5.2.1.4 GAS MICROSTRIPS
Gas microstrip detectors represent a new technology which has been the subject of intensive investigations at accelerators and for X-ray telescopes. The gas microstrip detector is a cross between the silicon microstrip and drift chamber detectors. The drift chamber anode cathode wires are replaced by thin metal traces photo-etched on an insulating substrate at a pitch of about 200 Fm. The field shaping wire grids of the drift chamber are replaced with a single plane electrode separated from the anode/cathode plane by 2-4 mm. These detectors have potentially large areas, limited by the photolithographic or direct etching fabrication equipment and not by the size of the substrate. They are, however, gas detectors which require a clean gas vessel and gas supply. The principal gamma-ray application of gas microstrips would be a pair telescope.
5.2.1.5 LIQUID XENON
Figure 5.2 - A prototype of a liquid xenon detector.
Liquid xenon, with its high density (3 g cm-3) and high atomic number (54) is a very good detector material for gamma rays. Liquid Xe detectors can be realized in a variety of configurations, using either its good ionization (64,000 electrons/MeV) or scintillation (similar yield as NaI, < 5 ns time response) properties, or both. In a liquid Xe Time Projection Chamber (LXeTPC), both the ionization electrons and the scintillation light signals are detected to infer the energy deposit (resolution of ~6% at 1 MeV) and the three-dimensional spatial coordinates (resolution < 1 mm) of each gamma-ray interaction. Events with multiple-site interactions are unambiguously recognized and the original gamma-ray direction reconstructed from kinematics, resulting in high detection efficiency for true source events. The characteristics of a LXeTPC for gamma-ray imaging and spectroscopy have been demonstrated with a prototype of 400 cm2 active area and 8 cm thickness. A double-scatter Compton telescope based on a coincidence of two liquid TPCs is being studied. With a Liquid Ar TPC as the low Z converter and Compton electron tracker, at a fixed distance from a LXeTPC as calorimeter and imager, a substantial improvement in sensitivity and angular resolution over COMPTEL in the 1-10 MeV range could be achieved. Moreover, such a configuration, with TPCs of ten times the area of the existing prototype, appears feasible within a MIDEX mission cost and weight constraints.
5.2.2 VLSI/ASIC
A characteristic common to many of the current and future technologies for gamma-ray detector systems is the large number of channels with relatively small signal outputs. Most such applications will, therefore, rely on Application Specific Integrated Circuits (ASICs) and Very Large Scale Integration (VLSI) techniques in order to keep the power consumption to a modest level for spaceflight. Although the specific circuits are tailored for individual applications, general approaches to designing and building such electronics can be improved. Any technology developments that allow faster or cheaper production of these electronics will directly benefit a broad range of gamma-ray telescope designs.
5.3 COMPUTATIONAL CAPABILITIES
The information explosion that will come with the next generation of satellite gamma-ray experiments will put severe demands on the current computing capabilities both on-board and ground-based. Increased data rates, even without better resolution, will demand computational upgrades. Increased angular resolution will force better image analysis, better time resolution will drive deeper pulsar and quasi-periodic searches and more sophisticated spectral techniques will require more complicated analysis routines. Fortunately, this is a field which can be expected to rapidly improve with time so that it is only necessary to ensure that state-of-the-art computer facilities are available for gamma-ray data analysis.