Current-generation solar observatories employ CCD image sensors to observe the Sun in the soft x-ray (SXR) and extreme ultraviolet (EUV) regimes. However, these observations are often compromised by pixel saturation and charge blooming in the CCD image sensors when observing large solar flares. To address these limitations, the Swift Solar Activity x-ray Imager Rocket (SSAXI-Rocket) program is developing CMOS image sensors (CIS) with low noise and high-speed readout (greater than 5Hz) for next-generation solar observatories. These CIS aim to enable the observation of large solar flares while significantly reducing the effects of pixel saturation and charge blooming. As a part of NASA’s 2024 solar flare sounding rocket campaign, the SSAXI-Rocket program demonstrated delta-doped CIS technology in a space environment by operating a novel camera as a sub-payload on board the High-Resolution Coronal Imager (Hi-C) sounding rocket. This paper describes the pre-launch laboratory tests performed with the SSAXI-Rocket CIS to characterize its linearity and soft x-ray spectral resolution.
The Swift Solar Activity X-ray Imager (SSAXI-Rocket) is a ride-along instrument to the High-Resolution Coronal Imager (Hi-C) Flare NASA sounding rocket launch campaign scheduled for the Spring 2024. In the short 5- minute rocket flight, SSAXI-Rocket will measure the soft X-ray near-peak emission phase of a large solar flare of GOES C-class or greater. The SSAXI-Rocket instrument has peak sensitivity to 10 MK solar plasma, similar to the current Hi-C flare extreme ultraviolet instruments, providing the exploration of the variability in heating and energy transport of solar flares. SSAXI-Rocket combines small X-ray focusing optic (Wolter-I) with onaxis imaging resolution of 9 arcseconds or better and high-speed readout CMOS detector, to image the flare soft X-rays at 5 hertz or faster, with minimized image saturation and pixel signal blooming. These high-time cadence measurements can help uncover the soft X-ray intensity variations which can provide constraints on the intermittent heating processes in the flare magnetic loops. SSAXI-Rocket is the testbed for technology that is planned for future heliophysics and astronomy SmallSat, CubeSat, and large satellite X-ray observatories.
Swift Solar Activity X-ray Image (SSAXI-Rocket), mounted on the High-Resolution Coronal Imager (Hi-C) as a sub-payload, is a wide field solar X-ray imager designed to image Solar X-ray flares at high cadence (>5 Hz). SSAXI-Rocket consists of a Wolter-I optic with a focal length of 1 m, coupled with a monolithic CMOS X-ray sensor at the focal plane. The optics for SSAXI-Rocket were fabricated using the Electroformed Ni Replication (ENR) technique at Center for Astrophysics, Harvard-Smithsonian. Each optic has both parabolic and hyperbolic sections with 62 mm diameter at the inflection plane with a total optic length of 18 cm. The performance of the flight and flight spare optic mounted on a spider was measured at the Marshall Space Flight Center (MSFC) Stray Light Testing Facility (SLTF) to characterize the Point Spread Function (PSF) and Effective Area (EA). The flight optic selected for SSAXI-Rocket shows on-axis 16′′ Half Power Diameter (HPD) and 5′′ Full Width Half Maximum (FWHM) at 4.5 keV, exceeding the 23′′ HPD and 9′′ FWHM requirement. The effective area is about 0.64 cm2 at 4.5 keV. Coupled with the fast readout of an X-ray CMOS sensor, this optic enables rapid high-resolution X-ray imaging over a wide field of view (> 20′ x 20′). Here we review the design, fabrication and testing of the SSAXI-Rocket optic and summarize its performance.
The Swift Solar Activity X-Ray Imager (SSAXI-Rocket) sounding rocket experiment is a direct-imaging, soft Xray telescope optimized for the observation of large (GOES C class-X class) solar flares. SSAXI-Rocket has high temporal sampling cadence (> 5 Hz) enabled by a fast-readout CMOS detector. A single Wolter-1 optic focuses light onto the detector plane. The optic has a 15.8′′ half-power diameter (HPD) angular resolution on-axis and an effective area of 0.64 cm2 at 4.5 keV. The SSAXI-Rocket camera reads out a spectrally integrated signal, and the system spatial resolution is designed to be < 16′′ HPD over the instrument field of view (> 55′ × 55′). The detector is a back-illuminated delta doped CMOS (2048 × 2048 pixels) with 10 μm pitch pixels. This manuscript details our instrument design, and overviews the processes employed in telescope alignment, testing, delivery, and integration onto the Hi-C Sounding Rocket. We present the “as-built” projected flight performance of the delivered SSAXI-Rocket flight system, obtained by synthesizing the results of pre-flight subsystem testing and measurements performed during system integration and alignment.
We are developing Wolter X-ray optics compatible with the short focal length and small size suitable for SmallSat missions. These X-ray Optics are compact lightweight Wolter-I X-ray optics which are affordable for low cost future missions. We are leveraging the recent and on-going development to build lightweight Wolter-I X-ray optics based on the electroformed nickel replication (ENR) technique. Recent results will be presented.
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.
Versatile Optics for X-ray Imaging (VOXI) is a technology that enables a wide range of missions and opens up new opportunities for scientific research over multiple disciplines including fundamental physics, heliophysics, astrophysics, lunar and planetary science, and laboratory physics. VOXI is well-suited to SmallSats, which have become powerful platforms from which to conduct leading scientific investigations and cutting-edge technology developments at low cost with rapid turn-arounds. At the Center for Astrophysics | Harvard and Smithsonian, in collaboration with other institutions, we have developed VOXI, a Wolter-I Xray telescope with a focal length of < 1.5 m. In this paper we describe the potential of these optics, and the applications for VOXI optics considered to date.
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 REgolith X-ray Imaging Spectrometer (REXIS) is a soft x-ray spectrometer and the student collaboration instrument aboard NASA’s OSIRIS-REx asteroid sample return mission. REXIS utilizes MIT Lincoln Laboratory CCID-41 x-ray detectors coated with a directly deposited optical blocking filter (OBF) with a thickness of 320 nm. The aluminum coating, developed at MIT Lincoln Laboratory, is designed to block visible light from the detector, to maintain high sensitivity to soft x-rays in the presence of reflected sunlight from the surface of the target asteroid Bennu. The scientific objective for the REXIS instrument is to measure the stimulated x-ray flux fluoresced from Bennu to discern elemental abundances present on the asteroid’s surface. The coating technique applied for blocking visible light had not previously been used on the CCD-41s in an extended space flight mission. The performance of the OBF on the flight detectors was not characterized before and after environmental stress testing. Therefore, to mature the OBF to technology readiness level (TRL) 6, the flight spare detectors were tested while the instrument was on the way to the asteroid. The flight spare hardware underwent vibration and thermal environmental stress testing to test the durability and effectiveness of the OBF. This testing informed our expectations of the in-flight OBF once it reached the asteroid and helped mature the TRL level of this directly deposited OBF. We discuss the setup and results of those tests and address the performance of the flight OBF at the asteroid. We conclude that depositing an aluminum OBF onto the surface of a charge-coupled device is able to withstand stresses of launch and an extended life-mission in interplanetary space.
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.
Wolter-I Optics for SmallSat Astronomy Mission (WOSAM) are a highly adaptable option for SmallSat missions for a number of astronomical uses. These compact Wolter-I optics with focal lengths on the order of 0.5 - 1 m are able to fit within strict mass and volume constraints and can be designed to fit the scientific requirements of exoplanet, solar, and lunar missions. In order to maximize Effective Area the telescope’s collecting area, graze angle, and vignetting need to be balanced. These factors are primarily affected by the optics’ focal length, outer diameter, shell length, and shell spacing. We show the modeling results of three SmallSat missions, the SmallSat Exosphere Explorer of hot Jupiters (SEEJ), the SmallSat Solar Axion and Activity X-ray Imager (SSAXI), and the Lunar X-ray Imaging Spectrometer (LuXIS). These missions have a range of Effective Area, Energy band, and Field of View requirements that can all be met with WOSAM telescopes.
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.
We are developing Wolter-I X-ray optics for use in SmallSat missions. These optics are being designed for telescope focal lengths on the order of 0.5 - 1 m, much shorter than typical Astrophysics missions. The various parameters of the optics module: diameter, length, number nested shells, coatings, etc., depend partly on the spacecraft bus but the final design is driven by the science and instrument requirements of the mission (effective area, resolution, and energy band of interest). Ray trace software was developed and used to project the performance of several optics configurations, which, designed for SmallSat missions, meet the instrument requirements for the SmallSat Exosphere Explorer of hot Jupiters (SEEJ) . Results of this modeling is presented.
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.
Axion is a promising dark matter candidate as well as a solution to the strong charge-parity (CP) problem in quantum chromodynamics (QCD). We describe a new concept for SmallSat Solar Axion and Activity X-ray Telescope (SSAXI) to search for solar axions or axion-like particles (ALPs) and to monitor solar activity over a wide dynamic range. SSAXI aims to unambiguously identify X-rays converted from axions in the solar magnetic field along the line of sight to the solar core, effectively imaging the solar core. SSAXI employs Miniature lightweight Wolter-I focusing X-ray optics (MiXO) and monolithic CMOS X-ray sensors in a compact package. The wide energy range (0.5 - 5 keV) of SSAXI can easily distinguish spectra of axion-converted X-rays from solar X-ray spectra, while encompassing the prime energy band (3 - 4.5 keV) of axion-converted X-rays. The high angular resolution (30 arcsec) and large field of view (40 arcmin) in SSAXI will easily resolve the enhanced X-ray flux over the 3 arcmin wide solar core while fully covering the X-ray activity over the entire solar disc. The fast readout in the inherently radiation tolerant CMOS X-ray sensors enables high resolution spectroscopy over a wide dynamic range with a broad range of operational temperatures. We present multiple mission implementation options for SSAXI under ESPA class. SSAXI will operate in a Sun-synchronous orbit for 1 yr preferably near a solar minimum to accumulate sufficient X-ray photon statistics.
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 first detected exoplanets found were "hot Jupiters"; these are large Jupiter-like planets in close orbits with their host star. The stars in these so-called "hot Jupiter systems" can have significant X-ray emission and the X-ray flux likely changes the evolution of the overall star-planetary system in at least two ways: (1) the intense high energy flux alters the structure of the upper atmosphere of the planet - in some cases leading to significant mass loss; (2) the angular momentum and magnetic field of the planet induces even more activity on the star, enhancing its X-rays, which are then subsequently absorbed by the planet. If the alignment of the systems is appropriate, the planet will transit the host star. The resulting drop in flux from the star allows us to measure the distribution of the low-density planetary atmosphere. We describe a science mission concept for a SmallSat Exosphere Explorer of hot Jupiters (SEEJ; pronounced "siege"). SEEJ will monitor the X-ray emission of nearby X-ray bright stars with transiting hot Jupiters in order to measure the lowest density portion of exoplanet atmospheres and the coronae of the exoplanet hosts. SEEJ will use revolutionary Miniature X-ray Optics (MiXO) and CMOS X-ray detectors to obtain sufficient collecting area and high sensitivity in a low mass, small volume and low-cost package. SEEJ will observe scores of transits occurring on select systems to make detailed measurements of the transit depth and shape which can be compared to out-of-transit behavior of the target system. The depth and duration of the flux change will allow us to characterize the exospheres of multiple hot Jupiters in a single year. In addition, the long baselines (covering multiple stellar rotation periods) from the transit data will allow us to characterize the temperature, flux and flare rates of the exoplanet hosts at an unprecedented level. This, in turn, will provide valuable constraints for models of atmospheric loss. In this contribution we outline the science of SEEJ and focus on the enabling technologies Miniature X-ray Optics and CMOS X-ray detectors.
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.
Wirebonds, although proven for space application and perceived necessary for hybrid sensors like CdZnTe (CZT) detectors, introduce assembly complexity and undesirable gaps between detector units. Thus, they pose a serious challenge in building a low cost large area detector. We are developing Through-Silicon Vias (TSVs) to make all connections (both power and data) through ASICs, which will eliminate wirebonds and enable simple direct flip-chip bonding between the ASIC and a substrate electronics layer. TSVs also enable a more compact layout of the ASIC, which reduces the inactive area of the detector plane, and thus enables nearly gaplessly tilable detector arrays. We demonstrate the first successful TSV implementation on ASICs used for CZT detectors onboard the Nuclear Spectroscopic Telescope Array (NuSTAR) mission as part of our program to develop large area CZT imagers for wide field coded aperture imaging.
OSIRIS-REx is the third spacecraft in the NASA New Frontiers Program and is planned for launch in 2016. OSIRIS-REx will orbit the near-Earth asteroid (101955) Bennu, characterize it, and return a sample of the asteroid’s regolith back to Earth. The Regolith X-ray Imaging Spectrometer (REXIS) is an instrument on OSIRIS-REx designed and built by students at MIT and Harvard. The purpose of REXIS is to collect and image sun-induced fluorescent X-rays emitted by Bennu, thereby providing spectroscopic information related to the elemental makeup of the asteroid regolith and the distribution of features over its surface. Telescopic reflectance spectra suggest a CI or CM chondrite analog meteorite class for Bennu, where this primitive nature strongly motivates its study. A number of factors, however, will influence the generation, measurement, and interpretation of the X-ray spectra measured by REXIS. These include: the compositional nature and heterogeneity of Bennu, the time-variable solar state, X-ray detector characteristics, and geometric parameters for the observations. In this paper, we will explore how these variables influence the precision to which REXIS can measure Bennu’s surface composition. By modeling the aforementioned factors, we place bounds on the expected performance of REXIS and its ability to ultimately place Bennu in an analog meteorite class.
Over the last few decades, grazing incidence X-ray optics have been a pivotal tool for advances in X-ray astronomy. They have been successfully employed in many great observatories such as ROSAT, Chandra X-ray Observatory and XMM-Newton. In planetary science, X-ray observations of Solar system objects are a great tool to understand the nature of the target bodies and the evolutionary history of the Solar system as a whole. To date, X-ray observations in near-target planetary missions have been limited to collimator-based instruments due to tight mass and volume constraints, arising from the multi-instrument nature of planetary missions. In addition, unlike observations of astrophysical sources at virtually infinite distances, near-target observations of planetary bodies introduce a unique set of challenges. While true focusing X-ray optics can overcome these challenges, a practical implementation of focusing X-ray optics for planetary missions depends on the feasibility of compact lightweight X-ray optics. We review scientific motivations for X-ray observations of planetary bodies and illustrate the unique challenges encountered in planetary missions through a few examples. We introduce a new metal-ceramic hybrid technology for X-ray mirrors that can enable compact lightweight Wolter-I X-ray optics suitable for resource limited planetary missions.
The OSIRIS-REx Mission was selected under the NASA New Frontiers program and is scheduled for launch in
September of 2016 for a rendezvous with, and collection of a sample from the surface of asteroid Bennu in 2019.
101955 Bennu (previously 1999 RQ36) is an Apollo (near-Earth) asteroid originally discovered by the LINEAR project in 1999 which has since been classified as a potentially hazardous near-Earth object. The REgolith X-Ray Imaging Spectrometer (REXIS) was proposed jointly by MIT and Harvard and was subsequently accepted as a student led instrument for the determination of the elemental composition of the asteroid's surface as well as the surface distribution of select elements through solar induced X-ray fluorescence. REXIS consists of a detector plane that contains 4 X-ray CCDs integrated into a wide field coded aperture telescope with a focal length of
20 em for the detection of regions with enhanced abundance in key elements at 50 m scales. Elemental surface distributions of approximately 50-200 m scales can be detected using the instrument as a simple collimator. An overview of the observation strategy of the REXIS instrument and expected performance are presented here.
The Energetic X-ray Imaging Survey Telescope (EXIST) is designed to i) use the birth of stellar mass black holes, as
revealed by cosmic Gamma-Ray Bursts (GRBs), as probes of the very first stars and galaxies to exist in the Universe.
Both their extreme luminosity (~104 times larger than the most luminous quasars) and their hard X-ray detectability over
the full sky with wide-field imaging make them ideal "back-lights" to measure cosmic structure with X-ray, optical and
near-IR (nIR) spectra over many sight lines to high redshift. The full-sky imaging detection and rapid followup narrowfield
imaging and spectroscopy allow two additional primary science objectives: ii) novel surveys of supermassive black
holes (SMBHs) accreting as very luminous but rare quasars, which can trace the birth and growth of the first SMBHs as
well as quiescent SMBHs (non-accreting) which reveal their presence by X-ray flares from the tidal disruption of
passing field stars; and iii) a multiwavelength Time Domain Astrophysics (TDA) survey to measure the temporal
variability and physics of a wide range of objects, from birth to death of stars and from the thermal to non-thermal
Universe. These science objectives are achieved with the telescopes and mission as proposed for EXIST described here.
The Infra-Red Telescope is a critical element of the EXIST (Energetic X-Ray Imaging Survey Telescope) observatory.
The primary goal of the IRT is to obtain photometric and spectroscopic measurements of high redshift
(≥6) gamma ray reaching to the epoque of reionization. The photometric and spectral capabilities of the IRT
will allow to use GRB afterglow as probes of the composition and ionization state of the intergalactic medium
of the young universe. A prompt follow up (within three minutes) of the transient discovered by the EXIST
makes IRT a unique tool for detection and study of these events in the infrared and optical wavelength, which
is particularly valuable at wavelengths unavailable to the ground based observatories. We present the results of
the mission study development on the IRT as part of the EXIST observatory.
ProtoEXIST1 is a pathfinder for the EXIST-HET, a coded aperture hard X-ray telescope with a 4.5 m2 CZT
detector plane a 90x70 degree field of view to be flown as the primary instrument on the EXIST mission and
is intended to monitor the full sky every 3 h in an effort to locate GRBs and other high energy transients.
ProtoEXIST1 consists of a 256 cm2 tiled CZT detector plane containing 4096 pixels composed of an 8x8 array
of individual 1.95 cm x 1.95 cm x 0.5 cm CZT detector modules each with a 8 x 8 pixilated anode configured
as a coded aperture telescope with a fully coded 10° x 10° field of view employing passive side shielding and
an active CsI anti-coincidence rear shield, recently completed its maiden flight out of Ft. Sumner, NM on the
9th of October 2009. During the duration of its 6 hour flight on-board calibration of the detector plane was
carried out utilizing a single tagged 198.8 nCi Am-241 source along with the simultaneous measurement of the
background spectrum and an observation of Cygnus X-1. Here we recount the events of the flight and report
on the detector performance in a near space environment. We also briefly discuss ProtoEXIST2: the next
stage of detector development which employs the NuSTAR ASIC enabling finer (32×32) anode pixilation. When
completed ProtoEXIST2 will consist of a 256 cm2 tiled array and be flown simultaneously with the ProtoEXIST1
telescope.
The hard X-ray sky now being studied by INTEGRAL and Swift and soon by NuSTAR is rich with energetic phenomena
and highly variable non-thermal phenomena on a broad range of timescales. The High Energy Telescope (HET) on the
proposed Energetic X-ray Imaging Survey Telescope (EXIST) mission will repeatedly survey the full sky for rare and
luminous hard X-ray phenomena at unprecedented sensitivities. It will detect and localize (<20", at 5σ threshold) X-ray
sources quickly for immediate followup identification by two other onboard telescopes - the Soft X-ray imager (SXI)
and Optical/Infrared Telescope (IRT). The large array (4.5 m2) of imaging (0.6 mm pixel) CZT detectors in the HET, a
coded-aperture telescope, will provide unprecedented high sensitivity (~0.06 mCrab Full Sky in a 2 year continuous
scanning survey) in the 5 - 600 keV band. The large field of view (90° × 70°) and zenith scanning with alternating-orbital
nodding motion planned for the first 2 years of the mission will enable nearly continuous monitoring of the full
sky. A 3y followup pointed mission phase provides deep UV-Optical-IR-Soft X-ray and Hard X-ray imaging and
spectroscopy for thousands of sources discovered in the Survey. We review the HET design concept and report the
recent progress of the CZT detector development, which is underway through a series of balloon-borne wide-field hard
X-ray telescope experiments, ProtoEXIST. We carried out a successful flight of the first generation of fine pixel large
area CZT detectors (ProtoEXIST1) on Oct 9, 2009. We also summarize our future plan (ProtoEXIST2 & 3) for the
technology development needed for the HET.
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed next generation multi-wavelength survey
mission. The primary instrument is a High Energy telescope (HET) that conducts the deepest survey for Gamma-ray
Bursts (GRBs), obscured-accreting and dormant Supermassive Black Holes and Transients of all varieties for immediate
followup studies by the two secondary instruments: a Soft X-ray Imager (SXI) and an Optical/Infrared Telescope (IRT).
EXIST will explore the early Universe using high redshift GRBs as cosmic probes and survey black holes on all scales.
The HET is a coded aperture telescope employing a large array of imaging CZT detectors (4.5 m2, 0.6 mm pixel) and a
hybrid Tungsten mask. We review the current HET concept which follows an intensive design revision by the HET
imaging working group and the recent engineering studies in the Instrument and Mission Design Lab at the Goddard
Space Flight Center. The HET will locate GRBs and transients quickly (<10-30 sec) and accurately (< 20") for rapid
(< 1-3 min) onboard followup soft X-ray and optical/IR (0.3-2.2 μm) imaging and spectroscopy. The broad energy
band (5-600 keV) and the wide field of view (~90º × 70º at 10% coding fraction) are optimal for capturing GRBs,
obscured AGNs and rare transients. The continuous scan of the entire sky every 3 hours will establish a finely-sampled
long-term history of many X-ray sources, opening up new possibilities for variability studies.
The EXIST (Energetic X-ray Imaging Survey Telescope) mission includes the 1.1 m optical Infra-Red Telescope
(IRT) which provides the capability to locate, identify, and obtain spectra of transient events, in particular GRB
afterglows at redshifts up to epoch of reionization. The instrument includes a high spatial resolution imager, low
spectral resolution spectrometer (R~ 30) and high resolution slit spectrometer (R~ 3000). This instrument, with
the observatory's rapid reaction response will quickly identify the GRB afterglow, measure its brightness curves,
redshift, measure spectral characteristics of the afterglows and measure absorption spectra of the intervening
intergalactic medium. With this instrument, high redshift GRBs become important tools for studying the growth
of structure, observing the processes through which the universe is reionized.
The primary instrument of the proposed EXIST mission is a coded mask high energy telescope (the HET),
that must have a wide field of view and extremely good sensitivity. In order to achieve the performance goals
it will be crucial to minimize systematic errors so that even for very long total integration times the imaging
performance is close to the statistical photon limit. There is also a requirement to be able to reconstruct images
on-board in near real time in order to detect and localize gamma-ray bursts, as is currently being done by the
BAT instrument on Swift. However for EXIST this must be done while the spacecraft is continuously scanning
the sky. The scanning provides all-sky coverage and is also a key part of the strategy to reduce systematic errors.
The on-board computational problem is made even more challenging for EXIST by the very large number of
detector pixels (more than 107, compared with 32768 for BAT). The EXIST HET Imaging Technical Working
Group has investigated and compared numerous alternative designs for the HET. The selected baseline concept
meets all of the scientific requirements, while being compatible with spacecraft and launch constraints and with
those imposed by the infra-red and soft X-ray telescopes that constitute the other key parts of the payload. The
approach adopted depends on a unique coded mask with two spatial scales. Coarse elements in the mask are
effective over the entire energy band of the instrument and are used to initially locate gamma-ray bursts. A finer
mask component provides the good angular resolution needed to refine the burst position and reduces the cosmic
X-ray background; it is optimized for operation at low energies and becomes transparent in the upper part of the
energy band where an open fraction of 50% is optimal. Monte Carlo simulations and analytic analysis techniques
have been used to demonstrate the capabilities of the proposed design and of the two-step burst localization
procedure.
We report our progress on the development of pixellated imaging CZT detector arrays for our first-generation balloon-borne
wide-field hard X-ray (20 - 600 keV) telescope, ProtoEXIST1. Our ProtoEXIST program is a pathfinder for the
High Energy Telescope (HET) on the Energetic X-ray Imaging Survey telescope (EXIST), a proposed implementation of
the Black Hole Finder Probe. ProtoEXIST1 consists of four independent coded-aperture telescopes with close-tiled (~0.4
mm gaps) CZT detectors that preserve their 2.5mm pixel pitch. Multiple shielding/field-of-view configurations are
planned to identify optimal geometry for the HET in EXIST. The primary technical challenge in ProtoEXIST is the
development of large area, close-tiled modules of imaging CZT detectors (1000 cm2 for ProtoEXIST1), with all readout
and control systems for the ASIC readout vertically stacked. We describe the overall telescope configuration of
ProtoEXIST1 and review the current development status of the CZT detectors, from individual detector crystal units
(DCUs) to a full detector module (DM). We have built the first units of each component for the detector plane and have
completed a few Rev2 DCUs (2x2 cm2), which are under a series of tests. Bare DCUs (pre-crystal bonding) show high,
uniform ASIC yield (~70%) and ~30% reduction in electronics noise compared to the Rev1 equivalent. A Rev1 DCU
already achieved ~1.2% FWHM at 662 keV, and preliminary analysis of the initial radiation tests on a Rev2 DCU shows
~ 4 keV FWHM at 60 keV (vs. 4.7 keV for Rev1). We therefore expect about ≤1% FWHM at 662 keV with the Rev2 detectors.
One of the key aspects of a detector material for space-borne hard X-ray and gamma-ray telescopes is the rate of
prompt and delayed background events generated inside the material by charged and neutral particles striking the detector. These particles are Cosmic Rays, particles trapped in Earth's magnetic field, and secondaries
from Cosmic Ray interacting with the atmosphere and the spacecraft. Here, we present a preliminary study of
Cadmium Zinc Telluride (CZT) and its behaviour in space environments. We have used the simulation package
MGGPOD to estimate the background of the CZT detectors in the proposed Energetic X-ray Imaging Survey
Telescope (EXIST) for possible orbital parameters. The EXIST mission will make use of ~6 m2 of >0.5 cm
thick CZT detectors to record cosmic X-rays in the energy range from 10 keV to 600 keV. The detectors will be
shielded by a fully or partly active shield. For the specific detector and shielding geometry considered here and
an orbit with a low (7°) inclination, the background rate is dominated by diffuse extragalactic photons below
~200 keV. Prompt and delayed hadronic backgrounds grow increasingly important above this energy, becoming
the main contributors to the total background above ~1 MeV. A fully active shield performs slightly better than
a half active/half passive shield.
We describe the detector development for a balloon-borne wide-field hard X-ray (20-600 keV) telescope, ProtoEXIST.
ProtoEXIST is a pathfinder for both technology and science of the proposed implementation of the Black Hole Finder
Probe, Energetic X-ray Imaging Survey telescope (EXIST). The principal technology challenge is the development of
large area, close-tiled modules of imaging CZT detectors (1000 cm2 for ProtoEXIST1). We review the updates of the
detector design and package concept for ProtoEXIST1 and report the current development status of the CZT detectors,
using calibration results of our basic detector unit - 2 x 2 x 0.5 cm CZT crystals with 2.5 mm pixels (8 x 8 array). The
current prototype (Rev1) of our detector crystal unit (DCU) shows ~4.5 keV electronics noise (FWHM), and the
radiation measurements show the energy resolution (FWHM) of the units is 4.7 keV (7.9%) at 59.5 keV, 5.6 keV (4.6%)
at 122 keV, and 7.6 keV (2.1%) at 356 keV. The new (Rev2) DCU with revised design is expected to improve the
resolution by ~30%.
KEYWORDS: Sensors, Signal to noise ratio, Telescopes, Coded aperture imaging, Crystals, Device simulation, Monte Carlo methods, Image compression, Hard x-rays, Photons
EXIST is being studied as the Black Hole Finder Probe, one of the 3 Einstein Probe missions under NASA's Beyond Einstein program. The major science goals for EXIST include highly sensitive full-sky
hard X-ray survey in a very wide energy band of 5 - 600 keV. The scientific requirements of wide energy band (10-600 keV for the High Energy Telescope considered for EXIST) and large field of view (approximately 130° × 60° in the current design, incorporating an array of 18 contiguous very large area coded aperture telescopes) presents significant imaging challenges. The requirement of achieving high imaging sensitivity puts stringent limits on the uniformity and knowledge of systematics for the detector plane. In order to accomplish the ambitious scientific requirements of EXIST, it is necessary to implement many novel techniques. Here we present the initial results of our extensive Monte-Carlo simulations of coded mask imaging for EXIST to estimate the performance degradation due to various factors affecting the imaging such as the non-ideal detector plane and bright partially coded sources.
We outline our plan to develop ProtoEXIST, a balloon-borne prototype experiment for the Energetic X-ray Imaging Survey Telescope (EXIST) for the Black Hole Finder Probe. EXIST will consist of multiple wide-field hard X-ray coded-aperture telescopes. The current design of the EXIST mission employs two types of telescope systems: high energy telescopes (HETs) using CZT detectors, and low energy telescopes (LETs) using Si detectors. With ProtoEXIST, we will develop and demonstrate the technologies required for the EXIST HETs. As part of our development efforts, we also present recent laboratory measurements of the spectral response and efficiency variation of imaging CZT detectors on a fine scale (~0.5 mm). The preliminary results confirm the need for multi-pixel readouts and small inter-pixel gaps to achieve uniform spectral response and high detection efficiency across detectors.
KEYWORDS: Sensors, Crystals, Imaging systems, X-rays, Telescopes, Monte Carlo methods, Hard x-rays, X-ray telescopes, Signal to noise ratio, Electronics
The proposed black-hole finder mission EXIST will consist of multiple wide-field hard X-ray coded-aperture telescopes. The high science goals set for the mission require innovations in telescope design. In particular, wide energy band coverage and fine angular resolution require relatively thick coded masks and thick detectors compared to their pixel size, which may introduce mask self-collimation and depth-induced image blurring with conventional design approaches. Previously we proposed relatively simple solutions to these potential problems: radial hole for mask selfcollimation and cathode depth sensing detector for image blurring. We have now performed laboratory experiments to explore the potential of these two techniques. The experimental results show that the radial hole mask greatly alleviates mask self-collimation and a ~1 mm resolution depth-sensitive detector scheme can be relatively easily achieved for the large scale required for EXIST.
We report our in-depth study of Cd-Zn-Te (CZT) crystals to determine an optimum pixel and guard band configuration for Hard X-ray imaging and spectroscopy. We tested 20x20x5mm crystals with 8x8 pixels on a 2.46mm pitch. We have studied different types of cathode / anode contacts and different pixel pad sizes. We present the measurements of leakage current as well as spectral response for each pixel. Our I-V measurement setup is custom designed to allow automated measurements of the I-V curves sequentially for all 64 pixels, whereas the radiation properties measurement setup allows for interchangeable crystals with the same XAIM3.2 ASIC readout from IDEAS. We have tested multiple crystals of each type, and each crystal in different positions to measure the variation between individual crystals and variation among the ASIC channels. We also compare the same crystals with and without a grounded guard band deposited on the crystal side walls vs. a floating guard band and compare results to simulations. This study was carried out to find the optimum CZT crystal configuration for prototype detectors for the proposed Black-Hole Finder mission, EXIST.
KEYWORDS: Sensors, Image resolution, Spatial resolution, Electrodes, Electrons, Astronomy, X-ray telescopes, Signal detection, X-rays, Signal to noise ratio
Determination of the photon interaction depth offers numerous advantages for an astronomical hard X-ray telescope. The interaction depth is typically derived from two signals: anode and cathode, or collecting and non-collecting electrodes. We present some preliminary results from our depth sensing detectors using only the anode pixel signals. By examining several anode pixel signals simultaneously, we find that we can estimate the interaction depth, and get sub-pixel 2-D position resolution. We discuss our findings and the requirements for future ASIC development.
Measuring the depth of interaction in thick Cadmium-Zinc-Telluride (CZT) detectors allows improved imaging and spectroscopy for hard X-ray imaging above 100 keV. The Energetic X-ray Imaging Survey Telescope (EXIST) will employ relatively thick (5 - 10 mm) CZT detectors, which are required to perform the broad energy-band sky survey. Interaction depth information is needed to correct events to the detector "focal plane" for correct imaging and can be used to improve the energy resolution of the detector at high energies by allowing event-based corrections for incomplete charge collection. Background rejection is also improved by allowing low energy events from the rear and sides of the detector to be rejected. We present experimental results of intereaction depth sensing in a 5 mm thick pixellated Au-contact IMARAD CZT detector. The depth sensing was done by making simultaneous measurements of cathode and anode signals, where the interaction depth at a given energy is proportional to the ratio of cathode/anode signals. We demonstrate how a simple empirical formula describing the event distributions in the cathode/anode signal space can dramatically improve the energy resolution. We also estimate the energy and depth resolution of the detector as a function of the energy and the interaction depth. We also show a depth-sensing prototype system currently under development for EXIST in which cathode signals from 8, 16 or 32 crystals can be read-out by a small multi-channel ASIC board that is vertically edge-mounted on the cathode electrode along every second CZT crystal boundary. This allows CZT crystals to be tiled contiguously with minimum impact on throughput of incoming photons. The robust packaging is crucial in EXIST, which will employ very large area imaging CZT detector arrays.
Imaging at hard X-ray energies (~10-600 keV) over very large fields of view (~60° per telescope) is required to conduct a high sensitivity all-sky and all-time survey for black holes. The proposed Energetic X-ray Imaging Survey Telescope (EXIST) could achieve the high sensitivity required for the mission science objectives by scanning an array of wide-field coded aperture telescopes with aperture mask holes radially aligned to minimize auto-collimation by the thick (~7mm) masks required for high energy imaging. Simulation results from a preliminary design study are reported which quantify the improvement in off-axis imaging sensitivity vs. the conventional case with mask holes all perpendicular to the mask. Such masks can be readily constructed from a stacked laminate of thin (1mm) laser-etched W sheets. An even more dramatic increase in coded aperture imaging sensitivity, and dynamic range, for a realistic telescope and imaging detector with typical systematic errors can be achieved by continuously scanning the field of view of the telescope over the source region to be imaged. Simulation results are reported for detectors with systematic errors 1-10%, randomly distributed but unknown in each detector pixel. For the simplified case of a 1-D coded aperture telescope scanning along its pattern, the systematics are removed identically. Results are also presented for the 2-D case with both 1-D and partial 2-D scanning which demonstrate the feasibility of a coded aperture scanning telescope with systematic errors achieving nearly Poisson-limited sensitivity for signal/background ratios S/B ~ 10-4, in constrast to limits typically ~10-100X worse that have been actually achieved by pointed or dithered coded aperture telescopes flown (or proposed) previously.
The importance of shields for suppressing neutron-induced background in new classes of (gamma) -ray detectors such as CZT has been emphasized for a variety of reasons. These include high cross-sections for neutron interactions in detector materials and inefficient vetoing of neutrons in conventional active shields. We previously demonstrated through Monte-Carlo simulation how our new approach, supershields, is superior to the monolithic, biatomic neutron shields which have been developed in the past. Here we show the construction of several prototype models for supershields using B and H. We verify the performance of these supershields through lab experiments using radioactive sources and monoenergetic neutron beam at the Radiological Research Accelerator Facility, and compare the results with monolithic neutron shields. We also discuss the implications of this experiment for designs of supershields in general.
We describe research on developing more effective means to shield gamma-ray detectors from neutron-induced background. Conventional active shields are very inefficient in vetoing neutrons. Neutron-induced background is exacerbated in new classes of gamma-ray detectors such as CZT, which have large neutron interaction cross-sections. An approach to this problem is supershields, which contain separate layers to moderate and absorb neutrons. We show how supershields are superior to monolayer neutron shields which have been studied previously.
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