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 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.
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.
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).
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 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.
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.
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