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Microdosimetry measures the stochastics of imparted energy at the micrometre scale, and is a reliable experimental technique to monitor complex radiation fields such as those used in hadron therapy. At the Legnaro National Laboratories of INFN, miniaturized gas-based microdosimeters were developed specifically for this kind of applications. However, their use outside research facilities has been hindered by the encumbrance of the gas-flow system which is used to preserve gas purity and of the high-resolution analog electronic chain. To overcome this drawback, a new detector designed to work without gas flow was developed recently. The stability and reproducibility of its response in sealed conditions were studied in two measuring shifts one year apart from each other, both with the analog electronic chain and with a compact digital acquisition system. Preliminary results confirm the possibility to operate the detector with a very compact experimental setup, which could be a major advantage in clinical facilities.

It is recognized today that the observable radiobiological effects of ionizing radiations are strongly correlated to the clustering of damages in micrometer- and nanometer-sized subcellular structures, hence to the particle track structure. The characteristic properties of track structure are directly measurable nowadays with bulky experimental apparatuses, which cannot be easily operated in a clinical environment. It is therefore interesting to investigate the feasibility of new portable detectors able to characterize the real therapeutic beams. With this in mind, a novel avalanche-confinement Tissue Equivalent Proportional Counter (TEPC) was constructed for simulating nanometric sites down to 25 nm. Experimental cluster size distributions measured with this TEPC were compared with Monte Carlo simulations of the same experiment and with cluster size distributions measured with the Startrack nanodosimeter.

Mono-energetic carbon ion scanning beams of 195.2 MeV/u at the Italian National Centre for Oncological Hadrontherapy (CNAO) have been used to study the microdosimetric features of an “active” carbon ion beam used in hadrontherapy. A 30x30 mm2 area has been scanned by a 6 mm beam with scanning steps of 2 mm. A mini TEPC of 0.57 mm3 has been used to perform measurements in a water phantom at different depths on the beam axis. The detector small size allowed for measuring, with good spatial resolution, also inside the relatively small Bragg peak region and inside the distal edge, where the radiation quality varies quickly. In spite of the high event rate (up to ~ 105 s-1), no pile-up effects were observed. Results showed that the frequency-mean lineal energy scaled well with the absorbed dose. Moreover, the dose-mean lineal energy itself seemed to be a good descriptor of the radiation quality.

A microdosimetric characterization of the 62 MeV proton beam line of CATANA has been performed all along the Spread Out Bragg Peak with three different detectors. Two silicon detectors and a Tissue Equivalent Proportional Counter measured at approximately the same depths of the SOBP. The TEPC is a new miniaturized gas counter developed at the Legnaro National Laboratories of INFN, modified to work without gas flow. The first silicon detector has been developed at the Politecnico of Milano and it is a monolithic telescope composed by a matrix of 2 µm thick cylindrical diodes with a diameter 9 µm. that compose the ΔE layer. The E and ΔE layers are fabricated on a single substrate of silicon. The third detector is the MicroPlus probe developed at the CMRP - University of Wollongong, it is an array of 3D sensitive volumes each with dimension 30x30 µm and 10 µm thick fabricated on SOI. Measurements performed with the three detectors are presented and discussed.

The tissue equivalent proportional counter (TEPC) is the most accurate device for measuring the microdosimetric properties of a particle beam, nevertheless no detailed information on the track structure of the impinging particles can be obtained, since the lower operation limit of common TEPCs is about 0.3 μm. On the other hand, the pattern of particle interactions is measured by track-nanodosimetry, which derives the single-event distribution of ionization cluster size at the nanometric scale. Anyway, only three nanodosimeters are available worldwide. A feasibility study for extending the performances of TEPC down to the nanometric region was performed and a novel avalanche-confinement TEPC was designed and constructed. This detector is constituted by a cylindrical chamber, based on a three-electrode structure, connected to a vacuum and gas flow system to ensure a continuous replacement of the tissue equivalent gas, thus allowing to simulate different biological site sizes in the range 300-25 nm. This TEPC can be calibrated by exploiting a built-in alpha source and a miniaturized solid-state detector as a trigger. Irradiations with photons, fast neutrons and two hadron beams demonstrated the good performances of the device. A satisfactory agreement with FLUKA simulations was obtained.

The tissue equivalent proportional counter (TEPC) is the most accurate device for measuring the microdosimetric properties of a particle beam, showing to properly assess the relative biological effectiveness by linking the physical parameters of the radiation with the corresponding biological response. Nevertheless no detailed information on the track structure of the impinging particles can be obtained, since the lower operation limit of the common TEPCs is about 0.3 ?m. On the other hand, the pattern of particle interactions at the nanometer level, which demonstrated to have a strong correlation with radiation-induced damages to the DNA, is directly measured by only three different nanodosimeters worldwide: practical instruments are not yet available. The gap between microdosimetry and track-nanodosimetry can be filled partially by extending the TEPC response down to the nanometric region. A feasibility study of a novel TEPC designed to simulate biological sites in the nanometric domain was performed. The present paper aims at describing the design, the development and the characterization of this avalanche-confinement TEPC. Irradiations with photons, fast neutrons and low-energy carbon ions demonstrated the capability of this TEPC of measuring in the range 0.3 μm - 25 nm.

The probability of the formation of ionization clusters by primary alpha-particles at 5.4 MeV in nanometric volumes of propane was studied experimentally and by Monte Carlo simulation, as a function of the distance between the center line of the particle beam and the center of the target volume. The volumes were of cylindrical shape, 3.7 mm in diameter and height. As the investigations were performed at gas pressures of 300 Pa and 350 Pa, the dimensions of the target volume were equivalent to 20.6 nm or 24.0 nm in a material of density 1.0 g/cm(3). The dependence of ionization-cluster formation on distance was studied up to values equivalent to about 70 nm. To validate the measurements, a Monte Carlo model was developed which allows the experimental arrangement and the interactions of alpha-particles and secondary electrons in the counter gas to be properly simulated. This model is supplemented by a mathematical formulation of cluster size formation in nanometric targets. The main results of our study are (i) that the mean ionization-cluster size in the delta-electron cloud of an alpha-particle track segment, decreases as a function of the distance between the center line of the alpha-particle beam and the center of the sensitive target volume to the power of 2.6, and (ii) that the mean cluster size in critical volumes and the relative variance of mean cluster size due to delta-electrons are invariant at distances greater than about 20 nm. We could imagine that the ionization-cluster formation in nanometric volumes might in future provide the physical basis for a redefinition of radiation quality.

In the framework of an advanced Exotic Ion Beam facility project, named SPES (Study and Production of Exotic Species), that will allow a frontier program in Nuclear and Interdisciplinary Physics, an intense thermal neutron beam facility, devoted to perform Boron Neutron Capture Therapy (BNCT) experimental treatments on skin melanoma tumor is currently under construction based on the SPES proton driver. A vast radiobiological investigation in vitro and in vivo has started with the new 10B carriers developed. Special microdosimetric detectors have been constructed to properly measure all the BNCT dose components and their qualities. Both microdosimetric and radiobiological measurements are being performed at the Enea-Casaccia TAPIRO reactor.

The probability of the formation of ionization clusters by primary α-particles at 5.4 MeV in nanometric volumes of propane was studied experimentally and by Monte Carlo simulation, as a function of the distance between the center line of the particle beam and the center of the target volume. The volumes were of cylindrical shape, 3.7 mm in diameter and height. As the investigations were performed at gas pressures of 300 Pa and 350 Pa, the dimensions of the target volume were equivalent to 20.6 nm or 24.0 nm in a material of density 1.0 g/cm^sup 3^. The dependence of ionization-cluster formation on distance was studied up to values equivalent to about 70 nm. To validate the measurements, a Monte Carlo model was developed which allows the experimental arrangement and the interactions of α-particles and secondary electrons in the counter gas to be properly simulated. This model is supplemented by a mathematical formulation of cluster size formation in nanometric targets. The main results of our study are (i) that the mean ionization-cluster size in the δ-electron cloud of an α-particle track segment, decreases as a function of the distance between the center line of the α-particle beam and the center of the sensitive target volume to the power of 2.6, and (ii) that the mean cluster size in critical volumes and the relative variance of mean cluster size due to δ-electrons are invariant at distances greater than about 20 nm. We could imagine that the ionization-cluster formation in nanometric volumes might in future provide the physical basis for a redefinition of radiation quality.[PUBLICATION ABSTRACT]

Results obtained with a new very compact detector for imaging with a matrix of Leak Microstructures (LM)are reported. Spatial linearity and spatial resolution obtained by scanning as well as the detection of alpha particles with 100% efficiency, when compared with a silicon detector, are stressed. Preliminary results recently obtained in detecting single electrons emitted by heated filament (Ec < 1 eV) at 1-3 mbar of propane are reported.