

A Large Area Detector proposed for the Large Observatory for X-ray Timing (LOFT)

The Large Observatory for X-ray Timing (LOFT) is one of the four candidate ESA M3 missions considered for launch in the 2022 timeframe. It is specifically designed to perform fast X-ray timing and probe the status of the matter near black holes and neutron stars. The LOFT scientific payload is composed of a Large Area Detector (LAD) and a Wide Field Monitor (WFM). The LAD is a 10 m2-class pointed instrument with 20 times the collecting area of the best past timing missions (such as RXTE) over the 2-30 keV range, which holds the capability to revolutionize studies of X-ray variability down to the millisecond time scales. Its ground-breaking characteristic is a low mass per unit surface, enabling an effective area of ~10 m2 (@10 keV) at a reasonable weight. The development of such large but light experiment, with low mass and power per unit area, is now made possible by the recent advancements in the field of large-area silicon detectors - able to time tag an X-ray photon with an accuracy <10 μs and an energy resolution of ~260 eV at 6 keV - and capillary-plate X-ray collimators. In this paper, we will summarize the characteristics of the LAD instrument and give an overview of its capabilities.

Large-area linear Silicon Drift Detector design for X-ray experiments

A large area, 120 × 72 mm2, linear Silicon Drift Detector (SDD) has been developed for X-ray spectroscopy in the 2-50 keV energy range. Elaborated via a number of prototypes, the final detector design, REDSOX1, features elements to meet the requirements of a modern space-borne X-ray detector with a power consumption per sensitive area below 0.5 mW/cm2, offering the possibility to perform timing and spectroscopy X-ray observations on a ten microseconds scale.

A Silicon Drift Detector-CMOS front-end system for high resolution X-ray spectroscopy up to room temperature

A system constituted by a Silicon Drift Detector (SDD), fabricated with an innovative technology for minimizing the anode current, and a new CMOS charge sensitive preamplifier (CSA), designed for ultimate low noise performance, has been realized and experimentally characterized. The SDD is hexagonal with an active area of 13 mm2. The current density measured at the anode with the detector in operating condition is 25 pA/cm2 at +20°C. The CSA—named SIRIO—has intrinsic Equivalent Noise Charge (ENC) ranging from 2.9 to 1.5 electrons r.m.s. at 0.8 μs and 11 μs peaking times at room temperature, respectively. With the SDD-SIRIO system at +21°C, an energy resolution of 141 eV FWHM on the 55Fe line at 5.9 keV and 74 eV FWHM on the pulser line with a noise threshold of 170 eV have been measured at 0.8 μs peaking time. The system has been tested from −30°C to +30°C with energy resolution from 124 eV to 148 eV FWHM at 5.9 keV. A moderate cooling at +10°C is sufficient to reach 133 eV FWHM at 5.9 keV.

The large area detector of LOFT: the Large Observatory for X-ray Timing

LOFT (Large Observatory for X-ray Timing) is one of the five candidates that were considered by ESA as an M3 mission (with launch in 2022-2024) and has been studied during an extensive assessment phase. It is specifically designed to perform fast X-ray timing and probe the status of the matter near black holes and neutron stars. Its pointed instrument is the Large Area Detector (LAD), a 10 m2-class instrument operating in the 2-30keV range, which holds the capability to revolutionise studies of variability from X-ray sources on the millisecond time scales. The LAD instrument has now completed the assessment phase but was not down-selected for launch. However, during the assessment, most of the trade-offs have been closed leading to a robust and well documented design that will be reproposed in future ESA calls. In this talk, we will summarize the characteristics of the LAD design and give an overview of the expectations for the instrument capabilities.

A setup for soft proton irradiation of X-ray detectors for future astronomical space missions

Abstract Protons that are trapped in the Earths magnetic field are one of the main threats to astronomical X-ray observatories. Soft protons, in the range from tens of keV up to a few MeV, impinging on silicon X-ray detectors can lead to a significant degradation of the detector performance. Especially in low earth orbits an enhancement of the soft proton flux has been found. A setup to irradiate detectors with soft protons has been constructed at the Van-de-Graaff accelerator of the Physikalisches Institut of the University of Tubingen. Key advantages are a high flux uniformity over a large area, to enable irradiations of large detectors, and a monitoring system for the applied fluence, the beam uniformity, and the spectrum, that allows testing of detector prototypes in early development phases, when readout electronics are not yet available. Two irradiation campaigns have been performed so far with this setup. The irradiated detectors are silicon drift detectors, designated for the use on-board the LOFT space mission. This paper gives a description of the experimental setup and the associated monitoring system.

X-Ray Silicon Drift Detector–CMOS Front-End System with High Energy Resolution at Room Temperature

We present a spectroscopic system constituted by a Silicon Drift Detector (SDD) coupled to a CMOS charge sensitive preamplifier, named SIRIO, specifically designed to reach ultimate low noise levels. The SDD, with an active area of 13 mm², has been manufactured by optimizing the production processes in order to reduce the anode current, successfully reaching current densities between 17 pA/cm² and 25 pA/cm² at + 20 ^° C for drift fields ranging from 100 V/cm to 500 V/cm. The preamplifier shows minimum intrinsic noise levels of 1.27 and 1.0 electrons r.m.s. at +20 ^°C and -30 ^°C, respectively. At room temperature (+ 20^° C) the ⁵⁵Fe 5.9 keV and the pulser lines have 136 eV and 64 eV FWHM, respectively, corresponding to an equivalent noise charge of 7.4 electrons r.m.s.; the noise threshold is at 165 eV. The energy resolution, as measured on the pulser line, ranges from 82 eV FWHM (9.4 electrons r.m.s.) at + 30 ^° C down to 29 eV FWHM (3.3 electrons r.m.s.) at - 30 ^° C.

Simulations of the X-ray imaging capabilities of the Silicon Drift Detectors (SDD) for the LOFT Wide Field Monitor

The Large Observatory For X-ray Timing (LOFT), selectyed by ESA as one of the four Cosmic Visiion M3 candidate missions to undergo an assessment phase, will revolutionize the study of compact objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. The Large Area Detector (LAD), carrying an unprecedented effective area of 10 m2, is complemented by a coded-mask Wide Field Monitor, in charge of monitoring a large fraction of the sky potentially accesesible to the LAD, to provide the history and context for the sources observed by LAD and to trigger its observations on their most interesting and extreme states. In this paper we present detailed simulations of the imaging capabilities of the Silicon Drift Detectors developed for the LOFT Wide Field Monitor detection plane. The simulations explore a large parameter space for both the detector design and the environmental conditions, allowing us to optimize the detector characteristcs and demonstrating the X-ray imaging performance of the large-area SDDs in the 2-50 keV energy band.

The design of the wide field monitor for the LOFT mission

LOFT (Large Observatory For x-ray Timing) is one of the ESA M3 missions selected within the Cosmic Vision program in 2011 to carry out an assessment phase study and compete for a launch opportunity in 2022-2024. The phase-A studies of all M3 missions were completed at the end of 2013. LOFT is designed to carry on-board two instruments with sensitivity in the 2-50 keV range: a 10 m2 class Large Area Detector (LAD) with a <1° collimated FoV and a wide field monitor (WFM) making use of coded masks and providing an instantaneous coverage of more than 1/3 of the sky. The prime goal of the WFM will be to detect transient sources to be observed by the LAD. However, thanks to its unique combination of a wide field of view (FoV) and energy resolution (better than 500 eV), the WFM will be also an excellent monitoring instrument to study the long term variability of many classes of X-ray sources. The WFM consists of 10 independent and identical coded mask cameras arranged in 5 pairs to provide the desired sky coverage. We provide here an overview of the instrument design, configuration, and capabilities of the LOFT WFM. The compact and modular design of the WFM could easily make the instrument concept adaptable for other missions.

A novel multi-cell silicon drift detector for Low Energy X-Ray Fluorescence (LEXRF) spectroscopy

The TwinMic spectromicroscope at Elettra is a multipurpose experimental station for full-field and scanning imaging modes and simultaneous acquisition of X-ray fluorescence. The actual LEXRF detection setup consists of eight single-cell Silicon Drift Detectors (SDD) in an annular configuration. Although they provide good performances in terms of both energy resolution and low-energy photon detection efficiency, they cover just about 4% of the whole photoemission solid angle. This is the main limitation of the present detection system, since large part of the emitted photons is lost and consequently a high acquisition time is required. In order to increase the solid angle, a new LEXRF detection system is being developed within a large collaboration of several institutes. The system, composed of 4 trapezoidal multi-cell silicon drift detectors, covers up to 40% of the photoemission hemisphere, so that this geometry provides a 10 times improvement over the present configuration. First measurements in the laboratory and on the TwinMic beamline have been performed in order to characterize a single trapezoidal detector, configured and controlled by means of two multichannel ASICs, which provide preamplification, shaping and peak-stretching, connected to acquisition electronics based on fast ADCs and FPGA and working under vacuum.

Characterization of the VEGA ASIC coupled to large area position-sensitive Silicon Drift Detectors

Low-noise, position-sensitive Silicon Drift Detectors (SDDs) are particularly useful for experiments in which a good energy resolution combined with a large sensitive area is required, as in the case of X-ray astronomy space missions and medical applications. This paper presents the experimental characterization of VEGA, a custom Application Specific Integrated Circuit (ASIC) used as the front-end electronics for XDXL-2, a large-area (30.5 cm^2) SDD prototype. The ASICs were integrated on a specifically developed PCB hosting also the detector. Results on the ASIC noise performances, both stand-alone and bonded to the large area SDD, are presented and discussed.