arXiv subjects

João F. Canhoto

Publications and source records attributed to João F. Canhoto.

3 recordsLinked to original sources

arXiv2026

Experimental and Monte Carlo Simulation Studies to Investigate the Working Principle of Compact Nanodosimeters

In recent years, compact nanodosimetric detectors based on ion multiplication in low-pressure gas have been developed and gained attention in the scientific community. These detectors use strong electric fields to collect and multiply positive ions produced by the incident radiation in mm-sized cell holes in dielectric materials, achieving a nm-equivalent spatial resolution of the localization of ionization events, when scaled to liquid water at unit density. Their design assumes that ion-impact ionizations of gas molecules within the cell holes dominate signal formation, yet this assumption has lacked direct physical verification. Electron emission from the cell hole walls or the cathode due to ion-impact could also contribute, requiring alternative designs to optimize efficiency. To investigate the relative importance of the possible mechanisms, a nanodosimetric detector featuring a single cell hole with a diameter of 1.5 mm in a dielectric plate was developed. Ion collection and multiplication were achieved by applying a negative high voltage to the glass cathode 0.5 mm below the cell hole, assisted by a low drift field above the plate. A grounded readout electrode with a 0.8 mm hole covers the cell hole to prevent interactions of collected ions with the hole walls. High signal yields in 1 mbar and 2 mbar propane gas were observed and indicated that ion-impact ionizations of the gas molecules could indeed be the primary mechanism for signal induction. Ion-induced secondary electron emission from the cathode was identified as another potential contribution. The compact nanodosimeter setup was further modeled with Geant4-DNA and Garfield++ for deeper insight. The results of these studies are important for understanding and developing a new class of nanodosimeters with potential applications in particle therapy, radiation protection, space dosimetry, and particle physics.

physics.ins-det↗

arXiv2025

The Associated Volume sampling algorithm as an alternative method for the calculation of ionisation cluster size distributions in computational nanodosimetry

In computational nanodosimetry, Monte Carlo Track Structure (MCTS) simulations are employed to calculate ionisation cluster size distributions (ICSDs), which are crucial for characterising mixed radiation fields at the nanoscale. The Uniform Sampling (US) algorithm, commonly used for this purpose, is inefficient when evaluating ICSDs conditioned on clusters exceeding a given size $ν$ threshold. This study investigates a more efficient alternative - the Associated Volume (AV) algorithm - against the US approach for computing conditional ICSDs ($ν\ge 1$) from proton tracks simulated with Geant4-DNA. Two configurations of the AV algorithm were evaluated: the standard AV-Overlap, allowing sensitive volumes to overlap (with a 1/$ν$ correction), and the novel AV-No Overlap, which prevents overlap. We compared the conditional ICSDs, mean cluster size ($M_1$), complementary cumulative frequencies ($F_k$ for $k \in [2,7]$), and overall execution time per run and per event. Deviations exceeding two standard deviations of the mean difference were considered statistically significant. Although statistically significant differences in ICSDs, $M_1$ and $F_k$ were observed, the relative differences in the AV-Overlap rarely exceeded 5%, whereas the No Overlap configuration they ranged from $\lt$ 1% to approximately 15% at the higher $F_k$. Both AV configurations achieved execution times one to two orders of magnitude shorter than the US algorithm. Our findings indicate that the AV-Overlap algorithm may be a preferred alternative for calculating conditional ICSDs and derived nanodosimetric quantities, offering a substantial gain in computational efficiency without compromising accuracy.

physics.comp-ph↗

arXiv2023

Repair kinetics of DSB-foci induced by proton and helium ion microbeams of different energies

In this work, the induction and repair of radiation-induced 53BP1 foci were studied in human umbilical vein endothelial cells irradiated at the PTB microbeam with protons and α-particles of different energies. The data were analyzed in terms of the mean number of 53BP1 foci induced by the different ion beams. The number of 53BP1 foci found at different times post-irradiation suggests that the disappearance of foci follows first order kinetics. The mean number of initially produced foci shows the expected increase with LET. The most interesting finding of this work is that the absolute number of persistent foci increases with LET but not their fraction. Furthermore, protons seem to produce more persistent foci as compared to α-particles of even higher LET. This may be seen as experimental evidence that protons may be more effective in producing severe DNA lesions, as was already shown in other work, and that LET may not be the best suited parameter to characterize radiation quality

physics.med-ph↗