CdZnTe (CZT) has been the material of choice and a successful commercial material for X-ray and gamma-ray radiation detector applications. However, despite continued research for last three decades, CZT still contains a high density of performance-limiting defects and compositional inhomogeneity in as-grown ingots. Recently, one option has been explored to add a small amount of selenium (Se) in the CZT matrix to improve compositional homogeneity and mitigate many performance-limiting defects. As a result, the quaternary CdxZn1−xTeySe1−y (CZTS) is emerging as a next-generation compound semiconductor for room-temperature gamma-ray detector applications. Nevertheless, it is well known that adding selenium in CdTe/CZT matrix imposes severe alloy/lattice disorder in the ternary and quaternary compounds, and the bowing of the bandgap with an increased Se content is the result of lattice disorder. Hence, the effect of lattice disorder is expected to degrade the crystalline quality of the material. In this presentation, we report our effort on evaluating the crystalline quality of THM-grown CZTS crystals with the optimized composition (Cd0.9Zn0.1Te0.98Se0.02). The as-grown samples were characterized by the low-temperature photoluminescence (PL) experiments and high-resolution X-ray diffraction using a synchrotron light source at Brookhaven National Laboratory (BNL). The full width at half maximum (FWHM) of both the PL and the X-ray rocking curve were observed to be broadened due to the lattice disorder of the quaternary compound, eventually degrading the crystalline quality. This was confirmed through Density Functional Theory (DFT) calculations.
Radiation detectors, especially for X- and gamma-rays, are being developed rapidly that utilize the advantages of semiconductor detectors operating at room temperature. CdTe and CdZnTe based detectors have successfully dominated the commercial market. Both the materials, however, face limitations due to the presence of high concentrations of intrinsic defects such as Te inclusions and sub-grain boundaries. In the recent years, we have observed that the addition of selenium circumvents many issues pertaining to CdTe/CdZnTe. As a result, the new quaternary material Cd1−xZnxTe1−ySey (CZTS) is emerging as a next-generation room temperature radiation detector material with the potential to supersede CdZnTe, competing in both cost and detector performance. In this presentation we will discuss the path toward optimization of the composition of the quaternary compound for the best detector performance.
Enormous effort has been exerted on research and development of CdZnTe (CZT) over the past two decades, as well as the pursuit of an alternative material to mitigate the disadvantages in today’s CZT material or provide comparable device performance at a lower cost of production. Although the quality of CdZnTe crystals has been improved drastically over the past few years and the material cost has steadily decreased, the yield of large-volume high-quality detector-grade CZT continues to be an issue due to its poor thermo-physical properties. TlBr was found to be a promising material to compete with CZT, but the contact degradation and device stability are still big issues and severely hinder the deployment of commercial TlBr-based devices for nonproliferation and national security applications. At BNL, we are developing a new compound Cd1-xZnx Te1-y Sey (CZTS) that holds promise as a potentially viable crystal for the replacement of CZT for some radiation detection and imaging applications. The addition of Se in the CZT compound has been found to be very effective in a drastic reduction of the sub-grain boundary network, leading to better compositional and charge-transport homogeneity. The new material has tremendous potential to increase the yield of high-quality detectors at a much lower cost of production. The reduction of the sub-grain boundary network can result in detectors with a lower voltage operation and increased detector thickness. Our efforts to develop CZTS for X- and gamma-ray radiation detector applications will be discussed in detail.
Intrinsic materials can offer advantages over doped materials for some important applications. The doped material might suffer from non-uniform distribution of the dopant, such as fine-scale striations and larger scale segregation, which might affect the overall device response, especially for large-volume detectors such as those in demand for homeland security applications for gamma spectroscopy. Cs2LiCeCl6 (CLCC), being an intrinsic scintillator, can be grown in large volume to produce large detectors with good performance, provided the crystals are free from unwanted scattering centers. CLCC belongs to the elpasolite family and the structure is cubic, so large-volume ingots can be grown without the strains resulting from anisotropic thermal expansion coefficients. In this presentation, we will discuss extensive material characterization and device response of CLCC for gamma and thermal neutron detector applications.
Alloying of CdZnTe (CZT) with selenium has been found to be very promising and effective in reducing the overall concentration of secondary phases (Te precipitates/inclusions) and sub-grain boundary networks in the crystals. These two types of defects are the main causes for incomplete charge collection, and hence they affect the yield of high-quality CZT, resulting in a very high cost for large-volume, high-quality detector-grade CZT detectors. The addition of selenium was also found to very effective in increasing the compositional homogeneity along the growth direction of the CdZnTeSe (CZTS) ingots grown by the traveling heater method (THM) technique. The compositional homogeneity along the growth direction can enhance the overall yield of detector-grade CZTS, which should therefore be possible to produce at a lower cost compared to CZT. The electrical properties and detector performance of the CZTS crystals will be presented and discussed.
Conventional gamma cameras are heavy and occupy large volume for all required hardware components. Direct-conversion solid-state detectors like cadmium zinc telluride (CZT) enable a compact design of gamma cameras. In addition, conventional gamma cameras require different collimators for different gamma-ray photon energies, and the change of collimators between studies sometimes disrupts the workflow. To ease this inconvenience (i.e., bulkiness and collimator change), we have recently developed a compact, CZT-based, energy-independent gamma camera that requires only one collimator to cover a broad range of photon energies. In this paper, we show our design parameters and system specifications as well as simulation studies that support our design principles.
High-resolution position-sensitive detectors have been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) crystals by virtually subdividing the detectors area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. One recent innovation proposed for pixelated detectors is to use the induced (transient) signals from neighboring pixels to achieve high sub-pixel position resolution while keeping large pixel sizes. The main hurdle in achieving this goal is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with the pixel dimensions of approximately 2 mm. We also present the sub-pixel resolution measurements at comparable energies from various gamma emitting isotopes.
KEYWORDS: Sensors, Electrodes, 3D metrology, Data acquisition, Crystals, Germanium, Hard x-rays, Gamma radiation, Physics, Current controlled current source
In this work, we reconfigured the design of the electrodes, incorporating the high-granularity position-sensitive 3D concept into a larger geometrical form factor, e.g., hemispheric detectors, to improve the uniformity of charge collection and the energy resolution. We designed and fabricated new position-sensitive hemispheric detectors and measured the pulse-height spectra and acquired charge transport data and other electrical measurements with different sealed radioactive sources before and after modifying the design, and compared their performance to identify the optimum configuration. We then applied charge-loss corrections by utilizing the x-y-z positional information from the charge-sensing pads for each event. Based on our simulations and experimental data, we optimized a new configuration for our position-sensitive hemispheric detectors that can effectively be fabricated as large as 20x20x15-mm3 size. Furthermore, our simulation suggests that we can achieve an energy resolution of <1% (FWHM) at 662 keV from even 10x10x5 mm3 sized position-sensitive hemispheric detectors using average-grade CZT crystals.
The traditional method for electron lifetime measurements of CdZnTe (CZT) detectors relies on using the Hecht equation. The procedure involves measuring the dependence of the detector response on the applied bias and applying the Hecht equation to evaluate the mu-tau product, which in turn can be converted into the carrier lifetime if the mobility is known. Despite general acceptance of this technique, which is very convenient for comparative testing of different CZT materials, the assumption of a constant electric field inside a detector is unjustified. In the Hecht equation, this assumption means that the drift time would be a linear function of the drift distance. This condition is rarely fulfilled in practice at low applied biases where the Hecht equation is most sensitive to the mu-tau product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low mu-tau material, <10-3 cm2/V, but give significantly underestimated values for the case of high mu-tau crystals. In this work, we applied the time-of-flight (TOF) technique to measure the electron lifetimes in long-drift-length (3 cm) standard-grade CZT detectors produced by Redlen Technologies. The TOF-based techniques are traditionally used for monitoring the electronegative impurity concentrations in noble gas detectors by measuring the electron lifetimes. We found the electron mu-tau product of tested crystals is in the range 0.1-0.2 cm2/V, which is an order of the magnitude higher than any value previously reported for CZT material. In this work, we reported the measurement procedure and the results. We will also discuss the applicability criteria of the Hecht equation for measuring the electron lifetime in high mu-tau product CZT.
During the transition period between closure of Beamline X27B at BNL’s NSLS and the opening of Beamline MID at NSLS-II, we began operation of LBNL’s ALS Beamline 3.3.2 to carry out our radiation detection materials RD. Measurements performed at this Beamline include, X-ray Detector Response Mapping and White Beam X-ray Diffraction Topography (WBXDT), among others. We will introduce the capabilities of the Beamline and present the most recent results obtained on CdZnTe and scintillators. The goal of the studies on CdZnTe is to understand the origin and effects of subgrain boundaries and help to visualize the presence of a higher concentration of impurities, which might be responsible for the deterioration of the energy resolution and response uniformity in the vicinity of the sub-grain boundaries. The results obtained in the second year of measurements will be presented.
Point defects and their concentrations play an important role in limiting the electrical and spectral properties of crystals. It is observed that the crystal-growth process causes the generation of different types of point defects, and these defects create non-uniformities that can be detrimental to device performance. In this research Cd1-xZnxTe1-ySey (CZTS) crystals grown by Bridgman and Travelling heater methods are studied for their point defects. The focus is on the types of defects, their concentrations and the variations with the selected growth method. In addition the effects of growth-related medium and deep energy traps and their corresponding densities are related to the resistivity, life-time of charge carriers and -product for electrons.
In our prior research we investigated room-temperature radiation detectors (CZT, CMT, CdMgTe, CTS, among other compound semiconductors) for point defects related to different dopants and impurities. In this talk we will report on our most recent research on newly grown CZT crystals doped with In, In+Al, In+Ni, and In+Sn. The main focus will be on the study of dopant-induced point defects using deep-level current transient spectroscopy (i-DLTS). In addition the performance, product, gamma-ray spectral response and internal electric field of the detectors were measured and correlated with the dopant-induced point defects and their concentrations. Characterization of the detectors was carried out using i-DLTS for the point defects, Pockels effect for the internal electric-field distribution, and -ray spectroscopy for the spectral properties.
The self-activated novel scintillating material, Cs2LiCeCl6, was grown and evaluated at BNL for dual gamma and thermal-neutron detector applications. Cs2LiCeCl6 belongs to the elpasolite family. Because of its cubic structure, Cs2LiCeCl6 has good potential for growth of large-volume ingots. The emission spectra showed doublet emission bands peaking at 384 nm and 402 nm, similar to CLYC. An energy resolution of ~ 6% at 662 keV was measured. Thermal neutrons were also detected with a resolution of ~ 4%. Results on the grown ingots using natural Li and enriched 6Li source materials will be presented and discussed.
Aluminum (Al) doped ZnO with very high Al concentration acts as metal regarding its electrical conductivity. ZnO offers many advantages over the commonly-known metals being used today as electrode materials for nuclear detector fabrication. Often, the common metals show poor adhesion to CdZnTe or CdTe surfaces and have a tendency to peel off. In addition, there is a large mismatch of the coefficients of thermal expansion (CTE) between the metals and underlying CdZnTe, which is one of the reasons for mechanical degradation of the contact. In contrast ZnO has a close match of the CTE with CdZnTe and possesses 8-20 times higher hardness than the commonly-used metals. In this presentation, we will explore and discuss the properties of CdZnTe detectors with ZnO:Al contacts.
Our prior investigations showed that alloying CdTe with selenium results in improved material characteristics, such as a reduction in the concentration of secondary-phase particles, better compositional uniformity and less sub-grain boundary networks, as compared to CdTe/CdZnTe. However, by alloying with Se, the band-gap of CdTeSe is significantly reduced from the value for CdTe, which is the main drawback for high-resistivity CdTeSe compounds useful for radiation detection. In order to increase the band-gap, we are now growing Cd1-xZnxSeyTe1-y crystals for detector applications. The effect of Se alloying with CdZnTe will be discussed in terms of the concentration of secondary phases, stress-related defects such as sub-grain boundaries and their networks. Characterization results for the transport properties of the as-grown materials will also be discussed.
Recently, Cadmium Manganese Telluride (CMT) emerged as a promising material for roomtemperature X- and gamma-ray detectors. However, our studies revealed several material defects primarily related to growth processes that are impeding the production of large single crystals with high resistivity and high mobility-lifetime product. In this work, we characterized various defects in materials grown by the floating zone method, including twins, Te inclusions, and dislocations, using our unique facilities. We also fabricated detectors from selected CMT crystals and tested their performance. This paper discusses our detailed findings on the material’s properties and the performance of fabricated CMT detectors.
CdTexSe1-x, with its several advantages over the conventional CdZnTe (CZT) material, offers potential as a roomtemperature radiation detector. Its main advantage is the near-unity segregation coefficient of Se in the CdTe matrix that results in higher compositional homogeneity of the grown ingot. In this paper, we discussed the growth of CdTeSe crystals by various techniques, such as the Traveling Heater method and the Vertical Bridgman technique. We analyzed the different defects in the grown ingots, including Te inclusions/precipitations, sub-grain boundaries and dislocation networks, and studied their effects on the materials’ charge-transport characteristics. Our experimental findings demonstrated several advantages of CdTeSe over CZT, in addition to the near-unity segregation coefficient of Se, including lower concentrations of Te-inclusions/precipitations and sub-grain boundaries and a higher degree of uniformity. Our findings on its charge-transport characteristics also are very encouraging.
KEYWORDS: Collimators, Sensors, 3D image reconstruction, Cameras, 3D image processing, Monte Carlo methods, Computer simulations, Tumors, Gamma radiation, Device simulation
Having the ability to take an accurate 3D image of a tumor greatly helps doctors diagnose it and then create a treatment
plan for a patient. One way to accomplish molecular imaging is to inject a radioactive tracer into a patient and then
measure the gamma rays emitted from regions with high-uptake of the tracer, viz., the cancerous tissues. In large,
expensive PET- or SPECT-imaging systems, the 3D imaging easily is accomplished by rotating the gamma-ray detectors
and then employing software to reconstruct the 3D images from the multiple 2D projections at different angles of view.
However, this method is impractical in a very compact imaging system due to anatomical considerations, e.g., the transrectal
gamma camera under development at Brookhaven National Laboratory (BNL) for detection of intra-prostatic
tumors. The camera uses pixilated cadmium zinc telluride (CdZnTe or CZT) detectors with matched parallel-hole
collimator. Our research investigated the possibility of using a collimator with slanted holes to create 3D pictures of a
radioactive source. The underlying concept is to take 2D projection images at different angles of view by adjusting the
slant angle of the collimator, then using the 2D projection images to reconstruct the 3D image. To do this, we first
simulated the response of a pixilated CZT detector to radiation sources placed in the field of view of the camera. Then,
we formulated an algorithm to use the simulation results as prior knowledge and estimate the distribution of a shaped
source from its 2D projection images. From the results of the simulation, we measured the spatial resolution of the
camera as ~7-mm at a depth of 13.85-mm when using a detector with 2.46-mm pixel pitch and a collimator with 60°
slant angle.
The near-unity segregation coefficient of Se in a CdTe matrix ensures the compositional homogeneity, both axial- and
radial, of the CdTeSe ternary compound, so making it a material of choice for room- temperature radiation detectors. In
this study, we grew CdTeSe crystals by the Traveling Heater Method (THM), using Te as the solvent, and characterized
the crystals by IR transmission microscopy, white-beam X-ray diffraction topography, and low-temperature
photoluminescence. The total average concentration of the secondary phases obtained for the CdTeSe sample was about
7x104 cm-3 for crystals grown at two different laboratories. The best resistivity registered was 5x109 ohm-cm, and the estimated μτ product for the electrons was 3-4x10-3 cm2/V.
We evaluated the effect of high-temperature treatment of Cd0.9Zn0.1Te:In single crystals using Hall-effect measurements,
medium- and high-temperature annealing under various deviations from stoichiometry, and infra-red (IR) transmission
microscopy Annealing at ~730 K sharply increased the electrical conductivity (by ~1-2 orders-of-magnitude). Plots of
the temperature- and cadmium-pressure dependences of the electrical conductivity, carrier concentration, and mobility
were obtained. Treating previously annealed Cd-samples under a Te overpressure at 1070 K allowed us to restore their
resistance to its initial high values. The main difference in comparing this material with CdTe was its lowered electron
density. We explained our results within the framework of Kröger’s theory of quasi-chemical reactions between point
defects in solids.
Data obtained with BNL's National Synchrotron Light Source (NSLS) has helped to elucidate, in detail, the roles of
non-uniformity and extended defects on the performance of CZT detectors, as well as the root cause of device
polarization during exposure to a high flux of incident X-rays. Measurements of carrier traps will be reported, including
their nature and relationships to different growth methods (conventional Bridgman, high-pressure Bridgman, traveling
heater, and floating zone methods). Most findings will be correlated with the performance of spectrometer-grade CZT Xray
and gamma detectors, and new directions to resolve the material deficiencies will be offered.
Although cadmium zinc telluride (CZT) is one of leading materials for fabricating room-temperature nuclear-radiation-
detectors, different defects in the crystals can degrade the performance of CZT detectors. Post-growth thermal
annealing potentially offers a satisfactory way to eliminate the deleterious influence of these defects. Here, we report that
the annealing of CZT in Cd vapor effectively lowers the density of Te inclusions. It takes a much longer annealing time
to eliminate separate large Te inclusions than small ones; however, the annealing time is greatly reduced when the large
Te inclusions are distributed along grain boundaries. We found that sub-grain boundaries still exist after the annealing at
500 °C, indicating that a higher annealing temperature might be needed.
KEYWORDS: Sensors, X-rays, X-ray detectors, Electrodes, Electric field sensors, Metals, Signal attenuation, Crystals, Signal detection, Gamma radiation
In our previous design of virtual Frisch-grid CdZnTe (CZT) detectors, the charge drift-lines can be terminated at the side
surfaces before the carriers reach the collecting anode; this results in a loss of signal from the interacting events near the
detector's edges. Here, we describe our new design for the anode contact that reduces these edge effects by focusing the
electric field towards the detectors' central axes. Four detectors were fabricated with the new hybrid anode contact, and
their performances were evaluated and compared to those from the previous design for our virtual Frisch-grid detectors.
The results obtained for all four showed similar improvement: therefore, we illustrate them with the findings from one
detector.
CdZnTe (CZT) crystals used for nuclear-radiation detectors often contain high concentrations of
subgrain boundaries and networks of poligonized dislocations that can significantly degrade the
performance of semiconductor devices. These defects exist in all commercial CZT materials,
regardless of their growth techniques and their vendor. We describe our new results from examining
such detectors using IR transmission microscopy and white X-ray beam diffraction topography. We
emphasize the roles on the devices' performances of networks of subgrain boundaries with low
dislocation densities, such as poligonized dislocations and mosaic structures. Specifically, we
evaluated their effects on the gamma-ray responses of thick, >10 mm, CZT detectors. Our findings
set the lower limit on the energy resolution of CZT detectors containing dense networks of subgrain
boundaries and walls of dislocations.
Cadmium Zinc Telluride (CdZnTe or CZT) is a very attractive material for room-temperature semiconductor detectors
because of its wide band-gap and high atomic number. Despite these advantages, CZT still presents some material
limitations and poor hole mobility. In the past decade most of the efforts developing CZT detectors focused on
designing different electrode configurations, mainly to minimize the deleterious effect due to the poor hole mobility. A
few different electrode geometries were designed and fabricated, such as pixelated anodes and Frisch-grid detectors
developed at Brookhaven National Lab (BNL). However, crystal defects in CZT materials still limit the yield of
detector-grade crystals, and, in general, dominate the detector's performance. In the past few years, our group's
research extended to characterizing the CZT materials at the micro-scale, and to correlating crystal defects with the
detector's performance. We built a set of unique tools for this purpose, including infrared (IR) transmission microscopy,
X-ray micro-scale mapping using synchrotron light source, X-ray transmission- and reflection- topography, current deep
level transient spectroscopy (I-DLTS), and photoluminescence measurements. Our most recent work on CZT detectors
was directed towards detailing various crystal defects, studying the internal electrical field, and delineating the effects of
thermal annealing on improving the material properties. In this paper, we report our most recent results.
Yonggang Cui, Terry Lall, Benjamin Tsui, Jianhua Yu, George Mahler, Aleksey Bolotnikov, Paul Vaska, Gianluigi De Geronimo, Paul O'Connor, George Meinken, John Joyal, John Barrett, Giuseppe Camarda, Anwar Hossain, Ki Hyun Kim, Ge Yang, Marty Pomper, Steve Cho, Ken Weisman, Youngho Seo, John Babich, Norman LaFrance, Ralph James
In this paper, we discuss the design of a compact gamma camera for high-resolution prostate cancer imaging using
Cadmium Zinc Telluride (CdZnTe or CZT) radiation detectors. Prostate cancer is a common disease in men.
Nowadays, a blood test measuring the level of prostate specific antigen (PSA) is widely used for screening for the
disease in males over 50, followed by (ultrasound) imaging-guided biopsy. However, PSA tests have a high falsepositive
rate and ultrasound-guided biopsy has a high likelihood of missing small cancerous tissues. Commercial
methods of nuclear medical imaging, e.g. PET and SPECT, can functionally image the organs, and potentially find
cancer tissues at early stages, but their applications in diagnosing prostate cancer has been limited by the smallness of
the prostate gland and the long working distance between the organ and the detectors comprising these imaging systems.
CZT is a semiconductor material with wide band-gap and relatively high electron mobility, and thus can operate at room
temperature without additional cooling. CZT detectors are photon-electron direct-conversion devices, thus offering high
energy-resolution in detecting gamma rays, enabling energy-resolved imaging, and reducing the background of
Compton-scattering events. In addition, CZT material has high stopping power for gamma rays; for medical imaging, a
few-mm-thick CZT material provides adequate detection efficiency for many SPECT radiotracers. Because of these
advantages, CZT detectors are becoming popular for several SPECT medical-imaging applications.
Most recently, we designed a compact gamma camera using CZT detectors coupled to an application-specific-integratedcircuit
(ASIC). This camera functions as a trans-rectal probe to image the prostate gland from a distance of only 1-5 cm,
thus offering higher detection efficiency and higher spatial resolution. Hence, it potentially can detect prostate cancers at
their early stages. The performance tests of this camera have been completed. The results show better than 6-mm
resolution at a distance of 1 cm. Details of the test results are discussed in this paper.
We present our new results from testing 15-mm-long virtual Frisch-grid CdZnTe detectors with a common-cathode
readout for correcting pulse-height distortions. The array employs parallelepiped-shaped CdZnTe (CZT) detectors of a
large geometrical aspect ratio, with two planar contacts on the top and bottom surfaces (anode and cathode) and an
additional shielding electrode on the crystal's sides to create the virtual Frisch-grid effect. We optimized the geometry of
the device and improved its spectral response. We found that reducing to 5 mm the length of the shielding electrode
placed next to the anode had no adverse effects on the device's performance. At the same time, this allowed corrections
for electron loss by reading the cathode signals to obtain depth information.
Dark currents, including those in the surface and bulk, are the leading source of electronic noise in X-ray and gamma
detectors, and are responsible for degrading a detector's energy resolution. The detector material itself determines the
bulk leakage current; however, the surface leakage current is controllable by depositing appropriate passivation layers. In
previous research, we demonstrated the effectiveness of surface passivation in CZT (CdZnTe) and CMT (CdMnTe)
materials using ammonium sulfide and ammonium fluoride. In this research, we measured the effect of such passivation
on the surface states of these materials, and on the performances of detectors made from them.
We characterized samples cut from different locations in as-grown CdZnTe (CZT) ingots, using Automated Infrared (IR)
Transmission Microscopy and White Beam X-ray Diffraction Topography (WBXDT), to locate and identify the extended
defects in them. Our goal was to define the distribution of these defects throughout the entire ingot and their effects on
detectors' performance as revealed by the pulse-height spectrum. We found the highest- and the lowest- concentration of
Te inclusions, respectively, in the head and middle part of the ingot, which could serve as guidance in selecting samples.
Crystals with high concentration of Te inclusions showed high leakage current and poor performance, because the
accumulated charge loss around trapping centers associated with Te inclusions distorts the internal electric field, affects
the carrier transport properties inside the crystal, and finally degrades the detector's performance. In addition, other
extended defects revealed by the WBXDT measurements severely reduced the detector's performance, since they trap
large numbers of electrons, leading to a low signal for the pulse-height spectrum, or none whatsoever. Finally, we fully
correlated the detector's performance with our information on the extended defects gained from both the IR- and the
WBXDT-measurements.
Cadmium Zinc Telluride (CZT) has attracted increasing interest with its promising potential as a room-temperature
nuclear-radiation-detector material. However, different defects in CZT crystals, especially Te inclusions and
dislocations, can degrade the performance of CZT detectors. Post-growth annealing is a good approach potentially to
eliminate the deleterious influence of these defects. At Brookhaven National Laboratory (BNL), we built up different
facilities for investigating post-growth annealing of CZT. Here, we report our latest experimental results. Cd-vapor
annealing reduces the density of Te inclusions, while large temperature gradient promotes the migration of small-size Te
inclusions. Simultaneously, the annealing lowers the density of dislocations. However, only-Cd-vapor annealing
decreases the resistivity, possibly reflecting the introduction of extra Cd in the lattice. Subsequent Te-vapor annealing is
needed to ensure the recovery of the resistivity after removing the Te inclusions.
We detail our new results from testing an array of 15-mm long virtual Frisch-grid CdZnTe detectors with a cathode
signal readout-scheme intended to improve spectral response by correcting for electron trapping. We designed a novel
electrode configuration for these long-drift detectors that ensures an energy resolution close to the statistical limit, and
high detection efficiency. However, in reality, the quality of the crystals limits the performance of this type of device.
Here, we describe the characterization of the array, show our preliminary results obtained with gamma-ray sources, and
expound on their relation to our material-characterization data.
Cadmium Zinc Telluride (CZT) is attracting increasing interest with its promise as a room-temperature nuclear-radiationdetector
material. The distribution of the electric field in CZT detectors substantially affects their detection performance.
At Brookhaven National Laboratory (BNL), we employed a synchrotron X-Ray mapping technique and a Pockels-effect
measurement system to investigate this distribution in different detectors. Here, we report our latest experimental results
with three detectors of different width/height ratios. A decrease in this ratio aggravates the non-uniform distribution of
electric field, and focuses it on the central volume. Raising the bias voltage effectively can minimize such nonuniformity
of the electric field distribution. The position of the maximum electric field is independent of the bias voltage;
the difference between its maximum- and minimum-intensity of electric field increases with the applied bias voltage.
CdZnTe (CZT) is the most promising semiconductor for room-temperature nuclear radiation detectors. At Brookhaven's
National Synchrotron Light Source (NSLS), we used a highly collimated synchrotron X-ray radiation to map different
CZT detectors. In this paper, the latest results from high spatial resolution X-ray mapping of CZT detectors are reported.
Effects of different internal defects on the performance of CZT detectors are discussed.
CdMnTe (CMT) can be a good candidate for producing gamma-ray detectors because of its wide band-gap, high
resistivity, and good electron transport properties. Further, the ability to grow CMT crystals at relatively low
temperatures ensures a high yield for manufacturing detectors with good compositional uniformity and few impurities.
Groups at Brookhaven National Laboratory and Institute of Physics are investigating several CMT crystals, selecting a
few of them to make detectors. In this paper, we discuss our initial characterization of these crystals and describe our
preliminary results with a gamma-ray source.
Generally, mechanical polishing is performed to diminish the cutting damage followed by chemical etching to
remove the remaining damage on crystal surfaces. In this paper, we detail the findings from our study of the effects of
various chemical treatments on the roughness of crystal surfaces. We prepared several CdZnTe (CZT) and CdMnTe
(CMT) crystals by mechanical polishing with 5 μm and/or lower grits of Al2O3 abrasive papers including final polishing
with 0.05-μm particle size alumina powder and then etched them for different periods with a 2%, 5% Bromine-Methanol
(B-M) solution, and also with an E-solution (HNO3:H20:K2Cr2O7). The material removal rate (etching rate) from the
crystals was found to be 10 μm, 30 μm, and 15 μm per minute, respectively. The roughness of the resulting surfaces was
determined by the Atomic Force Microscopy (AFM) to identify the most efficient surface processing method by
combining mechanical and chemical polishing.
Virtual Frisch-grid CdZnTe detectors potentially can provide energy resolution close to the statistical limit. However, in real detectors, the quality of the crystals used to fabricate the devices primarily determines energy resolution. In this paper, we report our findings on the spectral response of devices and their relation to material-characterization data obtained using IR microscopy and X-ray diffraction topography.
The performance of current long-drift-length Cadmium Zinc Telluride (CZT) detectors principally is determined by
the material's quality. Hence, the material's limitations must be better understood and potential solutions identified to
grow CZT crystals with the required qualities. Our efforts have focused on developing novel techniques and testing
methods that will allow us to explore the correlations between the crystal's defects and the detector's properties. Local
stoichiometric variations and other local disordering make it very hard to systematically correlate performance and
material defects on a macroscopic scale. Therefore, to delineate the factors limiting the energy resolution of CZT
detectors, we directed our efforts towards micron-scale material characterization and assessments of the detectors using
the National Synchrotron Light Source (NSLS). The NSLS offers us a highly collimated high-intensity X-ray beam that
we employed to undertake detector-performance mapping, and to investigate the association between microscopic
defects and fluctuations in collected charge. In this paper, we illustrate our techniques and results.
The excellent room temperature spectral performance of cadmium zinc telluride detectors grown via the Traveling
Heater Method (THM) makes this approach suitable for the mass deployment of radiation detectors for applications in
homeland security and medical imaging. This paper reports our progress in fabricating thicker and larger area detectors
from THM grown CZT. We discuss the performance of such 20x20x10 mm3, and 10x10x10 mm3 monolithic pixellated
detectors and virtual Frisch-Grid 4x4x12 mm3 devices, and describe the various physical properties of the materials.
In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (CZT) and other
compound semiconductor detectors. These include three-terminal, semi-spherical, CAPture, Frisch-ring, capacitive
Frisch-grid and pixel devices (along with their modifications). Among them, the Frisch-grid design employing a non-contacting
ring extended over the entire side surfaces of parallelepiped-shaped CZT crystals is the most promising. The
defect-free parallelepiped-shaped crystals with typical dimensions of 5x5x12 mm3 are easy to produce and can be
arranged into large arrays used for imaging and gamma-ray spectroscopy. In this paper, we report on further advances
of the virtual Frisch-grid detector design for the parallelepiped-shaped CZT crystals. Both the experimental testing and
modelling results are described.
KEYWORDS: Sensors, Electrodes, Electrons, Signal detection, Semiconductors, Ionization, Gamma radiation, Crystals, Energy efficiency, Signal generators
CdZnTe (CZT) is a very promising material for nuclear-radiation detectors. CZT detectors operate at ambient
temperatures and offer high detection efficiency and excellent energy resolution, placing them ahead of high-purity Ge
for those applications where cryogenic cooling is problematic. The progress achieved in CZT detectors over the past
decade is founded on the developments of robust detector designs and readout electronics, both of which helped to
overcome the effects of carrier trapping.
Because the holes have low mobility, only electrons can be used to generate signals in thick CZT detectors, so one must
account for the variation of the output signal versus the locations of the interaction points. To obtain high spectral
resolution, the detector's design should provide a means to eliminate this dependence throughout the entire volume of
the device. In reality, the sensitive volume of any ionization detector invariably has two regions. In the first, adjacent to
the collecting electrode, the amplitude of the output signal rapidly increases almost to its maximum as the interaction
point is located farther from the anode; in the rest of the volume, the output signal remains nearly constant. Thus, the
quality of CZT detector designs can be characterized based on the magnitude of the signals variations in the drift region
and the ratio between the volumes of the drift and induction regions. The former determines the "geometrical" width of
the photopeak, i.e., the line width that affects the total energy resolution and is attributed to the device's geometry when
all other factors are neglected. The latter determines the photopeak efficiency and the area under the continuum in the
pulse-height spectra.
In this work, we describe our findings from systematizing different designs of CZT detectors and evaluating their
performance based on these two criteria.
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