Objective
Calculations performed have confirmed that the fluctuations of the energy deposition in nanometre-sized targets traversed by charged particles are peculiar. Long-range delta-rays escape from very small targets, so that the delta-ray contribution to energy deposition is dominated by short-range delta-rays with energies not larger than about 100 eV. Thus the fluctuations of energy deposition is described by the Poissonian distribution of the number of the primary particle combined with the energy deposition distribution of the low-energy delta-rays. Since the low-energy delta-ray component results to be identical for all types and energies of the primary particle (invariance theorem), the absorbed energy fluctuations per traversing particle can be described by only one variable, namely the linear primary ionisation density of the primary particle. Monte Carlo calculations have shown that these findings are valid for a very wide range of proton energies, namely from few hundred of keV to 100 MeV.
As first step to extend the investigation to solid state targets, calculation have been performed to simulate delta-ray emission induced by ions passing thin foils of low-Z material. Calculations have been compared with experimental data. Comparison points out that the error induced by the assumption of gas phase cross-section for simulation of solid state media is tolerable in first order. These findings suggest that the energies fluctuation peculiarities calculated in nanometre gas targets could occur similarly in condensed target.
Calculations have been performed to evaluate the dose-mean values of restricted LET both for the core and the delta-ray halo of charged particle tracks.
Averaged values of physical track structure (linear primary ionisation density and restricted LET included) and microdose parameters, have been compiled as a ready reference for use in the interpretation of damage mechanisms and for quantifying radiation effects. Moreover, from study of quality parameters active at nanometre level, a new interpretation, based on the primary ionisation density, has been made of the main mechanisms of the radiation action. Validation has been found in the ability to explain unusual radiation effects. A new system of unified dosimetry has been suggested and described quantitatively.
Gas detectors have been designed and manufactured to investigate experimentally the ionisation distributions in nanometric volumes positioned at variable nanometric distances from a charged particle track. The mean ionisation yield in nanometric volumes around an alpha-particle track has been measured. This experimental investigation, which we call track-nanodosimetry, has been performed with two different detectors and a third detector is under preliminary experimental testing. The experimental results suggest that it is possible to perform track-nanodosimetry, with an electron counter, with sensitive volumes as small as 10-20 nanometres in diameter. For investigating the ionisation track with smaller volumes, a positive ion counter is necessary.
The experimental validation of calculated data at nanometre scale has been enriched by the work performed with a jet-counter-based device. This experimental set-up has been shown to be able to measure the mean energy deposition in nanometric gas volumes.
Parallel to ionisation measurements in gas devices, the possibility to investigate the track structure in solid DNA on a nanometre scale has been studied. This approach is based on local electron-spin densities measurements.
Finally some basic physics characteristics, on which the future risk assessment monitors will be designed, have been studied both for scintillator and gas devices.
The research aim is the best possible way of specifying radiation quality for radiobiology and radiation protection by a.single physical parameter expressing the track structure. The renewal of this old aim is motivated by new ideas which appear to describe more adequately the meaningful track parameters at nanometre level: restricted LET(L ) and linear primary ionization ( ).
Systematic correlation of existing yield data for radiation induced radicals, molecular lesions and products, cell inactivation and transformation, chromosome aberrations and mutations, will be accomplished towards L and . The data base will be extended to include results for transformations, incorporated radionuclides and double-strand breaks in DNA in mammalian cells. The modelling work and Monte-Carlo studies of trackstructures and its regularities Will be performed to investigate and as the physical parameters deputed to represent the quality of the radiation. Measured double differential cross-sections(in angle and in energy) of electron emis sion from gas and solid targets traversed bY heavy ions will implement the inputs of Monte-Carlo codes to study the track structures at nanometre level. The results will be compared with the models.
Ionization radial distribution measurements with nanometre resolution will be performed with light and heavy ions in order to study possible structures in the particle track core and their variation with mass and velocity of the particle. Research on novel detector for dosimetry in condensed phase will continue to assess the possibility of the new detector technologies to measure energy transfers at nanometre level. The ESR methods of continuous saturation'' and ''line broadening'' will be improve both in respect of hardware as well as software to measure the local radical densities on a nanometre scale. Irradiations with light and heavy ions will enable to correlate the spatial distributions of radiative interactions with L , and other track parameters.
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35020 LEGNARO
Italy