Stephen A. Soldner

CdZnTe Semiconductor Detectors for Spectroscopic X-ray Imaging

Next generation high-flux X-ray imaging technology is expected to advance towards multi-color or spectroscopic imaging and will significantly expand the capabilities of the technique in a multitude of applications. Spectroscopic X-ray imaging will require energy-sensitive detector arrays. In this work we evaluated the applicability of pulse-mode CdZnTe detector arrays to high-flux spectroscopic imaging. To study the material and device performance limitations of currently available CdZnTe detectors under high-flux X-ray irradiation we designed a 2D monolithic CdZnTe test array and associated test system. The detector arrays were 16 times 16 pixel devices with 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times 3.0 mm CdZnTe single crystals. We measured the high-flux performance of over 1200 such arrays with various bulk CdZnTe crystal properties using a 120 kVp X-ray source and our custom built test system. We studied the various static and dynamic charge collection effects typically not observed in low-flux applications. These included dynamic polarization, static charge steering and dynamic lateral polarization and charge steering. In parallel with the experimental effort we developed a dynamic charge transport and trapping model to describe the experimentally observed static, dynamic and transient phenomena. For the first time we demonstrated > 15 times 106 counts/s/mm2 count-rate for several hundred such CdZnTe detector arrays. In addition we demonstrated good < 1% short term count-rate stability of the detector arrays.

Interfacial chemistry and the performance of bromine-etched CdZnTe radiation detector devices

The interfacial chemistry and composition of Pt electrodes sputter deposited on bromine-etched CdZnTe surfaces was studied by X-ray photoelectron spectroscopy. The interfacial composition of a functioning and a nonfunctioning CdZnTe detector shows significant differences. The degree of cation out-diffusion into the Pt overlayer and the in-diffusion of Pt into the CdZnTe correlate with the degree of oxidation found at the metal-semiconductor interface. Most of the oxide present at the interface was found to be TeO/sub 2/. The results suggest that the interdiffusion of the atoms and associated charges contribute to stoichiometric variations at the metal-semiconductor interface and influence the electrical performance of the devices.

Ultra High Flux 2-D CdZnTe Monolithic Detector Arrays for X-Ray Imaging Applications

The performance of 2-D CdZnTe monolithic detector arrays designed for high flux X-ray imaging applications was studied. For the first time we have obtained 5 times 106 counts/s/mm2 count-rate for a CdZnTe pixelated detector array. This count-rate is more than twice the highest count-rate ever achieved using a CdZnTe detector array. Such excellent performance was demonstrated for more than 600 individual CdZnTe detector arrays. The 2-D CdZnTe monolithic arrays were 16 x 16 pixel devices with 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times 3.0 mm CdZnTe single crystals grown by the high-pressure, electro-dynamic gradient freeze technique. The CdZnTe detector arrays were bonded to a ceramic substrate with the Z-bondtrade technique. This enabled performance testing of the individual detector arrays before bonding to the read-out ASIC chip. The detector arrays were characterized in a custom designed test system. The measurement and data acquisition system consisted of a 16 times 16 pin probe head and 256-channel read-out electronics controlled by a host PC. We utilized our 8-channel fast bipolar ASIC chip and computer controlled 120 kVp X-ray source. In order to measure the true throughput of the CdZnTe devices a counts correction method was developed and implemented that compensates for the counting system non-linearity caused by pile-up and amplifier shaping time effects. Survey of detector array performance as a function of CdZnTe charge transport properties showed that the maximum achievable count-rate of these detectors strongly depends on the hole charge transport properties of the crystals.

Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and 4 Vesta

We present the design of the gamma-ray and neutron spectrometer (GR/NS) for Dawn, which is a NASA Discovery-class mission to explore two of the largest main-belt asteroids, 1 Ceres and 4 Vesta, whose accretion is believed to have been interrupted by the early formation of Jupiter. Dawn will determine the composition and structure of these protoplanetary bodies, providing context for a large number of primitive meteorites in our sample collection and a better understanding of processes occurring shortly after the onset of condensation of the solar nebula. The Dawn GR/NS design draws on experience from the successful Lunar Prospector and Mars Odyssey missions to enable accurate mapping of the surface composition and stratigraphy of major elements, radioactive elements, and hydrogen at both asteroids. Here, we describe the overall design of the GR/NS and compare the expected performance of the neutron spectrometer subsystem to the neutron spectrometer on Mars Odyssey. We also describe radiation damage studies carried out on CdZnTe detectors, which will be components of the primary gamma-ray spectrometer on Dawn. We conclude that provisions for annealing at moderate temperatures (40/spl deg/C to 60/spl deg/C) must be made to ensure that the spectrometer will function optimally over the nine-year mission.

CdZnTe gamma ray spectrometer for orbital planetary missions

We present the design and analysis of a new gamma ray spectrometer for planetary science that uses an array of CdZnTe detectors to achieve the detection efficiency needed for orbital measurements. The use of CdZnTe will provide significantly improved pulse height resolution relative to scintillation-based detectors, with commensurate improvement in the accuracy of elemental abundances determined by gamma ray and neutron spectroscopy. The spectrometer can be flown either on the instrument deck of the spacecraft or on a boom. For deck-mounted systems, a BGO anticoincidence shield is included in the design to suppress the response of the CdZnTe detector to gamma rays that originate in the spacecraft. The BGO shield also serves as a backup spectrometer, providing heritage from earlier planetary science missions and reducing the risk associated with the implementation of new technology.

A mechanism for dynamic lateral polarization in CdZnTe under high flux x-ray irradiation

It has been observed that pixillated CdZnTe detectors fabricated from crystals with low hole transport properties (μhτh<10−5cm2V−1) experience a dynamic lateral polarization when exposed to a high flux of x-rays. In this effect, counts are transferred from pixels near the edge of the irradiated region to pixels in the interior. In this letter, we propose a mechanism capable of explaining the observed dynamical effect. The mechanism is based on a transverse electric field that is generated due to space charge that builds within the material. This transverse field, in turn, is responsible for the altered carrier trajectories toward the center of the irradiated region.

Dynamic Lateral Polarization in CdZnTe Under High Flux X-Ray Irradiation

The dynamic lateral polarization and charge steering effect was studied in 2D pixilated CdZnTe monolithic detector arrays designed for high flux X-ray imaging applications. While these detectors have shown the ability to work at 15 times 106 counts s-1 mm-2 and higher count-rates in pulse mode, we observed some detectors that exhibited a dynamic lateral polarization and charge steering effect causing non-uniform spatial response to the radiation field. The dynamic nature of the effect is shown by its flux dependence and reversibility upon changing the X-ray flux without a requirement to turn off the bias voltage. The effect causes the induced charge that would normally move from the cathode towards the anode to instead move laterally causing counts to shift away from a flux boundary. We show that the effect is not related to the physical boundary of the detector but rather related to the boundary of the irradiated area of the device. The dynamic polarization and charge steering effect can be attributed to the limited hole transport in the bulk material causing a buildup of a dynamic space-charge region under the irradiated area. The resulting lateral (perpendicular to the irradiation direction) electric field causes the lateral drift (steering) of the X-ray injected charge clouds. The static version of such lateral steering is often observed for charged structural defects in CdZnTe crystals. The studied 2D CdZnTe monolithic arrays were 16 times16 pixel devices having 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times3.0 mm CdZnTe single crystals grown by the high-pressure electro-dynamic gradient freeze technique. The devices were probe tested in a system consisting of a custom 16 times16 pin probe head, 256 channel read-out electronics utilizing 8-channel fast bipolar ASIC chips, and a computer controlled 120 kVp X-ray source.

Characterization of the charge transport uniformity of CdZnTe crystals for large-volume nuclear detector applications

Mounting evidence indicates that charge transport nonuniformity is one of the most important obstacles limiting the energy resolution of large-volume (/spl ges/1 cm/sup 3/) CdZnTe nuclear detectors. Better understanding of the defects responsible for charge transport nonuniformity is needed to develop advanced crystal growth processes producing CdZnTe crystals with more uniform defect distribution and, hence, more uniform charge transport. Better material screening techniques are also required to determine the charge transport uniformity of the CdZnTe crystals at the ingot or slice level in order to maximize the detector fabrication yields in manufacturing large-volume CdZnTe nuclear detectors. Here, we present results from a program aimed at implementing a defect and charge transport uniformity characterization process to provide feedback to crystal growth process development efforts and material selection for large-volume CdZnTe detector manufacturing. We first focused on recent results suggesting that structural nonuniformity of CdZnTe crystals around Te inclusions is responsible for electron transport nonuniformity and diminished energy resolution of large-volume CdZnTe coplanar-grid detectors. The spatial size and density distribution of Te inclusions was measured with infra-red microscopy. A charge transport mapping station employing a collimated alpha source was used to study the uniformity of carrier transport in CdZnTe crystals.

Characterization of an ASIC for CPG sensors with grid-only depth of interaction sensing

A new version of the front-end application specific integrated circuit (ASIC) for co-planar-grid (CPG) sensors is presented. Compared to the first version, the ASIC is optimized for signal/noise performance, modified in peaking time, and it implements timing signals to measure the depth of interaction using the grids signals only. Experimental results using state-of the-art CPG sensors and different approaches for reducing the error due to electron trapping are reported. A new technique that makes use of the sum and difference of the grids signals is also presented.

CdZnTe Detectors Operating at X-ray Fluxes of 100 Million

CdZnTe single crystals grown with the traveling heater method have been specifically engineered to support high-flux x-ray photon counting applications. Pixelated detector devices absorb hardened 120 kVp fluxes up to 108 photons/(mm2·sec) without any sign of electric field polarization at room temperature (23-28) °C. It is shown that detector functionality can be verified using photon counting electronics even under conditions of strong pulse pile-up. A non-paralyzable counting model satisfactory describes the average output count rate of the detector-electronics system as a function of the absorbed flux. Good pixel-to-pixel count rate uniformity can be achieved.