Energy-sensitive X-ray radiography and charge sharing effect in pixelated detector

The semiconductor pixel detector TimePix (256×256 pixels with a pitch of 55 μm) is a newly developed successor of the Medipix2 device. Each Timepix pixel can be operated in Time-over-Threshold mode allowing direct energy measurement. The possibility of per pixel energy measurement presents a substantial advantage for X-ray radiography with polychromatic X-ray sources (tubes) allowing material identification (colour X-ray radiography) via analysis of transmission energy spectra. The energy resolution of the pixelated planar detector is, however, influenced by charge sharing. The ionization charge generated by the particle (X-ray photon) can be collected by several pixels, forming a cluster. If the fraction of the charge collected by a particular pixel drops below the energy threshold it is not counted and therefore it is lost. This fact, together with the presence of noise, limits the energy resolution of the device. A simplified model of the charge sharing effect is studied in this work and compared with experimental data. The application of a (per-pixel) energy-calibrated TimePix detector in the field of energy-sensitive X-ray radiography enables the recognition of soft biological materials (fat and muscle tissue) which is presented here.