LAUSR

Compact silicon on insulator (SOI) microdosimeters have been used to characterise the radiation field of many different hadron therapy beams. SOI devices are particularly attractive in hadron therapy fields due to their spatial resolution being well suited to the sharp dose gradients at the end of the primary beam's range. Due to the small size of SOI's sensitive volumes (SVs), which are usually $\sim$1-10 $\mu$m thick, the fabrication of these devices can present challenges which are not as common for more conventional thickness silicon devices such as silicon spectroscopy detectors. Microdosimetry is the study of the energy deposition in micrometre sized volumes representing biological sites and is a powerful approach to estimate the biological effect of radiation on the micron-scale level, in a cell. However, cell sizes vary extensively translating in different energy deposition spectra. This work studies SV thicknesses between 1 and 100 $\mu$m using Geant4 and examines the impact of SV dimensions on microdosimetric quantities. The quantities studied were the frequency mean lineal energy, $y_F$, and the dose mean lineal energy, $y_D$. Additionally the relative biological effectiveness (RBE), estimated by the microdosimetric kinetic model (MKM), is also investigated. To study the impact of the SV thickness, SOI microdosimeters were irradiated with proton, $^{4}$He and $^{12}$C ion beams with ranges of $\sim$160 mm, with the microdosimeter being set at various positions along the Bragg curve. It was found that $y_F$ was influenced the least in proton beams and increased for heavier ion beams. Conversely, $y_D$ was impacted by the SV thickness the most in proton beams and $^{12}$C was the least. Similar to $y_D$, protons were impacted the most by the SV thickness when estimating the RBE using the MKM. The cause of these differences was largely due to the different densities of the track structure for the case of $y_F$ and the energy transferred to the medium from the primary beam for $y_D$.