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Register a new userDesign, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission
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Due to be launched in late 2021, the Imaging X-Ray Polarimetry Explorer (IXPE) is a NASA Small Explorer mission designed to perform polarization measurements in the 2-8 keV band, complemented with imaging, spectroscopy and timing capabilities. At the heart of the focal plane is a set of three polarization-sensitive Gas Pixel Detectors (GPD), each based on a custom ASIC acting as a charge-collecting anode. In this paper we shall review the design, manufacturing, and test of the IXPE focal-plane detectors, with particular emphasis on the connection between the science drivers, the performance metrics and the operational aspects. We shall present a thorough characterization of the GPDs in terms of effective noise, trigger efficiency, dead time, uniformity of response, and spectral and polarimetric performance. In addition, we shall discuss in detail a number of instrumental effects that are relevant for high-level science analysis -- particularly as far as the response to unpolarized radiation and the stability in time are concerned.
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The High-Energy Particle Detector (HEPD) is one of the payloads of the CSES space mission. The CSES (China Seismo-Electromagnetic Satellite) mission will investigate the structure and the dynamic of the topside ionosphere, will monitor electric and magnetic field and high energy particle fluctuations, searching for their correlations with the geophysical activity, in order to contribute to the monitoring of earthquakes from space. The HEPD is built by the Italian collaboration and has different goals. It will study the temporal stability of the inner Van Allen radiation belts, the precipitation of trapped particles in the atmosphere and the low energy component of the cosmic rays (5 - 100 MeV for electrons and 15 - 300 MeV for protons). Here is presented a study of the performance of the apparatus to separate electrons and protons and identify nuclei up to iron.
We discuss a new class of Micro Pattern Gas Detectors, the Gas Pixel Detector (GPD), in which a complete integration between the gas amplification structure and the read-out electronics has been reached. An Application-Specific Integrated Circuit (ASIC) built in deep sub-micron technology has been developed to realize a monolithic device that is, at the same time, the pixelized charge collecting electrode and the amplifying, shaping and charge measuring front-end electronics. The CMOS chip has the top metal layer patterned in a matrix of 80 micron pitch hexagonal pixels, each of them directly connected to the underneath electronics chain which has been realized in the remaining five layers of the 0.35 micron VLSI technology. Results from tests of a first prototype of such detector with 2k pixels and a full scale version with 22k pixels are presented. The application of this device for Astronomical X-Ray Polarimetry is discussed. The experimental detector response to polarized and unpolarized X-ray radiation is shown. Results from a full MonteCarlo simulation for two astronomical sources, the Crab Nebula and the Hercules X1, are also reported.
X-ray Charge Coupled Devices (CCDs) have been the workhorse for soft X-ray astronomical instruments for the past quarter century. They provide broad energy response, extremely low electronic read noise, and good energy resolution in soft X-rays. These properties, along with the large arrays and small pixel sizes available with modern-day CCDs, make them a potential candidate for next generation astronomical X-ray missions equipped with large collecting areas, high angular resolutions and wide fields of view, enabling observation of the faint, diffuse and high redshift X-ray universe. However, such high collecting area (about 30 times Chandra) requires these detectors to have an order of magnitude faster readout than current CCDs to avoid saturation and pile up effects. In this context, Stanford University and MIT have initiated the development of fast readout X-ray cameras. As a tool for this development, we have designed a fast readout, low noise electronics board (intended to work at a 5 Megapixel per second data rate) coupled with an STA Archon controller to readout a 512 x 512 CCD (from MIT Lincoln Laboratory). This versatile setup allows us to study a number of parameters and operation conditions including the option for digital shaping. In this paper, we describe the characterization test stand, the concept and development of the readout electronics, and simulation results. We also report the first measurements of read noise, energy resolution and other parameters from this set up. While this is very much a prototype, we plan to use larger, multi-node CCD devices in the future with dedicated ASIC readout systems to enable faster, parallel readout of the CCDs.
The gas pixel detector (GPD) dedicated for photoelectric X-ray polarimetry is selected as the focal plane detector for the ESA medium-class mission concept X-ray Imaging and Polarimetry Explorer (XIPE). Here we show the design, assembly, and preliminary test results of a small GPD for the purpose of gas mixture optimization needed for the phase A study of XIPE. The detector is assembled in house at Tsinghua University following a design by the INFN-Pisa group. The improved detector design results in a good uniformity for the electric field. Filled with pure dimethyl ether (DME) at 0.8 atm, the measured energy resolution is 18% at 6 keV and inversely scales with the square root of the X-ray energy. The measured modulation factor is well consistent with that from simulation, up to ~0.6 above 6 keV. The residual modulation is found to be 0.30% +/- 0.15% at 6 keV for the whole sensitive area, which can be translated into a systematic error of less than 1% for polarization measurement at a confidence level of 99%. The position resolution of the detector is about 80 um in FWHM, consistent with previous studies and sufficient for XIPE requirements.
We have been developing monolithic active pixel sensors, known as Kyotos X-ray SOIPIXs, based on the CMOS SOI (silicon-on-insulator) technology for next-generation X-ray astronomy satellites. The event trigger output function implemented in each pixel offers microsecond time resolution and enables reduction of the non-X-ray background that dominates the high X-ray energy band above 5--10 keV. A fully depleted SOI with a thick depletion layer and back illumination offers wide band coverage of 0.3--40 keV. Here, we report recent progress in the X-ray SOIPIX development. In this study, we achieved an energy resolution of 300~eV (FWHM) at 6~keV and a read-out noise of 33~e- (rms) in the frame readout mode, which allows us to clearly resolve Mn-K$alpha$ and K$beta$. Moreover, we produced a fully depleted layer with a thickness of $500~{rm mu m}$. The event-driven readout mode has already been successfully demonstrated.
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