A next-generation medium-energy (100 keV to 100 MeV) gamma-ray observatory will greatly enhance the identification and characterization of multimessenger sources in the coming decade. Coupling gamma-ray spectroscopy, imaging, and polarization to neutrino and gravitational wave detections will develop our understanding of various astrophysical phenomena including compact object mergers, supernovae remnants, active galactic nuclei and gamma-ray bursts. An observatory operating in the MeV energy regime requires technologies that are capable of measuring Compton scattered photons and photons interacting via pair production. AstroPix is a monolithic high voltage CMOS active pixel sensor which enables future gamma-ray telescopes in this energy range. AstroPix’s design is iterating towards low-power (∼1.5mW/cm2), high spatial (500 μm pixel pitch) and spectral (<5 keV at 122 keV) tracking of photon and charged particle interactions. Stacking planar arrays of AstroPix sensors in three dimensions creates an instrument capable of reconstructing the trajectories and energies of incident gamma rays over large fields of view. A prototype multi-layered AstroPix instrument, called the AstroPix Sounding rocket Technology demonstration Payload (A-STEP), will test three layers of AstroPix “quad chips” in a suborbital rocket flight. These quad chips (2×2 joined AstroPix sensors) form the 4×4 cm2 building block of future large area AstroPix instruments, such as ComPair-2 and AMEGO-X. This payload will be the first demonstration of AstroPix detectors operated in a space environment and will demonstrate the technology’s readiness for future astrophysical and nuclear physics applications. In this work, we overview the design and state of development of the A-STEP payload.
ProtoEXIST2 (P2) was a prototype imaging x-ray detector plane developed for wide-field time-domain astrophysics (TDA) in the 5 to 200 keV energy band. It was composed of an 8 × 8 array of 5 mm thick, 2 cm × 2 cm pixelated (32 × 32) CdZnTe (CZT) detectors with a 0.6-mm pitch that utilize the NuSTAR ASIC (NuASIC) for readout. During the initial detector development process leading up to postflight examination of the entire detector plane, distortions in expected pixel positions and shapes were observed in a significant fraction of the detectors. The High Resolution Energetic x-ray Imager (HREXI) calibration facility (HCF) was designed and commissioned to improve upon these early experiments and to rapidly map out and characterize pixel nonuniformities and defects within CZT detector planes at resolutions down to 50 μm. Using this facility, the subpixel level detector response of P2 was measured at 100 μm5 resolution and analyzed to extract and evaluate the area and profile of individual pixels, their morphology across the entire P2 detector plane for comparison with previous measurements and to provide additional characterization. In this article, we evaluate the imaging performance of a coded-aperture telescope using the observed pixel morphology for P2 detectors. This investigation will serve as an initial guide for detector selection in the development of HREXI detector planes, for the future implementation of the 4pi X-Ray Imaging Observatory (4piXIO)6 mission, which aims to provide simultaneous and continuous imaging of the full sky (4π sr) in the 3 to 200 keV energy band with ≃2 arcmin angular resolution and ≃10 arcsec source localization, as well as other, future coded-aperture instruments.
Detector commanding, processing and readout of spaceborne instrumentation is often accomplished with application specific integrated circuits (ASICs). The ASIC designed for the nuclear spectroscopic telescope array (NuSTAR) mission enables future tiled CdZnTe (CZT) detector array readout for x-ray detectors, such as the high resolution energetic x-ray imager (HREXI). Modified NuSTAR ASIC (NuASIC) gain settings have been implemented for HREXI’s broader targeted imaging energy range (3 to 300 keV) compared with NuSTAR (2 to 79 keV), which may require updated NuASIC internal parameters for optimal energy resolution. To reach HREXI’s targeted low energy threshold, we have also enabled the NuASIC’s “charge pump mode,” which introduces an additional tuning parameter. We describe the mechanics of the NuASIC’s adjustable parameters and use our recently developed ASIC test stand to probe a “bare” NuASIC using its internal test pulser. We record the effects of parameter tuning on the device’s electronics noise and low energy threshold and report the optimal set of parameters for HREXI’s updated gain setting. We detail a semiautomated procedure to derive the optimal parameters for each of HREXI’s large area closely tiled NuASIC/CZT detectors to expedite instrument integration.
Application-specific integrated circuits (ASICs) are used in space-borne instruments for signal processing and detector readout. The electrical interface of these ASICs to frontend printed circuit boards is commonly accomplished with wire bonds. Through silicon via (TSV) technology has been proposed as an alternative interconnect technique that will reduce assembly complexity of ASIC packaging by replacing wire bonding with flip-chip bonding. TSV technology is advantageous in large detector arrays where TSVs enable close detector tiling on all sides. Wafer-level probe card testing of TSV ASICs is frustrated by solder balls introduced onto the ASIC surface for flip-chip bonding that hamper alignment. Therefore, we developed the ASIC test stand (ATS) to enable rapid screening and characterization of individual ASIC die. We successfully demonstrated ATS operation on ASICs originally developed for CdZnTe detectors on the Nuclear Spectroscopic and Telescope Array (NuSTAR) mission that were later modified with TSVs in a via-last process. We tested both backside blind-TSVs and frontside through-TSVs, with results from internal test pulser measurements that demonstrate performance equal to or exceeding the probe card wafer-level testing data. The ATS can easily be expanded or duplicated to parallelize ASIC screening for large area imaging detectors of future space programs.
The High-Resolution Energetic X-ray Imager SmallSat Explorer (HSE) is a proposed wide-field, hard X-ray (3-300 keV) coded aperture telescope. Operating a closely tiled array of pixelated CdZnTe (CZT) detectors, HSE seeks to rapidly localize short gamma ray bursts (GRBs) resulting from neutron star and black hole mergers and search for faint undiscovered black hole low mass x-ray binaries. The spectral signatures of these phenomena fall off as a power law, thereby motivating the improvement of HSE’s hard x-ray band coverage at lower energies. This is achievable by tuning HSE’s Nuclear Spectroscopic Telescope Array (NuSTAR) ASIC detector readout and operating in a charge pumping mode. This can extend energy band coverage to as low as 2-3 keV, but requires careful independent tuning of each of the instrument’s ASIC devices. An optimization procedure for efficiently tuning the detector readout via commandable ASIC registers is reported.
Application-specific integrated circuits (ASICs) are commonly used to efficiently process the signals from sensors and detectors in space. Wire bonding is a space-qualified technique of making interconnections between ASICs and their substrate packaging board for power, control, and readout of the ASICs. Wire bonding is nearly ubiquitous in modern space programs, but their exposed wires can be prone to damage during assembly and subject to electric interference during operations. Additional space around the ASICs needed for wire bonding also impedes efficient packaging of large arrays of detectors. Here, we introduce the through silicon vias (TSV) technology that replaces wire bonds and eliminates their shortcomings. We have successfully demonstrated the feasibility of implementing TSVs to existing ASIC wafers (a.k.a. a via-last process) developed for processing the x-ray signals from the x-ray imaging CdZnTe detectors on the Nuclear Spectroscopic Telescope Array small explorer telescope mission that was launched in 2012. While TSVs are common in the semiconductor industry, this is the first (to our knowledge) successful application for astrophysics imaging instrumentation. We expect that the TSV technology will simplify the detector assembly and thus will enable significant cost and schedule savings in assembly of large area CdZnTe detectors.
The High-Resolution Energetic X-ray Imager (HREXI) cadmium zinc telluride (CZT) detector development program at Harvard is aimed at developing tiled arrays of finely pixelated CZT detectors for use in wide-field coded aperture 3 to 200 keV x-ray telescopes. A pixel size of ∼600 μm has already been achieved in the ProtoEXIST2 (P2) detector plane with CZT readout by the NuSTAR application-specific integrated circuits. This paves the way for even smaller than 300-μm pixels in the next-generation HREXI detectors. We describe a new HREXI calibration facility (HCF) that enables a high-resolution subpixel-level (100 μm) two-dimensional (2D) scan of a 256-cm2 tiled array of 2 × 2 cm2 CZT detectors illuminated by a bright x-ray AmpTek Mini-X tube source at timescales of around a day. HCF is a significant improvement from the previous apparatus used for scanning these detectors, which took ∼3 weeks to complete a one-dimensional (1D) scan of a similar detector plane. Moreover, HCF has the capability to scan a large tiled array of CZT detectors (32 × 32 cm2) at 100-μm resolution in the 10- to 50-keV energy range, which was not possible previously. We describe the design, construction, and implementation of HCF for the calibration of the P2 detector plane.
HSP was selected for the NASA Astrophysics Science SmallSat Study (AS3) program
as a SmallSat mission concept that will be proposed for a 1 – 2 year science mission to demonstrate performance and cost goals to enable a future Explorer-class SmallSat Constellation mission for the first simultaneous full-sky imager with 2X finer resolution. HSP is a 36 x 36deg (FWHM) coded aperture telescope with 16 x 16 CdZnTe detectors, each 20 x 20 x 3mm with 32 x 32 0.6mm pixels and ~1.5keV energy resolution. The 1024 cm^2 HSP imaging detector array views the sky through the Tungsten coded aperture mask (0.7 mm pixels) at 68cm, providing 4’ imaging and <30” source positions over the 3 – 200 keV band. This is mounted on a Blue Canyon Technologies (BCT) SmallSat (S5) bus, with ~10arcsec pointing and star camera aspect, extends the capabilities of Swift/BAT and INTEGRAL/IBIS. HSP will promptly localize long and short GRBs and outbursts of X-ray transients: from nearby M dwarf flares, to BH-LMXB outbursts, Blazar flares and Jetted TDEs. HSP will daily-monitor the Galactic Bulge and adjacent Galactic plane and > 2 nearby OB association regions for 1 yr, providing high cadence light curves of black hole X-ray binaries (with low and high mass companions) in the Galaxy. HSP matches the on-axis sensitivity of Swift/BAT in the 15 – 200 keV band with 5X finer spatial resolution, and the simultaneous 3 – 15 keV imaging and spectra surpass MAXI with 15X finer spatial resolution, all within an ESPA class mission in LEO at ~500-600 km and <~30 deg inclination.
The High Resolution Energetic X-ray Imager (HREXI) is a coded-aperture imaging telescope that utilizes tiled CdZnTe (CZT) detectors to image cosmic x-ray sources and transients in the 3-200 keV energy band. A closely tiled array of 256 pixellated CZT detectors form the 1024 cm^2 detector plane of a proposed (Grindlay et al. 2019) SmallSat mission. This close tiling of the crystal units is achieved by Through-Silicon-Via (TSV) enabled readout ASICs that shrink the readout electronics footprint of the wire-bonded ASICs previously developed and incorporated on the Nuclear Spectroscopic Telescope Array (NuSTAR) mission. To close-tile large numbers of detectors, an efficient die-level ASIC screening method is required for the TSV-ASICs. The ASIC Test Stand (ATS) was developed (Violette et al. 2018, SPIE Proceedings) in order to enable rapid testing of die-level TSV-ASICs by precision alignment of a fixed array of spring-loaded pogo-pin probes to connect to the ASIC's 87 pads with a 225 micron pitch. Here we report ATS design improvements and results from testing ASIC energy resolution and stability using the commandable test pulser internal to the ASIC. Multiple ATS systems will enable rapid testing and selection of ASICs for large area detector arrays as needed for the HREXI SmallSat Prototype (HSP).
The High Resolution Energetic X-ray Imager (HREXI ) is a coded-aperture imaging telescope that utilizes a large closely-tiled array of CdZnTe (CZT) detectors, each 19.9 x 19.9 x 3mm with a 32 x 32 pixel (604μm) for coded aperture X-ray imaging (3 - 200 keV) of cosmic X-ray sources and transients. Each CZT crystal is read out by an ASIC incorporating, for the first time, Through Silicon Vias (TSVs). These TSVs replace the wire bonds for this ASIC, originally designed for the Nuclear Spectroscopic Telescope Array (NuSTAR) focusing hard X-ray telescope. The TSVs allow flip-chip bonding of the ASIC to the PCB board electronics for processing of the data. The new TSV-ASICs will enable closer tiling and larger imaging arrays which require faster, more efficient ASIC testing and calibration at the die level. We have designed and developed an ASIC Test Stand (ATS) for rapid ASIC testing prior to bonding to CZT. We demonstrate how ASIC die-level testing with the ATS can be performed rapidly with rigidly spaced micro-pogo pins supported by an FPGA readout.
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