[show abstract][hide abstract] ABSTRACT: Hybrid CdZnTe, CdTe, GaAs, selenium and PbI2 pixel detector arrays with 50×50 μm2 pixel sizes that convert X-rays directly into charge signals are under development at NOVA for application to digital mammography. These detectors have superior X-ray quantum efficiency compared to either emulsion-based film, phosphor-based detectors or other low-Z, solid-state detectors such as silicon. During this work, CdZnTe and CdTe pixel detectors gave the best results. The other detectors are at very early stages of development and need significant improvement. Among other detectors, selenium is showing the highest potential. The preliminary results show that single crystal CdZnTe detectors yield better results in Detective Quantum Efficiency (DQE) as well as in images obtained from phantoms, compared to the polycrystalline CdZnTe detectors. This is due to the non-uniformities in the polycrystaline CdZnTe that degrade the charge transport properties. In this paper, preliminary results from thin (0.15 to 0.2 mm) CdZnTe and CdTe detectors will be presented in terms of MTF, DQE and phantom images. Because of the charge-coupling limitation of the readout Application Specific Integrated Circuit (ASIC) that was originally designed for Si detectors, the detector is biased to collect holes from the input. This charge collection mode limits the CdZnTe detector performance. Their DQE measurements yield 25% and 65% for the polycrystal and single-crystal CdZnTe detectors, respectively. Polycrystal CdTe test detectors were also hybridized to the same type charge readout chip. Since CdTe has much longer hole-propagation lengths compared to CdZnTe, it shows better performance in the hole-collecting mode. However, it suffers from polarization. Excellent images were also obtained from the CdTe detectors. Future work to redesign the readout ASIC and thus improve the detector performance will be discussed. These detectors can also be used for other medical radiography with increased thickness and also for industrial imaging such as non-destructive evaluation and non-destructive inspection.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 01/2003;
[show abstract][hide abstract] ABSTRACT: A new high-resolution detector has been developed for use in a slot-scanned digital mammography system. The detector is a hybrid device that consists of a CCD operating in time-delay integration mode that is bonded to a 150-microm-thick CdZnTe photoconductor array. The CCD was designed with a detector element pitch of 50 microm. Two devices were evaluated with differing crystalline quality. Incomplete charge collection was a source of reduction in DQE. This occurs in both devices due to characteristically low mobility-lifetime products for CdZnTe, with the greatest losses demonstrated by the multicrystalline sample. The mobility-lifetime products for the multicrystalline device were found to be 2.4 x 10(-4) and 4.0 x 10(-7) cm2/V for electrons and holes, respectively. The device constructed with higher quality single crystal CdZnTe demonstrated mobility-lifetime products of 1.0 x 10(-4) and 4.4 x 10(-6) cm2/V for electrons and holes. The MTF and DQE for the device were measured at several exposures and results were compared to predictions from a linear systems model of signal and noise propagation. The MTF at a spatial frequency of 10 mm(-1) exceeded 0.18 and 0.56 along the scan and slot directions, respectively. Scanning motion and CCD design limited the resolution along the scan direction. For an x-ray beam from a tungsten target tube with 40 microm molybdenum filtration operated at 26 kV, the single crystal device demonstrated a DQE(0) of 0.70 +/- 0.02 at 7.1 x 10(-6) C/kg (27 mR) exposure to the detector, despite its relatively poor charge collection efficiency.
Medical Physics 01/2003; 29(12):2767-81. · 2.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hybrid CdZnTe and CdTe pixel detector arrays with 50 50 micron 2 pixel sizes that convert x-rays directly into charge signals are under development at NOVA for applications to digital mammography. CdZnTe and CdTe have superior x-ray quantum efficiency compared to either emulsion-based film, phosphor-based detectors or other low-Z, solid-state detectors such as silicon. In this paper, latest results from thin (0.15 to 0.2 mm) CdZnTe and CdTe detectors will be presented in terms of modulation transfer function (MTF), detective quantum efficiency (DQE), and phantom images. Single-crystal CdZnTe detectors yield better results in DQE as well as phantom images, compared to the polycrystalline CdZnTe detectors. This is due to the nonuniformities in the polycrystal that degrade the charge transport properties. Because of the charge-coupling limitation of the readout ASIC that was originally designed for Si detectors, the detector is biased to collect holes from the front side. This charge collection mode limits the CdZnTe detector performance. Their DQE measurements yield 25% and 65% for the poly-crystal and single-crystal CdZnTe detectors respectively. Poly-crystal CdTe test detectors were also hybridized to the same type charge readout chip. Since CdTe has much longer hole-propagation lengths compared to CdZnTe, it shows better performance in the hole-collecting mode. However, severe polarization effect degrades performance of the present device. Excellent images were also obtained from the CdTe detectors. Future work to redesign the readout ASIC and, thus, improve the detector performance is discussed. Application to industrial imaging such as nondestruc-tive evaluation (NDE) and nondestructive inspection (NDI) is a natural extension.
[show abstract][hide abstract] ABSTRACT: Hybrid pixel detector arrays that convert X-rays directly into charge signals are under development at NOVA for application to digital mammography. This technology also has wide application possibilities in other fields of radiology or in industrial imaging, nondestructive evaluation (NDE) and nondestructive inspection (NDI). These detectors have potentially superior properties compared to either emulsion-based film-screen systems which has nonlinear response to X-rays, or phosphor-based detectors in which there is an intermediate step of X-ray to light photon conversion (Feig and Yaffe, Radiol. Clinics North America 33 (1995) 1205–1230). Potential advantages of direct conversion detectors are high quantum efficiencies (QE) of 98% or higher (for 0.3 mm thick CdZnTe detector with 20 keV X-rays), improved contrast, high sensitivity and low intrinsic noise. These factors are expected to contribute to high detective quantum efficiency (DQE). The prototype hybrid pixel detector developed has 50×50 μm pixel size, and is designed to have linear response to X-rays, and can support a dynamic range up to 14 bits. Modulation Transfer Function (MTF) is measured on a 1-mm silicon detector system where 10% or better modulations are obtained at 10 lp/mm spatial frequency. Preliminary DQE measurements of the same detector yields a value of 75% at zero spatial frequency. In this paper, we report results obtained from our first full size prototype readout ASIC chips hybridized with both silicon and CdZnTe detector arrays and present preliminary MTF and DQE measurement results as well as some test images.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 01/2000;
[show abstract][hide abstract] ABSTRACT: Digital mammography has demanding imaging requirements, including very high spatial resolution (50 micrometer) and SNR. To make efficient use of the radiation dose, it is also desirable that the DQE of the image receptor is high. To achieve these requirements, a prototype CCD read-out has been designed, which is hybridized to a semiconductor array to form a direct conversion detector that can be employed in a slot- scanned digital x-ray imaging system. The image quality of the detectors in which the CCD is hybridized to either a silicon photodiode array or a CdZnTe photoconductor array has been measured. A 1 mm thick silicon device has shown a DQE(f) of 0.64 at 0 mm-1, falling to 0.14 at 10 mm-1 in the slot direction (20 keV). The CdZnTe hybrid device is very thin (150 micrometer) and has a theoretical DQE in excess of 0.9 at 20 keV. The resolution of the CdZnTe device is excellent, with an experimental MTF that is limited only by the detector element size, and the TDI scanning technique. However, the experimental DQE is lower than predicted, believed to be due to crystal non-uniformity, and excessive carrier trapping. Future work will investigate the improvement in image quality obtainable by using a very high purity single-crystal CdZnTe device.
[show abstract][hide abstract] ABSTRACT: A wide field-of-view, arcsecond imaging, high energy resolution x-ray and low energy gamma ray detector is proposed for a future space mission. It is specifically designed to monitor and study gamma ray bursts (GRBs) with high energy and angular resolution and also find counterparts at other wavelengths. Detection of GRBs requires wide field-of-view ((pi) to 2 (pi) field-of-view) and high sensitivity. This is achieved by using high quantum efficiency CdZnTe pixel detectors with a low energy threshold (few keV) to observe the larger flux levels at lower energies, and large effective area (625 to 1,000 cm(superscript 2)) per coded aperture imaging module. Counterpart searches can only be done with ultra high angular resolution detectors (10 to 30 arcsecond FWHM) which gives 1 to 5 arcsecond position determination especially for strong GRBs. A few arcsecond size error box is expected to contain at most one object observed at another wavelength. This will be achieved by using ultra high spatial resolution pixel detectors (100 by 100 microns) and a similar resolution coded aperture to achieve the required angular resolution. AXGAM also has two other important advantages over similar detectors: (1) excellent low energy response (greater than 1 keV) and (2) high energy resolution (less than 6% at 5.9 keV, less than 3% at 14 keV, less than 4% at 122 keV). The low energy range may provide important new information on GRBs and the high energy resolution is expected to help in the observation and identification of emission and absorption lines in the GRB spectrum. The effective energy range is planned to be 2 to 200 keV which is exceptionally wide for such a detector. AXGAM will be built in the form of a 'bucky ball' using a coded aperture mask in a semi-geodesic dome arrangement placed over a two-dimensional, high resolution CdZnTe pixel detector array using newly developed p-i-n detector technology. The p-i-n structure decreases the electron and hole trapping effect and increases energy resolution significantly. The major scientific goals of the proposed mission in addition to continuously monitoring gamma- ray bursts, is to observe AGNs, transient phenomena, isolated and binary pulsars, and solar flares. A space deployed AXGAM detector is expected to observe several hundred gamma ray bursts per year.
[show abstract][hide abstract] ABSTRACT: New, high spatial resolution CdZnTe (CZT) and silicon (Si) pixel detectors are highly suitable for x-ray astronomy. These detectors are planned for use in wide field of view, imaging x-ray, and low energy gamma-ray all-sky monitor (AXGAM) in a future space mission. The high stopping power of CZT detectors combined with low-noise front-end readout makes possible an order of magnitude improvement in spatial and energy resolution in x-ray detection. The AXGAM instrument will be built in the form of a fine coded aperture placed over two-dimensional, high spatial resolution and low energy threshold CZT pixel detector array. The preliminary result of CZT and silicon pixel detector test with low-noise readout electronics system are presented. These detectors may also be used with or without modification for medical and industrial imaging. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints