A novel Tissue Equivalent Proportional Counter for microdosimetry at nanometer scale

Microdosimetry and track-nanodosimetry belong to a recent branch of nuclear physics, which concerns the study of theoretical and experimental methodologies able to perform a detailed analysis of the radiation interaction with matter, in particular with biological tissue at cellular and sub-cellular level. In radiotherapy the treatment planning aims to deliver sufficient radiation to the tumour while sparing critical organs and minimizing the doze to healthy tissue. To date, the Tissue Equivalent Proportional Counter (TEPC) is the most accurate device able to measure the microdosimetric properties of ionizing processes, and it has showed to properly mimic the corresponding relative biological effectiveness (RBE). Since the track structure plays a significant role at the nanometer level, the microdosimetric approach is not able to provide detailed information about that: common TEPCs are able to measure single event spectra in sites down to about 300 nm. The nanodosimetric quantities demonstrated to have a strong correlation with radiation-induced damages to the DNA. The pattern of particle interactions at the nanometer level is measured by track-nanodosimeters, which derive the single event distribution of ionization cluster size for sites of a few nanometers up to 20 nm. One of this devices is the so-called StarTrack apparatus, installed at the Tandem-Alpi accelerator complex of LNL. Unfortunately all the three present nanodosimeters worldwide are cumbersome and not designed to operate in a clinical environment. In order to characterize the response function of the TEPC in the nanometric domain, simultaneous measurements with the StarTrack track-nanodosimeter should be performed. The aim of this thesis is the design and construction of a novel detector (a partially wall-less avalanche-confinement TEPC named MiMi2) able to work at the nanometer level, and the execution of the first function test measurements once installed inside the StarTrack apparatus. The operation of the detector, which exploits three electrodes (a central anode, a helix and an external cylindrical cathode) has been characterized by measurements with a 244Cm alpha source. The gas-filled detector, operating in pulse mode, has been successfully tested down to a simulated site size of 35 nm. In order to further decrease the site size, improvements in the performances of the preamplifier are necessary. Parallely, the possibility of biasing the anode electrode has to be investigated in order to achieve a higher gas gain. The detector is now ready for characterization in accelerated ion beams. The fluctuations of the signal detected by the TEPC result from the convolution of the stochastics of physical interaction processes with the stochastics of electronic avalanche formation. Pairwise measurements with the StarTrack counter will allow to study the possibility to extract, from TEPC spectra, physical quantities that are significant for characterizing the particle track structure at nanonmeter level, much more relevant in the frame of biological damage. If the comparison between micro- and nano-dosimetric spectra revealed an unfolding procedure able to separate the cumulative distribution informations from the stochastics of the avalanche formation, the exploitation of TEPCs in evaluating the therapeutic hadron beams quality could become way more accurate.