CdZnTe Radiation Detectors Research Articles

Evaluation of an automatic needle‐loading system

The purpose of this paper is to evaluate the dosimetric capabilities and the radiation protection (RP) performance of a new automatic needle‐loading system for permanent prostate implants, the Isoloader (Mentor Corp.). The unit has been used in more than 100 clinical cases at our institution.The Isoloader is a computerized workstation that allows automated seed testing by a solid‐state CdZnTe radiation detector and loading in surgical needles. The seeds are received in a shielded and ready‐to‐use cartridge. Radiation protection measurements were done on a cartridge filled with 67 I125 seeds and during dosimetric seed verification and needle loading. The reproducibility of the detector was tested and its accuracy was determined by comparison to specified activities of six calibration seeds and to their measurements in a calibrated well‐chamber (WC). Finally, the times required to complete dosimetric verification and needle loading were evaluated.The cartridge was found to be adequately shielded, since no significant amount of radiation was detected around it. Radiation during seed assay was found to be worst at the cartridge's bottom, where it has a value of 15.2 μSv/h (1.4 μSv/h at 10 cm). For the needle‐loading task, measurements were performed with a typical needle (three seeds) at the shielded needle holder surface yielding 307.2 μSv/h (8.3 μSv/h at 20 cm). Seed dosimetric verification takes an average of 15 s/seed, while it takes a mean time of 50 s/needle to complete the loading task. Measurements of the six seed activities were within 0.65% of the ordered activities and 1.9% higher on average than those from the WC (min=0.7%;max=3.5%). The reproducibility of the measurements of the CdZnTe detector was excellent, with an average of 0.01% of deviation from a reference measurement (N=120;σ=1.9%).We therefore conclude that the Isoloader is a safe, fast, and effective needle‐loading system.PACS number: 87.53.Jw

Digital IBIC—new spectroscopic modalities for ion-beam-induced charge imaging

The first implementation of digital ion-beam-induced charge (IBIC) imaging is presented, which permits spectroscopic time-resolved IBIC imaging of charge transport in semiconductors. A digital IBIC system has been developed which uses a high-speed waveform digitiser to capture the pulse shapes produced by interactions of a 1 μm resolution scanning microbeam in semiconductor samples. Using a variety of digital pulse shape analysis algorithms, quantitative images of charge signal amplitude, charge carrier lifetime, mobility and trapping phenomena can be acquired in real-time, with a time resolution of <20 ns. We report data from the commissioning experiment which shows digital IBIC images of electron and hole transport in CdZnTe radiation detectors, obtained by lateral scanning of the ion beam between the cathode and anode. The room temperature electron drift mobility in CdZnTe was measured with a value of 1.1×10 3 cm 2/Vs.

Microscopic evaluation of spatial variations in material and charge transport properties of CdZnTe radiation detectors

The material uniformity and charge transport properties in CdZnTe radiation detectors are investigated by various non-destructive room temperature techniques. High-resolution secondary electron microscopy shows evidence of Te-enriched regions near the metal contacts. Photoluminescence (PL) mapping is used to image material defects and variations in Zn composition. Close to the electrodes PL spectra show strong defect-activated emission which is not observed in the material bulk. Ion-beam-induced-charge (IBIC) microscopy using a 2 μm resolution focussed proton beam is used to investigate the spatial variation in charge transport across detectors. Localized regions of poor charge collection efficiency seen in the IBIC images correlate with material defects observed in PL such as tellurium precipitates. The effects of these material defects in terms of the spectroscopic performance of the detectors to X-ray and γ-ray irradiation is discussed.

Effects of excess tellurium on the properties of CdZnTe radiation detectors

Room-temperature radiation detectors have been fabricated on high-resistivity, indium-doped Cd0.90Zn0.10Te crystals grown under different amounts of excess Te. The effects of the excess Te on the properties of the detectors are explained by a simple model using only three parameters: the density of Cd vacancies, the density of Te antisites (Te at Cd sites), and the deep level of doubly ionized Te antisites. The best detectors, which can resolve the low-energy Np-L and Te-K peaks as well as Cd and Te escape peaks of 241Am, are produced from crystals grown with 1.5% excess Te. The detectors fabricated from crystals grown without excess Te are unable to resolve any characteristic-radiation peaks of 241Am and 57Co. This result is explained by a model of networked p-type domains in an n-type matrix or vice versa, which is caused by the lack of sufficient deep-level Te antisites. Such conduction-type inhomogeneity causes massive electron and hole trapping. As for the detectors fabricated from Cd0.90Zn0.10Te crystals grown with 2% and 3% excess Te, they are able to resolve the 241Am 59.5-keV, 57Co 122-keV, and 57Co 136-keV radiation peaks. However, the full-width at half-maximum (FWHM) values of these peaks are broadened, especially the high-energy 57Co peaks. These phenomena are attributed to the hole and, possibly, electron trapping by Cd vacancies and Te antisites, respectively. The result of the analysis indicates that sufficient Te antisites and a low density of carrier traps in Cd0.90Zn0.10Te are essential for producing high-quality radiation detectors. In the analysis, it was discovered that most of the excess Te, on the order of 1–2 × 1020 cm−3, remain electrically inactive. A possible explanation for this phenomenon is that the excess Te atoms form neutral Te-antisite and Cd-vacancy complexes, such as TeCd·(VCd)2, during the post-growth cooling process.

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.

Tellurium antisites in CdZnTe

The electrical properties of CdTe and Cd1−xZnxTe crystals grown under excess tellurium by a modified Bridgman technique are critically dependent on the zinc content. Below an x value of 0.07, the as-grown CdZnTe crystals are n type while, above this value, CdZnTe crystals are p type. The origin of the shallow donor level at 0.01 eV below the conduction band is most likely singly ionized Te antisites (Te at Cd sites). The origin of the deep donor level at 0.75 eV below the conduction band is therefore doubly ionized tellurium antisites. Based on this model, the conduction type of CdZnTe crystals is determined by the results of compensation between the shallow donors of Te antisites and the shallow acceptors of Cd vacancies. High resistivity Cd0.9Zn0.1Te crystals are produced by compensating the p-type crystals with indium impurity at a low doping level of 1–5×1015 cm−3. At room temperature, CdZnTe radiation detectors can resolve the six low energy peaks in the Am241 spectrum, a performance comparable to that of the best CdZnTe detectors reported.

Time of flight experimental studies of CdZnTe radiation detectors

A time of flight technique was used to study the carrier trapping time, τ, and mobility, μ, in CdZnTe (CZT) and CdTe radiation detectors. Carriers were generated near the surface of the detector by a nitrogen-pumped pulsed dye laser with wavelength ∼500 nm. Signals from generated electrons or holes were measured by a fast oscilloscope and analyzed to determine the trapping time and mobility of carriers. Electron mobility was observed to change with temperature from 1200 cm2/Vs to 2400 cm2/Vs between 293 K and 138 K, respectively. Electron mobilities were observed between 900 cm2/Vs and 1350 cm2/Vs at room temperature for various CZT detectors. Electron mobilities in various CdTe detectors at room temperature were observed between 740 cm2/Vs and 1260 cm2/Vs. Average electron mobility was calculated to be 1120 cm2/Vs and 945 cm2/Vs for CZT and CdTe, respectively. Hole mobilities in both CZT and CdTe were found to vary between 27 cm2/Vs and 66 cm2/Vs. Electron trapping times in CZT at room temperature varied from 1.60 µs to 4.18 µs with an average value of about 2.5 µs. Electron trapping time in CdTe at room temperature varied between 1.7 µs and 4.15 µs with an average value of about 3.1 µs.

Performance improvement of CdZnTe detectors using modified two-terminal electrode geometry

Recently, much has appeared in the literature concerning methods to improve the resolution and photopeak efficiency of CdZnTe radiation detectors operating at or around ambient temperature. These methods generally involve either the use of modified electrode structures (e.g. coplanar grids, three-terminal devices) or pulse processing techniques, both of which add complexity, and hence cost, to the production and operation of such devices. In this paper, we will report on results obtained with a simpler, modified two-terminal device. The detector structure combines a planar anode with an extended surface cathode, and relies on a standard, single-channel preamplifier/shaping amplifier system. The results obtained demonstrate that the charge collection efficiency of the device, as shown by the peak-to-valley ( P/ V) ratio, is significantly improved when compared to the standard planar geometry, especially at higher (>200 keV) photon energies.

Analyses on the measurement of leakage currents in CdZnTe radiation detectors

Models that place design constraints on devices which are used to measure the leakage currents in high-resistivity semiconductor materials are presented. If these design constraints are met, these models can then be used to quantitatively predict the surface sheet resistance of devices which are dominated by surface leakage currents. As a result, a means is provided to directly compare passivation techniques which are developed to decrease surface leakage currents. Furthermore, these models illustrate the necessity for inclusion of relevant geometrical data on sample size and shape and electrode configuration when reporting results of surface passivation techniques. These models specifically examine the case where a DC potential is applied across two electrodes on the surface of a semiconductor substrate which has a surface layer with lower resistivity than the bulk material. We describe several of the more common configurations used in analyzing passivation techniques for compounds of Cd/sub 1-x/Zn/sub x/Te (CZT) used for room-temperature radiation detection.

Effect of electron transport properties on unipolar CdZnTe radiation detectors: LUND, spectrumplus and coplanar grid

Device simulations of (1) the laterally contacted-unipolar-nuclear detector (LUND), (2) the SpectrumPlus, (3) and the coplanar grid made of Cd0.9Zn0.1Te (CZT) were performed for 137Cs irradiation by 662.15 keV gamma-rays. Realistic and controlled simulations of the gamma-ray interactions with the CZT material were done using the MCNP4B2 Monte Carlo program, and the detector responses were simulated using the Sandia three-dimensional multi-electrode simulation program (SandTMSP). The simulations were done for the best and the worst expected carrier mobilities and lifetimes of currently commercially available CZT materials for radiation detector applications. For the simulated unipolar devices, the active device volumes were relatively large and the energy resolutions were fairly good, but these performance characteristics were found to be very sensitive to the materials properties. The internal electric fields, the weighting potentials, and the charge induced efficiency maps were calculated to give insights into the operation of these devices.

Fine-scale spatial response of CdZnTe radiation detectors

Several studies have suggested that the uniformity of Cadmium Zinc Telluride (CZT) detectors play an important role in their performance when operated as gamma-ray spectrometers. However, the detailed gamma response of simple planar detectors as a function of position over the device area is largely unknown. To address this issue we have built a system capable of measuring the detector response with a resolution of /spl sim/250 /spl mu/m. The system consists of a highly collimated (/spl sim/200 /spl mu/m) photon source (<150 keV) scanned over the detector using a computer controlled, two-axis translation stage. Fifteen samples configured as planar detectors were examined with the new apparatus. The material grade of the detectors examined varied from counter to select discriminator. Two classes of spatial response variation were observed and are presented here. Infrared (IR) transmission images were also acquired for each sample and correlation between features in the pulse height spectrum and crystalline defects were observed.

Compensation and trapping in CdZnTe radiation detectors studied by thermoelectric emission spectroscopy, thermally stimulated conductivity, and current-voltage measurements

The thermal ionization energies of traps and their types, whether electron or hole traps, were measured in commercial CdZnTe crystals for radiation detectors. The measurements were done between 20 and 400K using thermoelectric emission spectroscopy (TEES) and thermally stimulated conductivity (TSC). For reliable results, indium ohmic contacts had to be used instead of gold Schottky contacts. For filling of the traps, photoexcitation was done at zero bias, at 20K, and at wavelengths which gave the maximum bulk photoexcitation. In agreement with theory, the TSC current was found to be on the order of \( \sim \tfrac{{eV}}{{k\Delta T}}\) times or even larger than the TEES current, where V is the applied bias in TSC and ΔT is the applied temperature difference in TEES. Large concentrations of hole traps at 0.1 and 0.6 eV were observed and a smaller concentration of electron traps at 0.4 eV was seen. The deep traps cause compensation in the material, which is desirable, but they also cause carrier trapping that degrades the spectral response of radiation detectors made from the material.

Conference

A series of experiments has determined the response of CdZnTe radiation detectors to electrons and alpha particles. Densely ionising particles result in an “S-shaped” pulse which is somewhat different in shape to those generated during photon interactions. A preliminary analysis of these pulse shapes is presented and the impact of this on possible schemes for pulse-shape discrimination is discussed. By altering the position of interaction of the alpha-particle beam in the detector, it is possible to analyse the non-uniformity of the electric field present in these detectors. Results are presented to demonstrate these effects, and the implications of these for operation of CdZnTe detectors in charged particle beams will be discussed.

The charged particle response of CdZnTe radiation detectors

A series of experiments has determined the response of CdZnTe radiation detectors to electrons and alpha particles. Densely ionising particles result in an “S-shaped” pulse which is somewhat different in shape to those generated during photon interactions. A preliminary analysis of these pulse shapes is presented and the impact of this on possible schemes for pulse-shape discrimination is discussed. By altering the position of interaction of the alpha-particle beam in the detector, it is possible to analyse the non-uniformity of the electric field present in these detectors. Results are presented to demonstrate these effects, and the implications of these for operation of CdZnTe detectors in charged particle beams will be discussed.

Transputer based acquisition/reconstruction system as a development tool for positron emission tomography

We present an acquisition and reconstruction system for positron emission tomography (PET), to be used as a development tool for testing PET reconstruction algorithms and for detector characteristics studies. It is based on a transputer network and modular electronics: acquired data are stored in memories associated with parallel processors and offline data analysis is performed, allowing for fast comparison of performances of different reconstruction algorithms. Its simple architecture and low cost makes it an easy-to-use and convenient tool to test new detectors for PET applications. We introduce here some results obtained with two CdZnTe radiation detectors on a rotating ring, as a prototype example of a small-animal PET camera.

CdZnTe semiconductor parallel strip Frisch grid radiation detectors

CdZnTe wide band gap compound semiconducting material offers promise as a room temperature operated gamma ray spectrometer. Position-dependent free charge carrier losses during transport can prevent efficient charge carrier extraction from semiconductor detectors and severely reduce energy resolution. Hole trapping losses in CdZnTe radiation detectors are far worse than electron trapping losses and resolution degradation in CdZnTe detectors results primarily from severe hole trapping during transport. Coplanar radiation detectors improve energy resolution by sensing the induced charge primarily from the motion of electrons. Demonstrated is an alternative approach to single free charge carrier sensing, in which a parallel strip Frisch grid is fabricated on either side of a parallelepiped block. The detectors are three terminal devices, but require only one preamplifier for the output signal. The prototype devices demonstrate a considerable increase in energy resolution when operated in the true Frisch grid mode rather than the planar mode, with a demonstrated room temperature energy resolution for 662 keV gamma rays of 5.91% at FWHM for a 10 mm/spl times/2 mm/spl times/10 mm device. Presently, high surface leakage currents prevent large voltages from being applied to the devices, which ultimately reduces their maximum achievable energy resolution. Further improvements are expected with the realization of reduced surface leakage currents.

Two-dimensional modeling of Cd(Zn)Te strip detectors

Although there has been significant recent progress in the development of CdTe and CdZnTe radiation detectors, these materials are still plagued by charge trapping and recombination problems. Recent work has shown that fabrication of arrays with small anodes can overcome these problems to some extent. The purpose of this simulation study was to investigate this small anode effect in relatively large strip detectors of various geometries. Unlike previous simulations, the current study includes the effects of both trapping and detrapping, significant inter-pixel spacing, and non-linear electric field profiles. In addition, the methods have the potential to account efficiently for spatial variations in trapping behavior and for multiple trapping levels. The results of the simulation predict that 8 mm thick strip detectors with 1 mm anode thickness and 0.3 mm anode spacing will provide adequate performance for our medical imaging application.

Study of contacts to CdZnTe radiation detectors

This study characterizes, for the first time, contacts to CdZnTe radiation detectors by measuring the dark noise spectra as a function of the applied bias. The noise currents are correlated with the dc dark current-voltage characteristics of CdZnTe x-ray and gamma-ray detectors. In order to identify and separate the role of the contacts in the overall performance, the measured noise phenomena is correlated with detector configuration and contact design as well as the growth method of the CdZnTe crystals, contact technology, and passivation. Several contact technologies (electroless gold, and a number of evaporated metallic contacts including gold, indium, zinc, titanium, aluminum, and platinum contacts) are compared. Contacts to CdZnTe crystals grown by high pressure Bridgman are compared with contacts to CdZnTe crystals grown by modified Bridgman. Contacts of resistive detectors as well as of Schottky detectors are reported. Large area symmetric contacts are compared with small area pixelized contacts. The role of the metallization used for contacts, the role of surface effects and passivation, and the role of contact design are discussed.

Fabrication and characterization of CdZnTe radiation detectors with a new P-I-N design

We report on the design, fabrication, and performance of CdZnTe gamma-ray detectors with a new P-I-N structure for spectroscopic applications. Highpressure and conventional vertical-Bridgman CdZnTe crystals were used for detector fabrication. P and n layers were deposited by thermal evaporation, and by optimizing the deposition conditions we achieved low leakage current (approximately 15 nA at 1000 V) and good performance. Spectral response data at high bias voltages showed improved energy resolution and peak-to-valley ratios for 241Am and 57Co compared to metal-semiconductor-metal detectors.

Simulated and Associated Experimental Results of CdZnTe Radiation Detector Response for Gamma-Ray Imaging Applications

AbstractSimulated and associated experimental results of a high efficiency CdZnTe (CZT) radiation detector response for gamma-ray imaging applications are presented. The model of a high efficiency semiconductor gamma ray detector takes into account several different physical phenomena involved in the detection and correction processes, namely the geometry of the irradiation, the gamma-ray's interaction with the crystal, the physics of the semiconductor's charge collection, the electric field distribution and the pulse height correction method. A few important decoupling assumptions allow us to use a one dimensional charge collection simulation with a two-dimensional field model and a full three dimensional Monte-Carlo calculation of the gamma ray interactions. The model allows calculation of charge collection and gamma ray spectra for non uniform electric field distribution in either planar, striped or pixellated detector.The model takes also into account the new CZT fast pulse correction method and its associated noise by considering the pulse height and the rise time of electron signals (Bi-Parametric spectrum) for all gamma ray interactions. Specific simulated and experimental spectra at 122 keV are presented for CZT. First, basic spectral changes are calculated for variations in crystal and detector properties like mobility, trapping lifetime and electric field profilesSecond, new experimental results of the fast pulse correction method applied to different CZT detector grades are presented. This method allows to achieve a high detection efficiency (&gt; 80 %) with a good energy resolution (&lt; 6 % FWHM) at 122 keV for a 4×4×6 mm3 CZT detector. No specific contact geometry is needed and the unusual low applied bias voltage allows to limit the ageing and break voltage effects and also the dark current and its associated noise. This fast correction method is expected to be useful for medical imaging and other applications.Finally, simulated Bi-Parametric (BP) spectra expected with the fast pulse correction method according to the detector properties (electric field profiles, electron lifetime) are simulated and a qualitative comparison is provided.