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A method to quantitatively evaluate the performances of a radiographic detection system consists in measuring the contrast, noise and modulation transfer functions. These functions have been evaluated for a digital radiographic system based on a single photon counting pixel detector. The X-ray detector is a Silicon sensor with one side segmented in a matrix of 256 by 256 square contacts with a pitch of 55 [micro]m. The active area is about 2 [cm.sup.2] The sensor is connected to the Medipix2 read-out chip by bump-bonding. As X-ray source we have used a tube for general radiography. To reproduce the conditions of a radiographic examination, a 4 cm thick lucite block positioned above the detector has been used to simulate a tissue sample. To study the Contrast Transfer Function we have measured the contrast of an 1 mm thick lead slab with respect to the background. To evaluate the scattering contribution from the lucite, the measurements have been performed with and without a collimator placed at the beam exit. To assess the efficiency and noise transfer properties, we have measured the Detective Quantum Efficiency (DQE) of the detector as a function of the tube voltage. The Modulation Transfer Function has been measured applying the slit method for different conditions of tube voltage and energy threshold. Index Terms--Contrast transfer function (CTF), detective quantum efficiency (DQE), digital radiography, medical imaging equipment, modulation transfer function (MTF), solid-state detectors.

To compare the characteristics of imaging systems based on pixel detectors of different thickness, we have measured the respective imaging capabilities, spatial resolution and spectroscopic characteristics. Each system consists of a single photon counting chip, developed in the framework of the Medipix Collaboration, bump bonded to a silicon detector. The detector is a matrix of 64 x 64 square pixels, with 170 [micro]m pitch and thickness ranging from 300 to 800 [micro]m. As expected, the intrinsic detection efficiency increases with the detector thickness; nevertheless the spatial resolution can be affected by a charge sharing mechanism between adjacent pixels due to charge diffusion. The aim of the work reported in this paper is to demonstrate that, optimizing the settings of the image systems, we can increase the detection efficiency without losing in spatial resolution.

We present the first tests of a digital imaging system based on a silicon pixel detector bump bonded to an integrated circuit operating in single-photon counting mode. The X-ray sensor is a 300-[micro]m-thick silicon, 14 x 14 [mm.sup.2] sensitive area, upon which a matrix of 256 x 256 (65536) square pixel, 55 [micro]m in side, has been built. The readout chip, named MEDIPIX2, has been developed at CERN within the MEDIPIX2 collaboration and it is composed by a matrix of 65536 readout channels exactly with the same geometry of the detector. The spatial resolution properties of the system have been assessed by measuring the square-wave resolution function and the first images of a standard mammographic phantom were acquired using a radiographic tube in the clinical irradiation condition. Index Terms--Mammography, signal-to-noise ratio (SNR), silicon pixel detector, single-photon counting chip, square-wave resolution function (SWRF).

In mammography, the first cause of image contrast reduction arises from the photons scattered inside the examined organ. The amount of Compton scattering strongly depends on the irradiation area and on the distance between the organ and the X-ray detector. We have experimentally evaluated how these geometrical conditions affect the scattering fraction. Our experimental setup includes a single photon counting device based on a silicon pixel detector as X-ray sensor, a lucite cylinder to simulate the breast tissue, and a lead collimator to define the irradiation area. We have evaluated the contrast and the signal-to-noise ratio for images acquired in different conditions. Index Terms--Contrast, mammography, pixel detector, scattered photons, signal-to-noise ratio (SNR), single photon counting chip.

High leakage currents and incomplete charge collection limit the performance of detectors obtained from bulk semi-insulating (SI) GaAs. In this contest, a crucial role is played by the electron limiting contact. We have investigated the effect of an epitaxial p-type layer deposited on the semi-insulating substrate in place of the usual Schottky contact. Electrical characterization evidences an effect of the p-layer only at low voltages, but radiation detection experiments show an improvement of the charge collection efficiency, which is function of the doping level of the p-layer. In particular both [Alpha] and x irradiations give evidence of gain mechanisms in the collected charge. From these results, the strong relation between the structure of the electron-blocking interface and the charge collection properties of these detectors comes out.

Semi-Insulating Gallium Arsenide (SI-GaAs) devices have been tested for radiation hardness with 3-4 MeV or 24 GeV proton beams. These devices can be operated in dc mode as optically activated electrical switches up to 1 kV. Both single switches (vertical Schottky diodes) and multiple (8) switches (planar devices) have been studied, by analyzing their current-voltage (I-V) reverse characteristics in the dark and under red light illumination, both before and after irradiation. We propose to use them in the system of high-voltage (-600 V) switches for the microstrip gas chambers for the CMS experiment at CERN. Low energy protons (3-4 MeV) were used in order to produce a surface damage below the Schottky contact: their fluence (up to 2.6x[10.sup.15]/[cm.sup.2]) gives a high-dose irradiation. The high energy proton irradiation (energy: 24 GeV, fluence: 1.1x[10.sup.14] p/[cm.sup.2]) reproduced a ten years long proton exposure of the devices in CMS experiment conditions. For low energy irradiation, limited changes of the I-V curves in the dark have been observed, with at most a fourfold increase of the leakage current: after exposure, however, the breakdown voltage decreases significantly. For high energy irradiation, we observed - for the vertical Schottky diodes biased at -600 V - an increase of the leakage current and a reduction of the photocurrent after irradiation, with respect to pre-irradiation conditions. For these diodes, the reduction of the photocurrent/dark current ratio was 25:1. At the same proton energy, an analogous behaviour was shown by the planar devices, but after irradiation the current gain may reduce over three orders of magnitude.

Solid state detectors made of Si (4.8x8 [mm.sup.2]) and GaAs (6.4x8 [mm.sup.2]) pixel matrices bump-bonded to the Omega2 and Omega3 electronic read-out systems, developed at CERN for H.E.P. experiments, have been used to obtain autoradiographic images of clusters of human epithelial cells and DNA fragments separated via electrophoresis, both labeled with 32p. The system has shown a good minimum detectable activity per unit area of 2. [10.sup.-4] cps [mm.sup.-2], and has proved linear for count rate in the range 0.2-20 cps, typical of autoradiography. The pixel dimensions are 75x500 [[[micro]meter].sup.2] (Si-Omega2) and 50x500 [[[micro]meter].sup.2] (GaAs-Omega3), respectively. We are able to clearly localize clusters of cells which have incorporated the radioactive tracer and DNA fragments on an electrophoretic gel on paper (blots).

A digital mammography system based on GaAs pixel detector has been developed by the INFN (Istituto Nazionale di Fisica Nucleare) collaboration MED46. The high atomic number makes the GaAs a very efficient material for low energy X-rays detection (10 - 30 keV is the typical energy range used in mammography). Low contrast details can be detected with a significant dose reduction to the patient. The system presented in this paper consists of a 4096 pixel matrix built on a 200 [micro]m thick Semi Insulating GaAs substrate. The pixel size is 170 x 170 [micro][m.sup.2] for a total active area of 1.18 [cm.sup.2]. The detector is bump-bonded to a VLSI Front-End chip which implements a single-photon counting architecture. This feature allows to enhance the radiographic contrasts detection with respect to charge integrating devices. The system has been tested by using a standard mammographic tube. Images of mammographic phantoms will be presented and compared with radiographs obtained with traditional film/screen systems. Monte Carlo simulations have been also performed to evaluate the imaging capability of the system. Comparison with simulations and experimental results will be shown.