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Orbiting photon-counting observatory for the Earth night-sky background: AURORA on MEGSAT-1

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A light photon-counting scientific payload, named AURORA, has been developed and launched on a commercial microsatellite in order to study the near-UV night-sky background emission (channel Notte) and the Aurora (Alba). AURORA is mapping, with the Notte channel, the night-side photon background in the 300-400 nm spectral range, together with a particular 2+ nitrogen line ((lambda) = 337nm). These measurements are required in the framework of the Extreme Universe Space Observatory (EUSO) experiment, approved by the European Space Agency (ESA) for the phase A and to be flown on the International Space Station (ISS) in 2009. The Alba channel studies the Aurora emissions in four different spectral bands centered on: 367nm, 391nm, 535nm, 560nm (OI). The instrument has been integrated on the MEGSAT-1 satellite and launched, on the September, 26th 2000, from the Baikonur cosmodrome. The nearly circular Low Earth Orbit (LEO), with inclination of 64.56 degrees, fully includes the ISS ground track envelope. The satellite overall mass is about 60 kg. An overview of the techniques adopted, including detectors, front-end electronics, Central Processor Unit (CPU), is given in this paper, together with a brief report on the mission status and plans.
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Thesis
Since the advent of space-based astronomy in the early 1960's, there has been a need for space-qualified detectors with sufficient sensitivity and resolution to detect and image single photons, ions or electrons. This thesis describes a research programme to develop detectors that fulfil these requirements. I begin by describing the role of detectors in space astronomy and follow with a review of detector technologies, with particular emphasis on imaging techniques. Conductive charge division image readouts offer high performance, simplicity, and flexibility and their potential is investigated in both theory and practice. I introduce the basic design concept and discuss the fundamental factors limiting performance in relation to physical design and to underlying physical processes. Readout manufacturing techniques are reviewed and a novel method presented. I describe specific space and ground-based readout applications which proved valuable in teaching lessons and raising questions. These questions initiated an experimental programme, whose goals were to understand limiting physical processes and find techniques to overcome them. Results are presented, and the innovation of the progressive geometry readout technique, which this programme also spawned, is described. Progressive geometry readout devices, such as the Vernier anode, offer dramatically improved performance and have been successfully flight-proven. I describe the development of a Vernier readout for the J-PEX sounding rocket experiment, and discuss the instrument calibration and the flight programme. First investigations into a next generation of charge division readout design are presented. These devices will use charge comparison instead of amplitude measurement to further enhance resolution and count rate capability. In conclusion, I summarize the advances made during the course of this research, and discuss ongoing technological developments and further work which will enable MCP detectors to continue to excel where characteristics such as true photon-counting ability, high spatial resolution, format flexibility, and high temporal resolution are required.
Conference Paper
Microchannel plate (MCP) photon counting detectors using image readout devices such as the Vernier and Cross-strip anodes can now achieve spatial resolutions limited by the pore geometry of commercially available MCPs. We describe progress in the development of a new readout system, part of our program to achieve MCP limited spatial resolution and larger format sizes using the small pore MCPs. We discuss the limitations of charge division devices that require high precision charge measurement and present a readout technique using charge comparison, with the potential to achieve large format readouts at high count rates. This scheme use a technique whereby the position coordinate of an event is represented by the order of amplitudes of a set of electrodes. Each coordinate is identified by a unique permutation of electrodes and is determined by comparing the charge collected on the electrodes. One of the major advantages offered by this scheme is a much lower signal-to-noise requirement. This will allow the detector to operate at substantially lower gain, raising the MCP limited count rate threshold. We present a simple and practical readout design to implement the charge comparison scheme, which uses the image charge technique to enhance performance in the areas of spatial resolution, linearity and image stability. The high count rate capability of the new design is augmented by an ability to capture events in parallel without the requirement for excessive numbers of electronic channels. We describe an electronics scheme specifically for the charge comparison readout and discuss how it can provide enhanced spatial resolution by using a charge centroiding technique based on pulse timing information. We support this with timing measurements obtained from a breadboarded electronic channel.
Article
The observation of twenty cosmic-ray air-showers at and above 1020 eV poses two problems for particle astrophysics: how the primary particles are accelerated to these energies, and how the primaries get here through the 2.7K microwave background filling the Universe. In addition, the highest-energy events exhibit a surprising small-scale clustering on the celestial sky. An overview of the puzzles is presented, followed by a brief discussion of many of the models proposed to solve these puzzles. Emphasis is placed on (i) the role neutrino primaries may play in resolving issues, and (ii) the signatures by which cosmic ray experiments in the near future will discriminate among the proposed models. It is an exciting prospect that highest-energy cosmic rays may have already presented us with new physics not accessible in terrestrial accelerator searches.
Article
The observation of twenty cosmic-ray air-showers at and above 1020 eV poses two problems for particle astrophysics: how the primary particles are accelerated to these energies, and how the primaries get here through the 2.7K microwave background filling the Universe. In addition, the highest-energy events exhibit a surprising small-scale clustering on the celestial sky. An overview of the puzzles is presented, followed by a brief discussion of many of the models proposed to solve these puzzles. Emphasis is placed on (i) the role neutrino primaries may play in resolving issues, and (ii) the signatures by which cosmic ray experiments in the near future will discriminate among the proposed models. It is an exciting prospect that highest-energy cosmic rays may have already presented us with new physics not accessible in terrestrial accelerator searches.
Article
The methods of detection of high-energy cosmic rays are briefly described and it is shown that results on the energy spectrum and arrival direction distribution obtained above confound theoretical expectation. There is an urgent need for better statistics as only about 12 events above have been detected and the limit to the energy which cosmic rays can reach is not known. The new, funded, instruments, the Pierre Auger Observatory and the Hi-Res fluorescence detector, are described and the plans for an ambitious satellite observatory (Airwatch/OWL) are outlined.
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