Medipix2 Device Research Articles

Medipix2 based CdTe microprobe for dental imaging

Medical imaging devices and techniques are demanded to provide high resolution and low dose images of samples or patients. Hybrid semiconductor single photon counting devices together with suitable sensor materials and advanced techniques of image reconstruction fulfil these requirements. In particular cases such as the direct observation of dental implants also the size of the imaging device itself plays a critical role. This work presents the comparison of 2D radiographs of tooth provided by a standard commercial dental imaging system (Gendex 765DC X-ray tube with VisualiX scintillation detector) and two Medipix2 USB Lite detectors one equipped with a Si sensor (300 μm thick) and one with a CdTe sensor (1 mm thick). Single photon counting capability of the Medipix2 device allows virtually unlimited dynamic range of the images and thus increases the contrast significantly. The dimensions of the whole USB Lite device are only 15 mm × 60 mm of which 25% consists of the sensitive area. Detector of this compact size can be used directly inside the patients' mouth.

Study of the charge sharing in silicon pixel detector by means of heavy ionizing particles interacting with a Medipix2 device

The charge sharing effect has been investigated in a 300 μm thick silicon pixel detector bump-bonded to a Medipix2 readout chip and exposed to heavy ionizing particles of various energies, namely protons and alpha-particles. The lateral spread of the charge carriers from the interaction of a heavy ionizing particle in the silicon detector causes a sharing of the charge among adjacent pixels, giving a signal in clusters of multiple pixels of the Medipix2 device. A model of charge sharing, including the plasma effect on top of diffusion and drift processes has been compared to the measured lateral spread of the charge for bias voltage values beyond the full depletion voltage. Results of the measurements of the cluster size as a function of the energy and incidence angle of incoming heavy ionizing particles are also reported and compared with the model.

Precise energy calibration of pixel detector working in time-over-threshold mode

The semiconductor pixel detector Timepix (256×256 pixels with pitch of 55 μm) is a successor of the Medipix2 device. Each Timepix pixel can be independently operated in one of three possible modes: (1) counting of the detected particles; (2) measurement of the particle energy; and (3) measurement of the time of interaction. The energy measurement in the second mode is performed via the determination of the “time-over-threshold” (TOT). The energy measurement with the Timepix detector in TOT mode requires knowledge of the energy calibration of each pixel of the matrix. Such calibration is very nonlinear in the low energy range and can be described by a surrogate function depending on four parameters. The determination of all these parameters can be performed by measurement and evaluation of the response of each pixel in at least four calibration points. The procedure is extremely demanding: it requires the analysis of at least 250 thousand spectra and the performance of 330 thousand least-squares fits. In this article, it is demonstrated that even better result can be achieved with only two or three calibration points halving the number of least-squares fits needed. The method is based on precise analysis of the shape of spectral peaks. The article also discusses the performance of energy calibrated device for spectrometry of heavy charged particles.

Measurement of pattern recognition efficiency of tracks generated by ionizing radiation in a Medipix2 device

Several experiments were performed to establish the Medipix2 device capabilities for track recognition and its efficiency at measuring fluxes. A Medipix2 device was exposed to 241Am, 106Ru and 137Cs radioactive sources, separately and simultaneously. It was also exposed to heavy particle beams (protons and alpha-particles), recoiled on a gold foil to reduce the incoming flux and allow the study of the detector response struck by incoming particles at different incidence angles. For three proton beams (400 keV, 4 and 10 MeV), the device was exposed to the radioactive sources on top of beam, giving a mixed radiation field. To test the reliability of track recognition with this device, the activities of the radioactive sources were extracted from the experimental data and compared to the expected activities. Rotation of the Medipix2 device allowed the test of the heavy tracks recognition at different incidence angles.

Study of charge sharing in a silicon pixel detector with heavy ionizing particles interacting with a Medipix2 device

The charge sharing effect among adjacent pixels of a Medipix2 pixel detector comes from the lateral spread of charges generated from the interaction of an ionizing particle in silicon. The charge sharing follows specific ionizing losses of particle along its track in silicon influenced by several effects that cause the lateral spread of the charge such as the plasma effect, charge column erosion and funneling on top of diffusion and drift processes in an electric field. A model that takes into account these effects has been used to fit results of cluster size measurements as a function of the bias voltage for a Medipix2 silicon pixel detector (256×256 pixels, each measuring 55 μm×55 μm×300 μm) exposed to a beam of heavily ionizing particles (protons, α-particles, carbon, and oxygen ions). Results on the response of the device to protons with different incident angles are also presented. In particular, the angular detection sensitivity measured for protons of 10 MeV is reported.

Study of the charge sharing in a silicon pixel detector by means of α-particles interacting with a Medipix2 device

The energy deposited in a silicon detector by a heavy charged particle, such as an α-particle, creates a large number of electron–hole pairs. Under the influence of an electric field, the carriers drift towards the corresponding electrode. Due to diffusion, the charge carriers are spread. Lateral spreading depends on the collection time; hence it is expected to be smaller for larger fields. In the case of a pixellated detecting structure, this lateral spread can cause a sharing of the charge between the electrodes and many pixels will have a signal: that is, charge carriers generate a cluster of adjacent pixels. Also influencing the charge collection and its spread is the large concentration of electron–hole pairs generated locally by the α-particle, which creates distortions of the electric field along the ionizing path, giving rise to the so-called plasma and funnelling effects. The results of the charge-sharing effect measured in the Medipix2 pixel detectors are shown as a function of the α-particle energy and applied bias voltage. A model describing the effects of plasma and diffusion on the charge collection and charge sharing is presented.

A noiseless kilohertz frame rate imaging detector based on microchannel plates read out with the Medipix2 CMOS pixel chip

A new hybrid optical imaging detector is described that is being developed for the next generation adaptive optics (AO) wavefront sensors (WFS) for ground-based telescopes. The detector consists of a photocathode and proximity focused microchannel plates (MCPs) read out by the Medipix2 CMOS pixel ASIC. Each pixel of the Medipix2 device measures 55 × 55 μ m 2 and comprises pre-amplifier, a window discriminator and a 14-bit counter. The 256 × 256 Medipix2 array can be read out noiselessly in 287 μ s . The readout can be electronically shuttered down to a temporal window of a few μ s . The Medipix2 is buttable on three sides to produce 512 × ( n * 256 ) pixel devices. Measurements with ultraviolet light yield a spatial resolution of the detector at the Nyquist limit. Sub-pixel resolution can be achieved using centroiding algorithms. For the AO application, very high continuous frame rates of the order of 1 kHz are required for a matrix of 512 × 512 pixels. The design concepts of a parallel readout board are presented that will allow this fast data throughput. The development status of the optical WFS tube is also explained.