In carbon ion beam radiotherapy, nuclear fragmentation in tissue leads to an attenuation of the primary particle flux and the build‐up of lighter projectile fragments. The radio‐biological effectiveness of these secondary particles differs from the primary ions. Therefore, fragmentation has to be considered in physical models used for therapy planning. While until now large apparatus have been used for ion spectroscopic measurements, we developed an experimental technique to measure lateral distributions of charged particles directly within or closely behind phantoms, using a small pixelated detector.