Browsing by Author "Spartiotis, K"

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A Cdte Real Time X-Ray Imaging Sensor and System
(Elsevier Science Bv, 2004) Spartiotis, K; Havulinna, J; Leppänen, A; Pantsar, T; Puhakka, K; Pyyhtiä, J; Schulman, T
A real time X-ray imaging sensor consisting of p-type CdTe pixel detectors with barrier type In front contacts (anode) and Pt pixel contacts (cathode) has been developed and tested. The 5 x 5 cm 2 sensor area is covered with four CdTe pixel detectors each bump bonded to two CMOS readout circuits and mounted side by side to a support board. Each CMOS has 127 x 253 pixels with a pitch of 100 mum. The sensor is operated in the charge integration mode. User selective digital video frame output rates up to 60 fps are possible. A detector bias voltage cycle which brings the voltage instantaneously to zero at a certain interval is applied to eliminate the effect of polarization. Real time image calibration is applied to compensate for leakage current variation and CMOS mismatch. The CdTe pixelization process and the sensor design and operation are described. Experimental results of the MTF, SNR and DQE are presented and X-ray images of printed circuit boards are shown. (C) 2004 Elsevier B.V. All rights reserved.
A Directly Converting High-Resolution Intra-Oral X-Ray Imaging Sensor
(Elsevier Science Bv, 2003) Spartiotis, K; Pyyhtiä, J; Schulman, T; Puhakka, K; Muukkonen, K
A digital intra-oral X-ray imaging sensor with an active area of 3.6 x 2.9 cm(2) and consisting of six charge-integrating CMOS signal readout circuits bump bonded to one high-resistivity silicon pixel detector has been developed and tested. The pixel size is 35 mum. The X-rays entering the sensor window are converted directly to electrical charge in the depleted detector material yielding minimum lateral signal spread and maximum image sharpness. The signal charge is collected on the gates of the input field effect transistors of the CMOS signal readout circuits. The analog signal readout is performed by multiplexing in the current mode independent of the signal charge collection enabling multiple readout cycles with negligible dead time and thus imaging with wide dynamic range. Since no intermediate conversion material of X-rays to visible light is needed, the sensor structure is very compact. The analog image signals are guided from the sensor output through a thin cable to signal processing, AD conversion and memory circuitry, which is realized as an intermediate electronic unit between the sensor and the computer. This unit controls the sensor, collects the data and transfers it via one of the standard computer buses for image processing, display and storage. The overall sensor design and operation are described and experimental MTF, SNR and DQE data are presented together with X-ray images. (C) 2003 Elsevier Science B.V. All rights reserved.
Improvement of the Radiation Hardness of a Directly Converting High Resolution Intra-Oral X-Ray Imaging Sensor
(Elsevier Science Bv, 2003) Spartiotis, K; Pyyhtiä, J; Schulman, T
The radiation tolerance of a directly converting digital intra-oral X-ray imaging sensor reported in Spartiotis et al. [Nucl. Instr. and Meth. A 501 (2003) 594] has been tested using a typical dental X-ray beam spectrum. Radiation induced degradation in the performance of the sensor which consists of CMOS signal readout circuits bump bonded to a high resistivity silicon pixel detector was observed already before a dose (in air) of I krad. Both increase in the leakage current of the pixel detector manufactured by Sintef, Norway and signal leakage to ground from the gate of the pixel input MOSFETs of the readout circuit were observed and measured. The sensitive part of the CMOS circuit was identified as the protection diode of the gate of the input MOSFET. After removing the gate protection diode no signal leakage was observed up to a dose of 5 krad (air) which approximately corresponds to 125.000 typical dental X-ray exposures. The radiation hardness of the silicon pixel detector was improved by using a modified oxidation process supplied by Colibrys, Switzerland. The improved pixel detectors showed no increase in the leakage current at dental doses. (C) 2003 Published by Elsevier B.V.