## 3 Results
### Secondary Particle Characterisation  
From the simulations performed in Geant4, a further characterisation of the secondary particles created within the primary beam axis was performed. It was estimated that, within the proton energy range used in this study, the number of secondary gammas (i.e., with a parent ID greater than or equal to 1 and for any value of kinetic energy) produced per primary proton ranges between 0.07 and 0.39, depending on the proton energy. In terms of the contribution of gammas and neutrons to the total amount of secondary particles produced inside the phantom, 43-63% of all secondaries correspond to gammas and 19-27% to neutrons, bearing in mind that the gamma percentage decreases with increasing beam energy, whereas the neutron percentage increases with increasing beam energy. To obtain information about the particles entering the detector area, a dummy volume was created right in front of the sensitive detector to read out the incoming flux, as well as the particle type, kinetic energy and parent ID. The secondary gamma yield in the detector area is much lower, ranging from 6.6 \(\times\) 10\({}^{-7}\) to 5.98 \(\times\) 10\({}^{-7}\) per primary proton. Moreover, the particle-type information evidences a dominance of gamma particles, where more than 80% of all interacting particles in the detector are gammas, in spite of the fact that, in reality, they comprise a lower percentage of all secondaries. This suggests that the detection system favours gamma detection over other particles, and the noise that may come from neutrons, electrons, positrons and isotopes is discarded by the system. In particular, neutrons, which are the most common secondary particles produced during treatment after gammas, have a low probability of interaction with the sensor material, but other particles, such as electrons and other isotopes, need a deeper analysis to be filtered out. A cluster size analysis was conducted to test the previously stated hypothesis where the noise reduction coming from other particles can be achieved by discriminating events in virtue of their cluster size. Figure 5 displays the result of such a discrimination process. Each bin value is obtained by calculating the fraction of particles with a determined cluster size over all secondaries. The values are calculated for each proton beam energy and then averaged. The reported data in the figure are the average, with the error bars representing the standard deviation. It can be seen that, of all secondary particles detected by HEXITEC, around 65% of them were one-pixel-cluster gammas, which is expected: for the pixel pitch of this detector, there is not a high level of charge-sharing for gamma particles. This implies that a simple threshold in the cluster size removes a significant proportion of background events that are not of interest. Furthermore, the graph suggests that other particles such as neutrons  
Figure 5: Cluster size histogram showing the differences in contribution between gammas and other particles. The sum of all bins (gammas and other particles together) equals 100%. The values are averaged over all proton energies and the error bars are obtained from the standard deviation. and electrons can be filtered from the data set via cluster size alone, given that their mean values are above three pixels. This is because, on the one hand, neutrons interact with the detector via multiple scattering, neutron capture (as Cd has a considerable cross-section for this physical process), among others, which causes new particles to be produced and deposit energy at further distances from the original hit. However, they can potentially be identified as they typically scatter before being captured. The subsequent relaxation emits a gamma that usually escapes the detector volume.