Investigation of CdTe, GaAs, Se and Si as Sensor Materials for Mammography.

Despite the benefits of mammography investigations, some studies have shown that X-ray exposure from the mammography screening itself can statistically cause breast cancer in a small fraction of women. Therefore, a dose reduction in mammography is desirable. At the same time, there is a demand for a higher spatial resolution in mammographic imaging. The most promising way to achieve these goals is the use of advanced photon-processing semiconductor X-ray detectors with optimum sensor materials. This study addresses the investigation of the optimum semiconductor sensor material for mammography in combination with the photon-processing detector Medipix3RX. The influence of K-shell fluorescence from the sensor material on the achievable contrast-to-noise ratio is investigated, as well as the attenuation efficiency. The three different sensor materials, CdTe, GaAs, and Si are studied, showing advances of CdTe-sensors for mammography. Furthermore, a comparison of the contrast-to-noise ratio between a clinical Se-detector and Medipix3RX detectors with Si- and CdTe-sensors is shown using a self-produced mammography phantom that is based on real human tissue.

Optimization of the Timepix chip to measurement of radon, thoron and their progenies.

Radon and thoron as well as their short-lived progenies are decay products of the radium and thorium series decays. They are the most important radionuclide elements with respect to public exposure. To utilize the semiconductor pixel radiation Timepix chip for the measurement of active and real-time alpha particles from radon, thoron and their progenies, it is necessary to check the registration and visualization of the chip. An energy check for radon, thoron and their progenies, as well as for (241)Am and(210)Po sources, was performed using the radon and thoron chambers at NIRS (National Institute of Radiological Sciences). The check found an energy resolution of 200 keV with a 14% efficiency as well as a linear dependency between the channel number (cluster volume) and the energy. The coefficient of determination r(2) of 0.99 for the range of 5 to 9 MeV was calculated. In addition, an offset for specific Timepix configurations between pre-calibration for low energy from 6 to 60 keV, and the actual calibration for alpha particles with energies from 4000 to 9000 keV, was detected.

Performance of a Medipix3RX spectroscopic pixel detector with a high resistivity gallium arsenide sensor.

High resistivity gallium arsenide is considered a suitable sensor material for spectroscopic X-ray imaging detectors. These sensors typically have thicknesses between a few hundred µm and 1 mm to ensure a high photon detection efficiency. However, for small pixel sizes down to several tens of µm, an effect called charge sharing reduces a detector's spectroscopic performance. The recently developed Medipix3RX readout chip overcomes this limitation by implementing a charge summing circuit, which allows the reconstruction of the full energy information of a photon interaction in a single pixel. In this work, we present the characterization of the first Medipix3RX detector assembly with a 500 µm thick high resistivity, chromium compensated gallium arsenide sensor. We analyze its properties and demonstrate the functionality of the charge summing mode by means of energy response functions recorded at a synchrotron. Furthermore, the imaging properties of the detector, in terms of its modulation transfer functions and signal-to-noise ratios, are investigated. After more than one decade of attempts to establish gallium arsenide as a sensor material for photon counting detectors, our results represent a breakthrough in obtaining detector-grade material. The sensor we introduce is therefore suitable for high resolution X-ray imaging applications.