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.
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
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
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.