KEYWORDS: Sensors, Weapons of mass destruction, Homeland security, System identification, Polarization, Platinum, Ions, Electric field sensors, Detector arrays, Defect detection
We present the results from testing over 100 5x5x12 mm3 TlBr detectors configured as 3D position-sensitive virtual Frisch-grid (VFG) detectors with platinum contacts. The primary objective was to comprehensively understand factors limiting performance and long-term response variations in these detectors. The incorporation of 3D position sensitivity allowed us to monitor internal changes in charge collection efficiency after applying voltage, and to correlate them with device performance changes. The biased detectors underwent defect distribution alterations due to electric field-enhanced ion migration. Our results are based on an extensive dataset obtained from TlBr crystals produced by Radiation Monitoring Devices (RMD). These measurements were part of our development of a handheld isotope identifier based on an array of position-sensitive TlBr detectors, supported by the Department of Homeland Security, Countering Weapons of Mass Destruction Office. The majority of the detectors exhibited a common trend of performance improvement within 1-2 weeks, stabilization for some period of time, then a slow degradation; however, some detectors deviated from this pattern.
The ComPair gamma-ray telescope is a technology demonstrator for a future gamma-ray telescope called the All-sky Medium Energy Gamma-ray Observatory (AMEGO). The instrument is composed of four subsystems, a double-sided silicon strip detector, a virtual Frisch grid CdZnTe calorimeter, a CsI:Tl based calorimeter, and an anti-coincidence detector (ACD). The CsI calorimeter's goal is to measure the position and energy deposited from high-energy events. To demonstrate the technological readiness, the calorimeter has flown onboard a NASA scientific balloon as part of the GRAPE-ComPair mission and accumulated around 3 hours of float time at an altitude of 40 km. During the flight, the CsI calorimeter observed background radiation, Regener-Pfotzer Maximum, and several gamma-ray activation lines originating from aluminum.
The NuSTAR (Nuclear Spectroscopic Telescope Array) mission was launched in 2012, and it has successfully deployed the first orbiting telescopes to focus high energy X-ray (3 - 79 keV) light, providing a wealth of new information on high-energy X-rays sources. Follow-up missions, such as the proposed HEX-P, BEST, and FORCE, could perform a deeper black hole census providing a more refined measurement of black hole spins, allowing for greater knowledge about supermassive black holes. These missions are motivated by the recent breakthroughs in the hard X-ray mirror technologies, where mirrors, either made of monolithic silicon segments, or made directly or via replication of shells, demonstrate the feasibility of making hard X-ray mirrors with angular resolutions of 5-10 arc-seconds Half Power Diameter (HPD) compared to the NuSTAR’s 1 arc-minute HPD. Such a high angular resolution requires matched detectors with higher degree of segmentation to fully benefit from the achievable improved spatial resolution. In the above framework, the HEXID ASIC, a novel pixelated front-end suitable for reading out a finely segmented CZT sensor with 150 μm pixel pitch in a hexagonal arrangement has been developed. This readout pixelated chip is capable of processing photon-generated charge packets over a large dynamic range (from 2 keV up to 180 keV), while keeping a low input noise (ENC <20 e-). In this work, the initial characterization of the ASIC prototype will be presented.
KEYWORDS: X-ray telescopes, X-rays, Tunable filters, Signal processing, Hard x-rays, Simulations, Sensors, Analog electronics, Spatial resolution, Design and modelling
The NuSTAR (Nuclear Spectroscopic Telescope Array) mission launched in 2012, and it has successfully deployed the first orbiting telescopes to focus light in the high-energy x-ray range (3 - 79 keV), providing a wealth of new information about the sources of high-energy x-rays. Follow-up missions such as the proposed HEX-P, BEST, and FORCE could perform a deeper black hole census providing a more refined measurement of black hole spins, allowing for greater knowledge about supermassive black holes. These missions are motivated by recent breakthroughs in hard x-ray mirror technologies where mirrors made of monolithic silicon segments and mirrors made directly or through replication of shells demonstrate the feasibility of making hard x-ray mirrors with angular resolutions of five to ten arc seconds Half Power Diameter (HPD) compared to NuSTAR’s one arc minute HPD. Such a high angular resolution requires matched detectors (higher pixel density) to fully benefit from the achievable improved spatial resolution. In the above framework, the development of the HEXID ASIC, embedding is a novel pixelated front-end suitable for reading out a finely segmented CZT sensor, is presented. The required large dynamic range (from 2 keV to 180 keV) and low input noise (ENC ⪅ 20 e−) together with a small pixel size (150 μm) pose several design challenges in chip implementation. The chosen architecture of the front-end circuit and in-pixel processing blocks, together with the readout architecture of the registered signals and other adopted design solutions, driven by the quoted requirements, will be reviewed.
We present the results from testing the performance of CdZnTe (CZT) position-sensitive virtual Frisch-grid (VFG) detectors for gamma-ray imaging. Large-volume CZT detectors with dimensions up to 10x10x30 mm3 recently became available from CZT crystal vendors. Such devices improve detection efficiency and position resolution when integrated into position-sensitive photon counting cameras proposed for nonproliferation, nuclear security, and gamma-ray astronomy. It is important to evaluate the factors affecting the response uniformity and limiting the performance of these detectors. In general, the response non-uniformities could be caused by detector geometries, materials inhomogeneity, and crystal defects. Several techniques have been developed to correct response non-uniformities and improve detector performance. Among them are the high-granularity position-sensitive detectors, which provide the most accurate and robust corrections. Position sensitivity can also be used to reveal response non-uniformities and understand their causes during the detector development or fabrication stages. Here, we describe a technique that we developed for position-sensitive virtual Frisch-grid detectors employing CdZnTe (CZT) and other semiconductors. To illustrate our experimental technique, we measured responses from the selected detectors of different qualities acquired from different vendors and grown by different methods.
TlBr is a promising material for room-temperature semiconductor gamma-ray detectors currently under development by several groups around the world. TlBr has the optimal combination of properties: high atomic number, high density, high mu-tau product, low Fano factor, and lower fabrication cost compared to other materials. The presence of crystal defects and ionic drift-diffusion enchained by the electric field affects the performance of today’s TlBr detectors. As a bias is applied across a detector, a defect distribution inside starts changing due to ion migration. The changes appear to be most pronounced in the first weeks of applying a bias to newly-manufactured crystals during the “conditioning” period. The 3-D position-sensitive detectors provide an opportunity to investigate these processes and their effects on the device performance and on corrections applied to the spectrum. Here, we present results from analyzing response changes in TlBr crystals under applied biases using position-sensitive capacitive Frisch-grid detectors.
This work has been supported by the U.S. Department of Homeland Security, Countering Weapons of Mass Destruction Office, under competitively awarded contract 70RDND18C00000024. This support does not constitute an express or implied endorsement on the part of the Government.
In this paper we describe ProtoGECCO–a prototype instrument for the Galactic Explorer with a Coded aperture mask and Compton Telescope (GECCO). ProtoGECCO is comprised of two main imaging calorimeters. The top calorimeter is an array of cadmium zinc telluride (CZT); the bottom calorimeter is an array of high-light yield Gd3Al2Ga3O12:Ce (GAGG) fingers with silicon photomultiplier (SiPM) readout. The calorimeters are surrounded by a thallium-doped cesium iodide (CsI:Tl) active shield. ProtoGECCO employs the techniques of both coded aperture imaging and a Compton telescope. The main goals of the prototype are to further develop the instrument technology, thereby raising the technical readiness level (TRL), and to fly on a high-altitude balloon from Fort Sumner, NM. The results of this work are directly applicable to future space instruments that require detectors with large area; excellent spatial, energy, and angular resolution; and high detection efficiency. Such future missions will address problems in the MeV domain of gamma-ray astronomy—one of the most underexplored windows on the universe.
We report on the results from testing CdZnTe (CZT) position-sensitive virtual Frisch-grid (VFG) detectors and a prototype of a 16x16 detector array proposed for a high-energy gamma ray imaging space telescope. Previously, we evaluated the spectroscopic performance of these detectors. Here, we present results from our detector performance studies with an emphasis on position resolution. We employed digital waveform capturing and analog ASIC based approaches to read out the signals from the detectors and evaluate their spectral- and spatial-resolution. The VFG arrays allow for the flexibility to scale-up the dimensions of the detectors for the desired efficiency, while the position information allows for correcting the detectors’ response non-uniformities caused by crystal defects and device geometry, thereby reducing the instrument cost and making them more feasible for emerging applications in gamma-ray astronomy, nonproliferation, portal screening and nuclear safeguards, where large
It has been recently suggested that optical interferometers may not require a phase-stable optical link between the stations if instead sources of quantum-mechanically entangled pairs could be provided to them, enabling extra- long baselines and benefiting numerous topics in astrophysics and cosmology. We developed a new variation of this idea, proposing that two photons from different sources could be interfered at two decoupled stations, requiring only a slow classical information link between them. We show that this approach could allow high- precision measurements of the relative astrometry of the two sources, with a basic calculation giving angular precision of 10 µas in a few hours’ observation of two bright stars. We also give requirements on the instrument for these observations, in particular on its temporal and spectral resolution. Finally, we discuss possible technologies for the instrument implementation and first proof-of-principle experiments.
The absolute power of the Linac Coherent Light Source pulses has been measured accurately in the hard X-ray beamline by using simultaneously two detectors: an X-ray Gas Monitor Detector (XGMD) in tandem with a radiometer. From these measurements, we were also able to characterize and calibrate in details our sets of beamline attenuators, in addition to extending an absolute calibration for our beamline intensity monitors. Similarly, we demonstrate that commercial optical power meter has a response in the hard X-ray regime, that can be cross-correlated with the absolute power of the LCLS beam.
The Linac Coherent Light Source (LCLS) poses a number of daunting and often unusual challenges to maintaining X-ray detectors, such as proximity to liquid-sample injectors, complex setups with moving components, intense X-ray and optical laser light, and Electromagnetic Pulse (EMP). The Detector and Sample Environment departments at LCLS are developing an array of engineering, monitoring, and administrative controls solutions to better address these issues. These include injector improvements and monitoring methods, fast online damage recognition algorithms, EMP mapping and protection, actively cooled filters, and more.
KEYWORDS: Prototyping, Sensors, Analog electronics, High dynamic range imaging, Liquid crystal lasers, Photons, Cameras, Signal processing, Image resolution, Free electron lasers
ePix10k is a variant of a novel class of integrating pixel ASICs architectures optimized for the processing of signals in second generation LINAC Coherent Light Source (LCLS) X-Ray cameras. The ASIC is optimized for high dynamic range application requiring high spatial resolution and fast frame rates. ePix ASICs are based on a common platform composed of a random access analog matrix of pixel with global shutter, fast parallel column readout, and dedicated sigma-delta analog to digital converters per column. The ePix10k variant has 100um×100um pixels arranged in a 176×192 matrix, a resolution of 140e- r.m.s. and a signal range of 3.5pC (10k photons at 8keV). In its final version it will be able to sustain a frame rate of 2kHz. A first prototype has been fabricated and characterized. Performance in terms of noise, linearity, uniformity, cross-talk, together with preliminary measurements with bump bonded sensors are reported here.
ePix100 is the first variant of a novel class of integrating pixel ASICs architectures optimized for the processing of signals in second generation LINAC Coherent Light Source (LCLS) X-Ray cameras. ePix100 is optimized for ultra-low noise application requiring high spatial resolution. ePix ASICs are based on a common platform composed of a random access analog matrix of pixel with global shutter, fast parallel column readout, and dedicated sigma-delta analog to digital converters per column. The ePix100 variant has 50μmx50μm pixels arranged in a 352x384 matrix, a resolution of 50e- r.m.s. and a signal range of 35fC (100 photons at 8keV). In its final version it will be able to sustain a frame rate of 1kHz. A first prototype has been fabricated and characterized and the measurement results are reported here.
KEYWORDS: Cameras, Sensors, Liquid crystal lasers, Data acquisition, X-rays, Photons, Free electron lasers, Stanford Linear Collider, Solar concentrators, Imaging systems
The Linear Coherent Light Source (LCLS), a free electron laser operating from 250eV to10keV at 120Hz, is opening windows on new science in biology, chemistry, and solid state, atomic, and plasma physics1,2. The FEL provides coherent x-rays in femtosecond pulses of unprecedented intensity. This allows the study of materials on up to 3 orders of magnitude shorter time scales than previously possible. Many experiments at the LCLS require a detector that can image scattered x-rays on a per-shot basis with high efficiency and excellent spatial resolution over a large solid angle and both good S/N (for single-photon counting) and large dynamic range (required for the new coherent x-ray diffractive imaging technique3). The Cornell-SLAC Pixel Array Detector (CSPAD) has been developed to meet these requirements. SLAC has built, characterized, and installed three full camera systems at the CXI and XPP hutches at LCLS. This paper describes the camera system and its characterization and performance.
Over the past three years NASA Marshall Space Flight Center has been collaborating with Brookhaven National
Laboratory to develop a modular Silicon Drift Detector (SDD) X-Ray Spectrometer (XRS) intended for fine surface
mapping of the light elements of the moon. The value of fluorescence spectrometry for surface element mapping is
underlined by the fact that the technique has recently been employed by three lunar orbiter missions; Kaguya,
Chandrayaan-1, and Chang'e. The SDD-XRS instrument we have been developing can operate at a low energy threshold
(i.e. is capable of detecting Carbon), comparable energy resolution to Kaguya (<150 eV at 5.9 keV) and an order of
magnitude lower power requirement, making much higher sensitivities possible. Furthermore, the intrinsic radiation
resistance of the SDD makes it useful even in radiation-harsh environments such as that of Jupiter and its surrounding
moons.
We present new results from our studies of defects in current single-crystal CdZnTe material. Our previous measurements, carried out on thin (~1 mm) and long (>12 mm) CZT detectors, indicated that small (1-20 μm) Te inclusions can significantly degrade the device's energy resolution and detection efficiency. We are conducting detailed studies of the effects of Te inclusions by employing different characterization techniques with better spatial resolution, such as quantitative fluorescence mapping, X-ray micro-diffraction, and TEM. Also, IR microscopy and gamma-mapping
with pulse-shape analysis with higher spatial resolution generated more accurate results in the areas surrounding the micro-defects (Te inclusions). Our results reveal how the performance of CdZnTe detectors is influenced by Te inclusions, such as their spatial distribution, concentration, and size. We also discuss a model of charge transport through areas populated with Te inclusions.
Te-rich secondary phases existing in CdZnTe (CZT) single crystals degrade the spectroscopic performance of
these detectors. An unpredictable number of charges are trapped, corresponding to the abundance of these
microscopic defects, thereby leading to fluctuations in the total collected charge and strongly affecting the
uniformity of charge-collection efficiency. These effects, observed in thin planar detectors, also were found to
be the dominant cause of the low performance of thick detectors, wherein the fluctuations accumulate along
the charge's drift path. Reducing the size of Te inclusions from a virtual diameter of 10-20 μm down to less
than 5 μm already allowed us to produce Frisch-ring detectors with a resolution as good as ~0.8% FWHM at
662 keV. Understanding and modeling the mechanisms involving Te-rich secondary phases and charge loss
requires systematic studies on a spatial scale never before realized. Here, we describe a dedicated beam-line
recently established at BNL's National Synchrotron Light Source for characterizing semiconductor detectors
along with a IR system with counting capability that permits us to correlate the concentration of defects with
the devices' performances.
The effects of Te precipitates on the performances of CdZnTe nuclear radiation detectors were investigated with
wide X-ray or alpha-particle beams. In these measurements, the degradations in the device performances were correlated
with the areas of high concentrations of Te precipitates aggregated around the crystalline defects or with large-size (>100
μm) individual precipitates. The critical role of the small-size, 1-20 μm in diameter, precipitates was not clearly
established, because of the large size of the particle beams used in prior studies (300-400-μm diameter). Our recent
measurements conducted with a highly collimated, <10 μm, x-ray beam and thin, ~1 mm, CZT crystals proved that even
a single small-size precipitate affects the local charge transport properties in CdZnTe detectors. The next step is to
determine the extent of the potential cumulative effect of randomly distributed Te precipitates on the properties of CZT
detectors. In this work, we report on the modeling of the effects of Te precipitates on measurable characteristics such as
energy resolution, detection efficiency, electron mobility-lifetime product, and others and compare with the experimental
results.
CZT material quality improvement has been achieved by optimizing the crystal growth process. N-type conductivity has
been measured on as-grown, undoped Cd0.9Zn0.1Te. Cd 0.85 Zn 0.15Te crystals have been grown for producing high
resistivity CZT radiation detectors. The best FWHM of 57Co 122KeV spectrum was measured to be 3.7% and (µτ)e was
3x10-3 cm2V-1. The microscopic gamma ray response using a beam size of 10µm has been used to map the entire 4 mm
x 4 mm detector. Several black spots indicating no signal responses were observed while all other areas showed an
average of 65-70% collection efficiency. The black spots suggest that at those locations, the Te precipitates are larger
than 10μm. Detailed microscopic infrared transmission measurement on the sample found that most Te precipitates have
sizes of 4-6μm. Theoretical analysis of the results suggests that singly and doubly ionized TeCdVCd2 might be the
shallow and deep donors previously assigned to TeCd by us.
New results from studies of coplanar-grid CdZnTe (CZT) detectors are presented. The coplanar-grid detectors were investigated by using a highly collimated X-ray beam available at Brookhaven's National Synchrotron Light Source and by applying a pulse-shape analysis. The coplanar-grid detector operates as a single-carrier device. Despite the fact that its operational principle is well known and has been investigated by many groups in the past, we found some new details that may explain the performance limits of these types of devices. The experimental results have been confirmed by extensive computer modeling.
A high-intensity X-ray beam collimated down to a 10-micrometer spot size, available at Brookhaven's National Synchrotron Light Source (NSLS), was employed to perform X-ray mapping to measure the correlation between microscopic defects (precipitates) and variations in the collected charges in long-drift CdZnTe (CZT) detectors. First, we use X-ray diffraction topography (XDT) measurements at the high-energy beamline and IR microscopy to identify the defects distribution and strains in the bulk of CZT crystals. Then, we perform X-ray raster scans of the CZT detectors to measure their responses with 10-micrometer spatial resolution. The brightness of the source allows for good statistics in very short times. Precipitates that were singled out with X-ray scans are locally investigated by applying pulse-shape analysis. The presentation discusses how precipitates affect the device performance.
KEYWORDS: Sensors, Signal detection, X-rays, Shape analysis, Signal attenuation, Clouds, Signal generators, Crystals, Detection and tracking algorithms, Photons
Pulse shape analysis is proved to be a powerful tool to characterize the performance of CdZnTe devices and understand their operating principles. It allows one to investigate the device configurations, electron transport properties, effects governing charge collection, electric-field distributions, signal charge formation, etc. This work describes an application of different techniques based on the pulse shape measurements to characterize pixel, coplanar-grid, and
virtual Frisch-grid devices and understand the electronic properties of CZT material provided by different vendors. We report new results that may explain the performance limits of these devices.
CdZnTe detectors demonstrated great potentials for detection of gamma radiation. However, energy resolution of CdZnTe detectors is significantly affected by uncollected holes which have low mobility and short lifetime. To overcome this deleterious effects upon energy resolution special detector designs have to be implemented. The most practical of them are the small pixel effect device, the co-planar grid device, and the virtual Frisch-grid device. We routinely use a highly collimated high-intensity X-ray beams provided by National Synchrotron Light Source (NSLS) facility at Brookhaven National Laboratory to study of CdZnTe material and performances of the different types of devices on the micron-scale. This powerful tool allows us to evaluate electronic properties of the material, device performance, uniformity of the detector responses, effects related to the device's contact pattern and electric field distribution, etc. In particular, in this paper we present new results obtained from the performance studies of 15 x 15 x 7.5 mm3 coplanar-grid devices coupled to readout ASIC. We observed the effect of the strip contacts comprising the grids on the energy resolution of the coplanar-grid device.
New data regarding performance studies of Frish-grid CdZnTe (CZT) detectors are presented. The Frisch-grid detector configuration under investigation is a bar shaped CZT crystal with teh side surfaces coated with an insulating layer. A Frisch grid is fashioned by inserting the CZT bar into a metallic sleeve, or by depositing the metal directly upon the insulator; hence the semiconductor material does not come in contact with the metal grid. The simple design operates well as a single-carrier-sensitive device. Despite the simplicity of this device, its performance depends on the balanced combinations of several factors, including the bulk and surface conductivity, μτ product, and geometrical aspect ratio. Described are several effects that determine charge collection in such drift devices and, consequently, the performance of the non-contacting Frisch-grid configuration.
The main problems involved in applying Cadmium Zinc Telluride (CZT) to detectors are the crystal perfection required and the difficulty in making reliable surface electrical contacts to the material. Our efforts have focused on the development of interconnect techniques and testing methods which will allow us to explore the interaction of defects with detector properties. Local stoichiometry variations and other local disordering make it very hard to find a systematic correlation between performance and material defects in the macroscopic scale. In order to understand the factors limiting the energy resolution of CZT detectors, our efforts were directed to the area of material characterization and detector testing using the National Synchrotron Light Source (NSLS). NSLS provides us with a highly collimated high intensity X-ray beam, which is employed to investigate micron-scale detector performance mapping and the correlation between microscopic defects and fluctuations in collected charge. Some results were already published and more are presented and correlated to X-ray diffraction topography (XDT) measurements. XDT at the beamline X17B1 is used to investigate more systematically the origins of the mosaicity that can give us information about the defect distribution and strains in bulk CZT crystals.
The characteristics of a defect counting tool, COMNET< based on scattering light measurement, is presented. This prototype supports the development of defect-free EUV blanks. Thanks to new improvements, it becomes possible to detect PSL particles having a diameter as low as 100 nm, with a video CCD camera, on silicon substrates or on EUV blanks. To reach this sensitivity, one of the enhancements consists in a laser irradiation close to 65°. The present configuration and the use of a CCD camera, with a variable exposure time, should lead to the detection of 80 nm PSL particles deposited on silicon substrate. This extrapolation is based on experimental results and on a simple model. To detect 100 nm particle and smaller particles, it is essential to reduce the level of stray light and to increase the signal to noise ratio. In our application, the stray light essentially comes from three sources: the noise induced by the roughness of the sample, the Rayleigh scattering of the atmosphere, and the stray light in the room. The restrictions induced by these phenomena are described in some detail. All the improvements are not only available for the characterization of silicon substrates but also for transparent blank substrates and for EUV mask blanks. The additional noise induced by the tranparent substrate is analyzed. The defects, whatever the compoent sizes and the component shapes can be detected. A cross characterization achieved with a commercial tool on silicon substate is reported. Counting measurements performed on EUV blanks are shown. Furthermore, a more explicit definition of added defects is proposed.
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