Measurements on the influence of magnetic fields on charge sharing in Medipix detectors

The position and energy resolution of hybrid photon counting pixel detectors like the Timepix detector can suffer from charge sharing: Due to diffusion the initial charge cloud of electrons (or holes) generated by ionizing radiation becomes a gaussian like distribution when arriving at the pixel electrodes. This leads to the loss of charge information in edge pixels (if the amount charge in the pixel falls below the discriminator threshold). In this work we investigated the reduction of charge sharing by applying a magnetic field parallel to the electric drift field inside the sensor layer. A reduction of the diffusion radius due to a magnetic field is well known in gases. With realistic assumptions for the mean free path of charge carriers in semiconductors, a similar effect should be observable in solid state materials. We placed a Medipix-2 detector in the magnetic field of a medical MR device with a maximum field strength of 3 T and illuminated it with photons and alpha-particles from an ²⁴¹Am source. As expected the cluster-size distribution shows a shift to smaller cluster sizes.