Quantum-imaging detection of secondary neutrons in proton radiotherapy fields

Secondary radiation fields encountered in proton radiotherapy environments contain different particle species produced in a broad range of energies and directions. Experimental knowledge of the composition and spectral characteristics of such complex fields is valuable for operation and protection of instruments and personnel, design and optimization of irradiations as well as planning and validation of treatment plans. The neutron component, which are produced with non-negligible yield, is in particular challenging to measure and discriminate from other radiations by conventional detectors. In order to measure in such complex fields the neutron component, both fast and thermal, we make use of the semiconductor pixel detector Timepix3 equipped with a silicon sensor and a neutron converter mask. The detector was before calibrated with well-defined neutron fields. In this work, we characterize the secondary radiation field and examine in particular the neutron component behind a large water-equivalent phantom irradiated by a 190 MeV clinical proton beam. The detected neutrons have a predominant fast neutron component. No thermal neutrons are observed in the measured data. The neutron-induced interactions in the detector are resolved in a high background with enhanced discrimination by quantum-imaging visualization, micrometer scale pattern recognition and high-resolution spectral-sensitive tracking of single particles. Detailed results are provided in wide range in terms of composition of the mixed-radiation field, total and partial fluxes and dose rates as well as particle deposited dose and linear-energy-transfer (LET) spectra.