High-sensitivity hybrid semiconductor pixel detectors can provide precise and wide-range spectrometric and directional information of energetic charged particles in mixed radiation fields. The high-granularity, small pixel size and integrated signal processing at the pixel level allow performing single-particle tracking with high-spatial resolution and spectral response. Through detailed pattern recognition analysis of the pixelated clusters created by single particles, an enhanced resolving power of particle-event type can be achieved. The miniaturized radiation camera MiniPIX-Timepix proves to be particularly useful for characterizing primary and secondary radiation in particle therapy. Wide-range data can be obtained such as deposited energy, linear energy transfer (LET) spectra and angular distribution of particles in a wide-field of view . LET is mainly crucial for particle therapy and is an important parameter in assessing the biological effectiveness of the treatment. Using Timepix detectors, the calculation of LET is based on the deposited energy and the particle’s track length which depends on the elevation angle. However, the existing methodology for the assessment of the elevation angle and thus the path length and the LET using a single Timepix chip is limited to particles with incident angles greater than 20⁰ and with a limited angular resolution for incident angles smaller than 28⁰ .
In this work, we present a new model to derive the proton's incident angle based on a morphological analysis of the cluster track parameters, namely, track path length, roundness and linearity. Using this method, we have extended the angular sensitivity of the detector down to an elevation angle of 0⁰ (normal incidence) for Timepix detectors equipped with 300 and 500 µm thick silicon sensors. This enables the reconstruction of the particle's incident angle with an improved angular resolution of a few degrees over the full solid angle (2𝜋). As a result, the calculation of the track length across the sensor is extended and further improved, which is an essential parameter for estimating the LET. The model is applicable to protons with sufficient energy to cross the sensitive layer of the detector. By using this method, the LET spectra of a wide-range of proton energies (10 MeV to 200 MeV) were measured and compared with Monte Carlo simulations using TOPAS. A very good agreement was found between measurements, simulations and the electronic stopping power based on PSTAR (see figure 1). At low energies, approaching the Bragg peak region, variations in the measured LET values arise due to a greater difference in the energy loss registered along the single tracks especially for low-energy particles incident at large angles – e.g. 12 MeV protons at 60°. Our method shows that precise LET calculation and directional response with accurate angular resolution in extended range can already be obtained with a single layer Timepix detector reducing the need for a stacked telescope array. In future work, we will investigate how a similar approach can be applied to ions heavier than protons.