Charge collection from proton and alpha particle tracks in silicon pixel detector devices

The lateral spread of charge carriers under the influence of the electric field in a pixellated silicon detector hit by a heavy charged particle, such as a proton or an alpha-particle, causes a sharing of the charge between the electrodes and many pixels have a signal. The results of the charge sharing effect measured in the Medipix2 and Timepix pixel detectors of 300 mum thicknesses is shown as a function of particle energy and applied bias voltage. A model describing the effects of funneling, plasma and diffusion on the charge collection and its sharing will be also presented. Using Timepix, it is possible to measure directly the quantity of charge deposited in each pixel within the cluster and to follow changes in charge collection as a function of collection time. This allows 3D-visualization of individual tracks of charged particles in silicon with Timepix.