Composition and Spectral Characterization of Mixed-Radiation Fields with Enhanced Discrimination by Quantum Imaging Detection

Mixed radiation fields in environments such as particle radiotherapy and outer space exhibit large complexity in terms of composition and spectral distribution which are difficult to measure in detail. For this purpose, we present a high-sensitivity technique using the pixel detector Timepix3 to measure the composition and spectral-tracking characterization of secondary fields produced in proton radiotherapy. Particle-event classes are resolved into broad groups of high-energy transfer particles (HETP), such as protons, ions and neutrons, as well as low-energy transfer particles (LETP), such as electrons, X rays and, partly, low-energy gamma rays. The quantum-imaging capability of Timepix3 is exploited to enhance the resolving power for particle-type classification. The particle tracks are analyzed by spectral-sensitive pattern recognition algorithms. The response matrix for Timepix3 is newly derived and is based from experimental calibrations in well-defined radiation fields including in-beam rotational scans of protons performed at various energies and directions. Clinical proton beams of radiotherapeutic intensities and energies in the range 225 - 12 MeV were used in experimental configurations with and without a tissue-equivalent phantom. Detailed results of radiation components can be used to produce total and partial particle fluxes, dose rate, absorbed dose, deposited energy and linear-energy-transfer (LET) spectra. Dedicated Monte Carlo (MC) simulations are compared with experimental results of field composition, particle fluence, and deposited energy. The numerical information aids the interpretation of experimental data which includes also secondary neutrons. The technique and developed methodology can be applied for research and routine measurements in environments of varying complexity.