J. Watt

Characterisation of a single photon counting pixel system for imaging of low-contrast objects

In the framework of the Medipix1 collaboration the PCC, a single photon counting pixel chip, has been developed with the aim of improving the contrast resolution in medical imaging applications. The PCC consists of a matrix of 64]64 square pixels with 170 lm side length, each pixel comprising a 15-bit counter and a pulse-height discriminator. The chip has been bump bonded to equally segmented 200 lm thick SI-LEC GaAs detectors showing a very high absorption energy for X-rays used in diagnostics. An absolute calibration of the system with a radioactive source and a synchrotron beam are described resulting in the value of the test input capacitance of &24.7 fF. Using this value a full characterisation of the system from electrical measurements is presented. The entire system can reach a minimum threshold of &2100e~ with &250e~ rms noise. One of the characteristic features of the PCC is the possibility to adjust the thresholds of all pixels on a pixel-by-pixel basis with 3-bit precision. The threshold distribution after adjustment is &120e~ rms. The spatial resolution of the system has been measured to be 3.6 lp/mm. A comparison of a tooth image taken with the PCC and with a screen-CCD-system demonstrates its imaging capabilities. ( 2001 Elsevier Science B.V. All rights reserved.

X-ray imaging using a hybrid photon counting GaAs pixel detector

Abstract The performance of hybrid GaAs pixel detectors as X-ray imaging sensors were investigated at room temperature. These hybrids consist of 300 μm thick GaAs pixel detectors, flip-chip bonded to a CMOS Single Photon Counting Chip (PCC). This chip consists of a matrix of 64 × 64 identical square pixels (170 μm × 170 μm) and covers a total area of 1.2 cm 2 . The electronics in each cell comprises a preamplifier, a discriminator with a 3-bit threshold adjust and a 15-bit counter. The detector is realized by an array of Schottky diodes processed on semi-insulating LEC-GaAs bulk material. An IV-characteristic and a detector bias voltage scan showed that the detector can be operated with voltages around 200 V. Images of various objects were taken by using a standard X-ray tube for dental diagnostics. The signal to noise ratio (SNR) was also determined. The applications of these imaging systems range from medical applications like digital mammography or dental X-ray diagnostics to non destructive material testing (NDT). Because of the separation of detector and readout chip, different materials can be investigated and compared.

Development of low-pressure vapour-phase epitaxial GaAs for medical imaging

Abstract A summary is given of progress accomplished with the development of low-pressure vapour-phase epitaxial GaAs as a material for X-ray detectors. As the III–V concentration ratio is altered from Ga-rich to As-rich, the material is shown to improve from p-type, to n-type with compensation via deep levels, to n-type with a doping density of 1.7×1014 atoms cm−3. The measured barrier height is 0.8 V, as expected for the Ti contact used. Overdepletion was obtained before breakdown, enabling a layer thickness of 41 μ m to be deduced for the final sample. For the later samples, charge collection for 60 keV Am-241 gammas was bias independent at a value of 100±8%. Spectra were also obtained from Sr-90 electrons. The most probable value of the charge collected as a function of the bias reached a plateau and from this value a depletion width of 40 μ m was found for the final sample, equal to the epitaxial layer thickness. Results from detailed alpha and low-energy proton spectroscopy are shown for diodes fabricated from this material. A charge collection efficiency of 100% was obtained when the diode could be depleted sufficiently. The concept of a charge collection depth was introduced, since a significant amount of charge was collected without bias. The minimum depth of such a region was shown to be 10.8 μ m at 0 V reverse bias, far greater than the 1.1 μ m predicted for the depletion depth. Charge coupling between the guard ring and the pad was observed and successfully modelled.

X-ray imaging with photon counting hybrid semiconductor pixel detectors

Semiconductor pixel detectors, originally developed for particle physics experiments, have been studied as X-ray imaging devices. The performance of devices using the 3 read-out chip bump-bonded to pixellated silicon semiconductor detectors is characterised in terms of their signal-to-noise ratio when exposed to 60 kVp X-rays. Although parts of the devices achieve values of this ratio compatible with the noise being photon statistics limited, this is not found to hold for the whole pixel matrix, resulting in the global signal-to-noise ratio being compromised. First results are presented of X-ray images taken with a gallium arsenide pixel detector bump-bonded to a new read-out chip, (MEDIPIX), which is a single photon counting read-out chip incorporating a 15-bit counter in every pixel.

Applications of pixellated GaAs X-ray detectors in a synchrotron radiation beam

Hybrid semiconductor pixel detectors are being investigated as imaging devices for radiography and synchrotron radiation beam applications. Based on previous work in the CERN RD19 and the UK IMPACT collaborations, a photon counting GaAs pixel detector (PCD) has been used in an X-ray powder diffraction experiment. The device consists of a 200 μm thick SI-LEC GaAs detector patterned in a 64×64 array of 170 μm pitch square pixels, bump-bonded to readout electronics operating in single photon counting mode. Intensity peaks in the powder diffraction pattern of KNbO3 have been resolved and compared with results using the standard scintillator, and a PCD predecessor (the Ω3). The PCD shows improved speed, dynamic range, 2-D information and comparable spatial resolution to the standard scintillator based systems. It also overcomes the severe dead time limitations of the Ω3 by using a shutter based acquisition mode. A brief demonstration of the possibilities of the system for dental radiography and image processing are given, showing a marked reduction in patient dose and dead time compared with film.

Dose-dependent X-ray measurements using a 64 64 hybrid GaAs pixel detector with photon counting

New developments in medical imaging head towards semiconductor detectors flip-chip bonded to CMOS readout chips. In this work, detectors fabricated on SI-GaAs bulk material were bonded to Photon Counting Chips. 1 This PCC consists of a matrix of 64 � 64 identical square pixels (170mm � 170mm) with a 15-bit counter in each cell. We investigated the imaging properties of these detector systems under exposure of a dental X-ray tube. First, a dose calibration of the X-ray tube was performed. Fixed pattern noise in flood exposure images was determined for a fixed dose and an image correction method, which uses a gain map, was applied. For characterising the imaging properties, the signal-to-noise ratio (SNR) was calculated as function of exposure dose. Finally, the dynamic range of the system was estimated. # 2001 Elsevier Science B.V. All rights reserved.

Detective quantum efficiency of the Medipix pixel detector

We have measured the intrinsic performance of a digital X-ray detector, the Medipix1, by examining the total detective quantum efficiency (DQE). We studied how the DQE depends on both the incident photon energy and spatial frequency. Reported here is the calculation of the detective quantum efficiency for the case of a 300 /spl mu/m thick silicon diode detector attached to the Medipix1 readout chip. This was done by determining the modulation transfer function and the noise power spectrum; together these allow the frequency component of the DQE to be calculated. The X-ray absorption efficiency in the detector gives the dependence on incident energy. This system was found to have a DQE that peaked at 0.118, using a dental X-ray source, and dropped to 0.049 at the Nyquist frequency of 2.94 line pairs per mm.

The simulation of charge sharing in semiconductor X-ray pixel detectors

Two simulation packages were used to model the sharing of charge, due to the scattering and diffusion of carriers, between adjacent pixel elements in semiconductors X-ray detectors. The X-ray interaction and the consequent multiple scattering was modelled with the aid of the Monte Carlo package, MCNP. The resultant deposited charge distribution was then used to create the charge cloud profile in the finite element semiconductor simulation code MEDICI. The analysis of the current pulses induced on pixel electrodes for varying photon energies was performed for a GaAs pixel detector. For a pixel pitch of 25 μm, the charge lost to a neighbouring pixel was observed to be constant, at 0.6%, through the energies simulated. Ultimately, a fundamental limit on the pixel element size for imaging and spectroscopic devices may be set due to these key physical principles.

Simulated and experimental results from a room temperature silicon X-ray pixel detector

Simulated and experimental results are presented from a silicon X-ray pixel detector which is bump bonded to a PAC5 pixel array of read-out electronics. When coupled to a matching, fully depleted silicon detector the pre-amplifier is observed to have a linear response up to 80 keV, and a pulse height resolution of around 1 keV FWHM over the range 13–60 keV. The Monte-Carlo N-Particle code has been used to simulate the detector response under illumination from a variety of energies. The excellent agreement observed between simulation and experiment illustrates the predictive abilities of such packages.

Image quality of the Medipix pixel detector: detective quantum efficiency

We have measured the intrinsic performance of a digital X-ray detector, the Medipix1, by examining the total detective quantum efficiency (DQE). We studied how the DQE depends on both the incident photon energy and spatial frequency. Reported here is the calculation of the detective quantum efficiency for the case of a 300 /spl mu/m thick silicon diode detector attached to the Medipix1 readout chip. This was done by determining the modulation transfer function and the noise power spectrum; together these allow the frequency component of the DQE to be calculated. X-ray absorption efficiency in the detector gives the dependance on incident energy. This system was found to have a DQE that peaked at 0.118, using a dental X-ray source, and dropped to 0.049 at the Nyquist frequency of 2.94 line pairs per mm.