Towards fragment distinction in therapeutic carbon ion beams: A novel experimental approach using the Timepix detector

Radiotherapy with carbon ion beams is a highly precise method for cancer treatment. This is due to the finite range and the relatively low lateral scattering of the carbon ions in comparison to protons. On their path through tissue, the carbon ions can undergo nuclear fragmentation, resulting in lighter projectile fragments. Since the biological effect of the fragments differs from the primary particles, it is important to consider fragmentation in beam models used for therapy planning. Until now, large apparatus have been utilized for ion spectroscopic measurements. Employing a small detector would allow investigations directly within therapy relevant phantoms. In this contribution we report on the development of such a method using the Timepix detector. Its high spatial resolution enables to visualize and distinguish tracks of single particles. Experiments were performed at the Heidelberg Ion-Beam Therapy Center, using a carbon ion pencil beam of E=271 MeV/u. To investigate fragments in different material depths, the detector was placed perpendicular to the beam with 12 to 45 cm thick PMMA slabs in front of it. The pixel-wise energy calibration allows to directly determine the particle energy loss in the sensitive layer. Charge released in the detector by an ion spreads out during charge collection and is collected by several adjacent pixels forming signal clusters. Pattern recognition analysis of the signal shows a clear dependence of cluster parameter distributions on the PMMA depth. Based on this information the particular particle species in the obtained spectra could be identified. In this way, discrimination between primary carbon ions and hydrogen, helium and heavier fragments is possible. The presented novel method enables fragment distinction in mixed particle fields. Its main advantage lies in the flexibility and small size of the set-up.