Microdosimetry Maps in Continuous and Pulsed Alpha Beams

Abstract

The experimental assessment of the linear energy transfer (LET) at micrometric scale with high resolution is mandatory to achieve biologically optimized treatment plans in Hadron therapy. Therefore, it is crucial to create active radiation sensors able to quantify microdosimetry distributions in three-dimensions under clinical conditions. We have quantified the first microdosimetry 2-D-maps delivered by alpha beams with the highest resolution so far ( $400~\mu $ m $\times 600~\mu $ m) using a customized sensor. It consists of 3 multiarrays of $50~3\times 3$ microdetectors with a pitch of $200~\mu $ m that cover a total radiation-sensitive area of 9 cm $\times 0$ .4 mm. The microdetectors are based on a novel 3-D-cylindrical silicon architecture with 25- $\mu $ m diameter and 20- $\mu $ m thickness. The back-side of these sensors is etched to avoid back-scattering contributions in heavy ion fields. This system was calibrated with alpha beams from 5.75 to 20 MeV in the Accélérateur Linéaire et Tandem à Orsay (ALTO), France, and irradiated with pulsed beams (from $1.4~\mu $ s to $3.3~\mu $ s pulse widths) of 7.5 MeV alphas in the Centro de Micro-Análisis de Materiales (CMAM), Spain. We worked with clinical equivalent fluence rates in both continuous irradiation mode ( $\sim ~10{^{{8}}}$ cm ${^{-}2 } \cdot $ s ${}^{-}1 $ ) and in pulsed regimes with higher fluence rates up to $\sim ~10{^{{10}~}}$ cm $\cdot {^{-}2 }$ s ${}^{-}1 $ . Measurements of pulse height spectra were collected with the sensor in both modes. The experimental results were crosschecked with Monte Carlo simulations using the GATE code. The simulated spectral shapes and peak positions showed good agreement with the experimental data, with relative differences of less than 3.5% for all the energies studied. Results show that the new microdosimetry array, specially manufactured for ions, is a reliable tool for LET characterization of ions in both continuous and pulsed modes.