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Technology R&D

Group members: Monica Alderighi, Fabio Casini, Sergio D'Angelo, Mauro Fiorini, Salvatore Incorvaia, Marcello Mancini, Sandro Pastore, Francesco Perotti, Michela Uslenghi

Detectors and Front-End Electronics

MARC (Millisecond ASIC for Readout of CCDs)

The aim of the project is to develop low noise Front End Electronics implemented in ASIC (Application-Specific Integrated Circuit) for reading Charge Coupled Devices (CCDs) with highly segmented architectures and multiple parallel outputs, with the primary target of realizing photon counting x-ray detectors with time resolution in the order of ∼ms, still retaining the almost Fano-limited energy resolution achievable with sensors readout at low frequency.

High frame rate is a key characteristic for using CCDs in photon counting in the x-ray range because not only it defines the time resolution of the detector, but also the maximum tolerable count rate because whenever the separation of two or more X-ray photons incident on a CCD is less than a few CCD pixels, and their arrival time lies within the same CCD frame readout, they are regarded as a single event, affecting both spatial and spectral resolution.

This is still a weak point for CCDs, because the readout frame time is limited by the large number of pixels to be readout by each node sequentially and by the necessity of limiting the bandwidth for low noise readout. In standard CCD architecture, in fact, data are shifted vertically into a structure that acts as a horizontal shift register. The horizontal shift register is then clocked out one pixel at a time.

In order to reach higher frame rates without affecting the energy resolution and the low energy threshold, the number of readout node have to be increased, allowing the parallel read out of pixels. The extreme case is the so-called "column-parallel" architecture, with one output node for each column and no serial register at all.

A major development item for a high-speed CCD is then the associated readout electronics. In fact, as the number of output nodes rises, implementing readout channels with standard discrete electronics become a problem due to the complexity, size and power consumption, and the development of an ASIC is mandatory. This program, funded by INAF with a Tecno-PRIN, is aimed to develop an ASIC optimized for thick, fully depleted sensors on high resistivity Silicon (providing improved QE in x-ray and near Infrared). In prospect, this could lead up to devices with higher spatial resolution, better energy resolution and even higher radiation hardness than solutions presently available.

In addition to the application for X-ray instruments, high frame rate, thick CCDs can be used in other wavelength ranges (for example for high speed optical photometry or as wavefront sensors for adaptive optics systems.

This program is carried out in collaboration with Politecnico di Milano, Osservatorio Astrofisico di Catania e IASF Roma. High resistivity CCDs are provided by Lawrence Berkeley National Laboratory.

PICO PIcosecond Cherenkov Observatory

This project aims at the realization of novel detectors with picoseconds timing accuracy based on Single Photon Avalanche Diodes (SPADs), for application to both ground and space based new generation telescopes. In particular we will investigate ultra-fast application of SPADs to future Cherenkov telescopes on ground, and to active anti-coincidence shields with time-of-flight capabilities in space.

There is a growing demand for detectors with sub-nanosecond time resolution in the field of experimental high-energy astrophysics. The electromagnetic showers detected on ground by Imaging Air Cherenkov Telescopes (IACTs) have characteristic development times of few nanoseconds, and must be selected among the background signals produced by a much larger flux of cosmic rays. In the MeV-range gamma-ray astronomy, the currently best sensitivity limits were set in the '90s by the COMPTEL experiment onboard the CGRO mission, which employed time-of-flight (TOF) capabilities between its two main detectors to identify the incoming direction of the detected photon. All space instruments for gamma-ray astronomy in the GeV range (EGRET,AGILE, Fermi) have anti-coincidence shields for charged particle rejection, but none of them with TOF capabilities. A TOF system is foreseen for the GAMMA-400 space observatory, currently under development.

Up to now focal plane detectors for IACTs as well as TOF systems have been based on photomultiplier tubes (PMTs). In the frame of this project we propose the use of SPADs for light collection instead of PMTs. SPADs are silicon devices operating in Geiger-mode which feature sensitivity to the single photon level, high photon detection efficiency (PDE) around 490nm, a remarkable timing accuracy as good as 40ps, without spectroscopic capabilities. These timing performances make these devices particularly promising for applications where a high timing accuracy is mandatory.

This program, funded by INAF with a Tecno-PRIN, is carried out in collaboration with IASF Bologna.

Photon Counting UV detectors based on MCP

Photon counting detectors based on micro-channel plates are the most used in a number of applications in UV/EUV astronomy and solar physics, since no other technology, up to now, can provide the same combination of large format (both in terms of sensitive area and number of pixels), photon counting capability, virtually null readout noise, possibility of optimizing the detector for different wavelength ranges with appropriate choice of the photocathode (with solar blindness option, with very high rejection of the visible light, characteristics very appealing for astronomical applications, since astronomical sources are typically orders of magnitudes brighter in the visible than in the UV), radiation hardness and operation at room temperature.

An R&D program on MCP-based detectors, readout by means of silicon image sensors, has been carried out at IASF-Milano, originally funded by a specific ASI contract and then continued as support activity for mission concept studies. Prototypes of detectors readout with CCD and APS have been realized and one of them, optimized for ground-based optical observations, is employed at the Asiago Observatory for high speed photometry and spectroscopy of Cataclysmic Variables. Specific study aimed to optimize this kind of detectors for solar physics applications, with particular emphasis on operations in harsh radiation environment, is ongoing.

Currently, the results of this activity are used for the UV detector of the METIS instrument on board of the ESA mission Solar Orbiter(see the related paragraph).

METIS Multi Element Telescope for Imaging and Spectroscopy (Solar Orbiter)

METIS is one of the six remote sensing instruments of the Solar Orbiter mission of the European Space Agency (ESA), currently under implementation for a launch planned in 2017. The objective of Solar Orbiter is the exploration of the Sun-heliosphere connection with a combination of remote sensing and in-situ instruments from a distance that will reach 0.28 AU and from solar latitudes that will reach up to 34°.

METIS is an inverted-occultation coronagraph that will image the solar corona in two different wavelengths (visible light between 590 nm and 650 nm, and the Lyman-a lines of the hydrogen at 121.6 nm) by a combination of multilayer coatings and spectral bandpass filters. The visible channel also includes a polarimeter to observe the linearly polarized component of the K corona. Moreover, METIS provides the capability of collecting spectra of the hydrogen and helium Lyman-a lines simultaneously at three different heights (accomplished by a multiple slit) in an angular sector of the corona.

IASF-Milano is mainly involved in the study of the two detectors and associated FEE:

The METIS consortium includes: INAF IASF MIlano, IFSI, OACN, OACt, OAPa, OATo, OATs (IT), CNR Institute for Photonics and Nanotechnology (IT), Politecnico di Torino (IT), Universita' di Firenze (IT), Universita' di Padova (IT), CNRS-IAS (FR), Institute of Astronomy Czech Academy of Science (CZ), Max-Planck-Institute für Sonnensystemforschung (DE), Naval Research Laboratory (USA), University of Athens (GR)

High-Reliability Computing Systems

The objective of this research is the development of high reliability computing systems for space applications. Programmable logic devices, specifically SRAM-based Field Programmable Gate Arrays (SRAM-FPGAs), have been chosen as reference implementation technology. They offer the advantages of a high gate density, i.e. implementation of complex systems in a single chip, -- available solutions often include internal processors, reduced development costs and unlimited device reconfiguration. Unfortunately, this last comes at the expense of a potentially high susceptibility to radiation, due to the presence of a large number of configuration memory cells, whose functioning might be compromised by radiation.

In particular, the dominant effect of particle interaction with this kind of technology (SRAM memory) is the Single Event Upset (SEU) or soft error. As the name "soft" denotes, the error is temporary and not permanent: when a particle hits a memory cell, its energy may be high enough to provoke the temporary change of the status of the cell from "0" to "1" or viceversa. However, as the memory cells hold the configuration of a circuit, the effect (the error) persists until the device is reconfigured.

The activity faces the following main topics:

Concerning the evaluation of the radiation sensitivity, within the context of an on-going study contract funded by ESA and started in 2004, a platform (h/w and s/w) for fault injection, FLIPPER, has been designed and implemented for Xilinx SRAM-FPGA devices, based on device reconfiguration. FLIPPER was experimentally validated by means of radiation ground testing in accelerator. The platform initially developed for Xilinx Virtex II devices, has been recently upgraded to the newer and more powerful Virtex 4 device family.

As a complementary approach, an on-going study concerns the definition of a tool for the static evaluation of the radiation vulnerability of circuits implemented in SRAM-FPGAs. A probabilistic model of occurrences of soft errors in programmable interconnections of SRAM-FPGAs has been defined and tested. The rationale behind this is that the majority of the configuration memory cells rules the interconnection setting; thus it is expected that an upset on those cells might have a great impact on the SEU sensitivity of the implemented designs.

Finally, within the context of a project funded by ESA, a high reliability on board computer is being developed that is based on Commercial-Off-The-Shelf (COTS) technology; the system will be used in future missions of the Agency. The expected duration of the project is three years, and the team is composed by: Thales Alenia Space Italia (Prime Contractor), Politecnico di Torino, Universita' di Roma Tor Vergata, Sanitas EG srl, and INAF. INAF is in charge of: i) a technological survey of candidate reprogrammable logic devices, ii) the definition of the evaluation environment and test campaigns, and iii) the design and implementation of the EGSE for the COTS based computer.

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