Gradient sparsity regularization is an effective way to mitigate artifacts due to sparse-view sampling or data noise in computed tomography (CT) image reconstruction. The effectiveness of this type of regularization relies on the scanned object being approximately piecewise constant. Trabecular bone tissue is also technically piecewise constant, but the fine internal structure varies at a spatial scale that is smaller than the resolution of a typical CT scan; thus it is not clear what form of sparsity regularization is most effective for this type of tissue. In this conference submission, we develop a pixel-sparsity regularization model, which is observed to be effective at reducing streak artifacts due to sparse-view sampling and noise. Comparison with gradient sparsity regularization is also shown.
The change in bulk resistivity of CdZnTe (CZT) crystals was measured during infrared (IR) light between 950 and 1000 nm. The crystals are grown using one of the state-of-the-art methods either the traveling heating method or the modified Bridgman method. The change resistivity was evaluated using the steady-state current with and without light. Additionally, the change in current with both IR sources were correlated to the influence of secondary phases (SP) in each crystal using IR transmission microscopy to determine whether the number and size of the impurities has a drastic effect based on the current-voltage (IV) characteristics. SP at various depths within CZT are connected to the existence of variable depth, IR-excitable traps that lie within the bandgap. The release of these traps will significantly affect the overall current in the system. However, the current increase may not match the overall energy of the light utilized are more dependent on the size and quantity for each energy range.
Cadmium Zinc Telluride (CdZnTe/CZT) crystals were grown using a modified vertical Bridgman growth technique with
10 % Zn concentration at Washington State University (WSU). Analyses of the effects of volume (vol.) %, number
density (cm-3), mean diameter (μm) of secondary phases (SPs) and thickness (mm) of the CZT crystals on single crystal
properties such as carrier mobility lifetime (μτe) and resistivity (ρ) were performed. Some correlations were observed
between μτe values of different CZT crystals and vol. %, number density, mean diameter of SPs and thickness of the
crystals. High μτe and lower SP vol. % values were obtained for the ingots grown with rapid cool down times and with
no intentional amounts of excess Te/Cd. For the selected samples, the effects of the SPs on the μτe values were
established for the SPs whose mean diameters were ≤4 μm and >4 μm. These studies indicate vol. % and mean diameter
of SPs are the important parameters for CZT crystal performance as a radiation detector.
CdZnTe (CZT) semiconducting crystals are of interest for use as room temperature X- and γ-ray spectrometers. Several
studies have focused on understanding the various electronic properties of these materials, such as the surface and bulk
resistivities and the distribution of the electric field within the crystal. Specifically of interest is how these properties are
influenced by a variety of factors including structural heterogeneities, such as secondary phases (SPs) and line defects as
well as environmental effects. Herein, we report the bulk current, surface current, electric field distribution and
performance of a spectrometer-grade CZT crystal exposed to above band-gap energy illumination.
Semiconducting CdZnTe or "CZT" crystals are very suitable for use as a room temperature-based gamma
radiation spectrometer. During the last decade, modifications in growth methods for CZT have significantly improved
the quality of the produced crystals however there are material features that can influence the performance of these
materials as radiation detectors. For example, various structural heterogeneities within the CZT crystals, such as, pipes,
voids, polycrystallinity, and secondary phases (SP) can have a negative impact on the detector performance. In this
study, a CZT material was grown by the modified vertical Bridgman growth (MVB) method with zone leveled growth
in the absence of excess Te in the melt. Numerous SP were imaged using transmission IR at a volume % of 0.002.
Samples from this material were analyzed using various analytical techniques to evaluate its electrical properties, purity
and detector performance as radiation spectrometers and to determine the morphology, dimension and elemental
/structural composition of one of the SP in this material. This material was found to have a high resistivity and good
radiation spectrometer performance. It had SPs that were rich in calcium (Ca), carbon (C) and oxygen (O) (possibly
CaCO3) or only C and O that were 5 μm or less in diameter.
CdZnTe or "CZT" crystals are highly suitable for use as a room temperature based spectrometer for the
detection and characterization of gamma radiation. Over the last decade, the methods for growing high quality CZT
have improved the quality of the produced crystals however there are material features that can influence the
performance of these materials as radiation detectors. For example, various structural heterogeneities within the CZT
crystals, such as twinning, pipes, grain boundaries (polycrystallinity), and secondary phases (SP) can have a negative
impact on the detector performance. In this study, a CZT material was grown by the modified vertical Bridgman
growth (MVB) method with zone leveled growth without excess Te in the melt. Visual observations of material from
the growth of this material revealed significant voids and SP. Samples from this material were analyzed using various
analytical techniques to evaluate its electrical properties, purity and detector performance as radiation spectrometers and
to determine the morphology, dimension and elemental /structural composition of one of the SP in this material. This
material was found to have a high resistivity but poor radiation spectrometer performance. It had SP that were rich in
polycrystalline aluminum oxide (Al2O3), metallic Te and polycrystalline CdZnTe and 15 to 50 μm in diameter. Bulk
elemental analyses of sister material from elsewhere in the boule did not contain high levels of Al so there is
considerable elemental impurity heterogeneity within the boule from this growth.
Semi-conducting CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. CZT
shows great promise for use as a gamma radiation spectrometer. However, its performance is adversely
affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning,
pipes, grain boundaries (polycrystallinity), secondary phases and in some cases, damage caused by external
forces. One example is damage that occurs during characterization of the surface by a laser during Raman
spectroscopy. Even minimal laser power can cause Te enriched areas on the surface to appear. The Raman
spectra resulting from measurements at moderate intensity laser power show large increases in peak intensity
that is attributed to Te. Atomic Force Microscopy (AFM) was used to characterize the extent of damage to the
CZT crystal surface following exposure to the Raman laser. AFM data reveal localized surface damage in the
areas exposed to the Raman laser beam. The degree of surface damage to the crystal is dependent on the laser
power, with the most observable damage occurring at high laser power. Moreover, intensity increases in the Te
peaks of the Raman spectra are observed even at low laser power with little to no visible damage observed by
AFM. AFM results also suggest that exposure to the same amount of laser power yields different amounts of
surface damage depending on whether the exposed surface is the Te terminating face or the Cd terminating face
CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. This large band gap material
shows great promise for use as a gamma radiation spectrometer. Historically, the performance of CZT has typically been
adversely affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning,
pipes, grain boundaries (polycrystallinity) and secondary phases (SP). The synthesis of CZT material has improved
greatly with the primary performance limitation being attributed to mainly SP. In this presentation, we describe the
extensive characterization of detector grade material that has been treated with post growth annealing to remove the SPs.
Some of the analytical methods used in this study included polarized, cross polarized and transmission IR imaging, I-V
curves measurements, synchrotron X-ray topography and electron microscopy.