The interaction of proton and hadron beams of relativistic energy in matter is accompanied by the production of penetrating highly energetic secondary particles and nuclear reaction products which can affect the desired highly localized deposition of energy of the primary high LET particles for radiotherapy purposes. The observation of all ionizing particles arising from and accompanying the primary radiation as well as the direct measurement of particle energy loss, trajectory, local energy deposition and lateral straggling can be directly provided by the semiconductor pixel detector Timepix. This device operates as an active nuclear emulsion providing on-line visualization of particle traces. Results of a pilot experiment are presented with proton and carbon ion beams in the energy range 48–220 MeV/u and 88–430 MeV, respectively produced at the Heavy Ion Therapy HIT facility in Heidelberg.