Derek S. Bale

A Wave Propagation Method for Conservation Laws and Balance Laws with Spatially Varying Flux Functions

We study a general approach to solving conservation laws of the form qt+f(q,x)x=0, where the flux function f(q,x) has explicit spatial variation. Finite-volume methods are used in which the flux is discretized spatially, giving a function fi(q) over the ith grid cell and leading to a generalized Riemann problem between neighboring grid cells. A high-resolution wave-propagation algorithm is defined in which waves are based directly on a decomposition of flux differences fi(Qi)-f-1(Qi-1) into eigenvectors of an approximate Jacobian matrix. This method is shown to be second-order accurate for smooth problems and allows the application of wave limiters to obtain sharp results on discontinuities. Balance laws

CdZnTe Semiconductor Detectors for Spectroscopic X-ray Imaging

q_t+f(q,x)_x=\psi(q,x)

Wave Propagation Methods for Conservation Laws with Source Terms

are also considered, in which case the source term is used to modify the flux difference before performing the wave decomposition, and an additional term is derived that must also be included to obtain full accuracy. This method is particularly useful for quasi-steady problems close to steady state.

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

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.

Fast High-Flux Response of CdZnTe X-Ray Detectors by Optical Manipulation of Deep Level Defect Occupations

An inhomogeneous system of conservation laws will exhibit steady solutions when flux gradients are balanced by source terms. These steady solutions are difficult for many numerical methods (e.g., fractional step methods) to capture and maintain. Recently, a quasi-steady wave-propagationalgorithm was developed and used to compute near-steady shallow water flow over variable topography. In this paper we extend this algorithm to near-steady flow of an ideal gas subject to a static gravitational field. The method is implemented in the software package CLAWPACK. The ability of this method to capture perturbed quasi-steady solutions is demonstrated with numerical examples.

Fabrication of high-performance CdZnTe quasi-hemispherical gamma-ray CAPture plus detectors

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.

Homogenization theory for the cumulative effect of Te inclusions in CdZnTe radiation detectors

We experimentally investigate the possible correlation between high hole-trap concentrations in wide-bandgap semiconductors and delayed temporal response of high-flux x-ray detector devices to changing photon fluxes. We show that fast photo-current response can be achieved with (1) CdZnTe detectors with high hole mobility-lifetime products, (2) temperature increased detrapping, and (3) constant below-bandgap energy light illumination that modifies the dark defect occupation towards a steady-state with a reduced concentration of active hole traps. This way, the detector signal stabilizes immediately upon flux onset, independent of details of the semiconductors point defect structure. Quasi-instantaneous response stabilization (<; 3 ms) to x-ray flux changes > 107 photons mm-2 s-1 is demonstrated.

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

In this paper we report on the simulation, design, and testing of high performance CdZnTe quasi-hemispherical CAPtureTM Plus radiation detectors. Quasi-hemispherical CdZnTe detectors offer a cost effective alternative to other single-polarity (electron-only) detector configurations such as co-planar grid, pixilated or Frish ring CdZnTe detectors with comparable energy resolution both in the high (>500 keV) and low energy range (<500 keV). We have used the device simulation package eVDSIM to design optimal electrode geometry together with the necessary material selection criteria for charge transport properties of the CdZnTe crystals. A test set of 10x10x5mm3 CAPtureTM Plus detectors has been built using state-of-the art fabrication technology. The measured spectral and efficiency performance of these detectors is compared to the theoretical predictions of simulation. Our results demonstrate that with careful selection of uniform material and high-quality fabrication, this design of CAPtureTM Plus detectors is capable of regularly achieving FWHM @ 81keV of 5%, FWHM @ 122keV of <3%, and FWHM @ 662keV of <2%.

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

In this paper, homogenization theory based on a multiple scale perturbation of the charge-transport equation is used to derive a mathematical framework for modeling the cumulative effect of Te inclusions in radiation detectors based on semi-insulating cadmium zinc telluride (CdZnTe). The derived framework naturally incorporates a wide range of physical models that may posit either a reduced electron lifetime due to enhanced trapping at inclusions, or an altered carrier speed due to a distorted electric field at inclusions, or both. The new framework is applied to a simplified version of the geometric model introduced by Bolotnikov et al. [Nucl. Instrum. Methods Phys. Res. A 571, 687 (2007)], and it is shown that this results in a closed-form approximation to the reduced electron trapping time that depends in a rather simple way on fundamental inclusion parameters such as their mean size and number density. It is also demonstrated that this effective trapping time compares well with previously published si...

Electron transport and charge induction in cadmium zinc telluride detectors with space charge build up under intense 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.