From micro to nanodosimetry with an avalanche-confinement TEPC: Characterization with He-4 and Li-7 ions

The measurable radiobiological effects of ionizing radiation strongly depend on the clustering of damages in subcellular sites, which are related to the particles track structure. The characteristic properties of track structure are directly measurable nowadays with bulky experimental apparatuses, which are not easily transportable and are not suited for the clinical environment. For this reason, the feasibility of new transportable detectors capable of characterizing real therapeutic beams was investigated in recent years. In particular, two novel avalanche-confinement Tissue Equivalent Proportional Counters (TEPCs) were designed and constructed for simulating nanometric sites down to 25 nm: a sealed version for operation in plain air and a new prototype with perforated walls and without external encapsulating cap for operation in a vacuum chamber with accelerated particle beams. This work is focused on the response of the open TEPC, directly installed in the vacuum chamber of the STARTRACK nanodosimeter of the Italian Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali di Legnaro (INFN-LNL), against He-4 particles from a 244Cm isotopic source and 26.7 MeV Li-7 ions accelerated by the Tandem accelerator of LNL. Experimental cluster size distributions of He-4 and Li-7 ions measured with this TEPC are compared with nanodosimetric Monte Carlo simulations and with cluster size distributions measured with the STRATRACK nanodosimeter. Both comparisons highlight a very good agreement: the relative variance added by the electron multiplication process results negligible if compared to the variability of the ionization process. This encourages the use of the avalanche-confinement TEPC as a portable detector for nanodosimetric characterization of particle tracks.