“Analysis of the physico-chemical processes within the Bragg peak of continuous ion beams in water”

The legacy of the French nuclear industry has led to an increase in the dismantling of Basic Nuclear Installations (INB). In this context, the development of new techniques for detecting radioactive pollution based on the interaction between radiation and water is becoming increasingly relevant. However, to achieve this goal, it is imperative to have a thorough understanding of the chemistry that occurs within the ionization tracks generated by ionizing particles. The chemistry that takes place within the Bragg Peaks (PB) is poorly understood, mainly due to the extremely narrow width of these PBs (less than 1 mm). To better understand the chemical processes occurring within them, high-resolution techniques are required to directly determine yields within the PB. The results obtained also hold great interest in fields such as hadrontherapy or the effects of cosmic radiation on humans. Measurements carried out with a high-resolution spectrometer designed during this thesis have allowed us to probe the PB of proton and carbon ion beams. Using different sensor systems, the aim of this study was to investigate the yields of major radiolytic species both globally and individually, combining measurements in absorbance and fluorescence. Spectrometry measurements were completed with chemiluminescence measurements to study the yield of H2O2 and photon counting measurements to assess the impact of an ionizing beam on the dynamic fluorescence properties of fluorescent sensors. Finally, simulations conducted with Geant4-DNA were performed to provide additional understanding. The results obtained at the end of the irradiation campaigns conducted during the thesis have revealed interesting and previously unobserved phenomena within the PB and have led to several conclusions: (1) With the Fricke dosimeter, the yields exhibit a decrease within the PB, followed by an increase after the PB. This increase is attributed to the longitudinal energy dispersion of the ion beam, leading to a decrease in the average linear energy transfer (LET) at the end of the track. This increase was observed with other sensor systems (methylviologen-tert-butanol, resazurin (RNO), and AmplexRed (AR)), but it was not observed with the methylviologen-formate and ferrocyanide systems. (2) The results suggest that organic sensors are sensitive to attacks by reactive species produced within ion tracks, especially in the case of resorufin. Therefore, the use of pulsed beams is recommended when using such systems. (3) Salt-based systems, such as ferrocyanide, have shown the ability to provide yields in agreement with Geant4-DNA simulations in pure water, making them a relevant option for yield measurements under continuous irradiation. (4) Preliminary photon counting measurements did not reveal a significant impact of ionizing beams on the fluorescence lifetime properties of resorufin formed from RNO and AR. (5) The use of Geant4-DNA enhances the understanding of chemical reactions occurring within ion tracks. However, the software is limited to simulating ion beams at very low dose rates, preventing it from reproducing phenomena that occur under continuous irradiation. The integration of multithreaded calculations could partially address this issue. (6) The results generated by this thesis represent an additional step towards a complete understanding of the chemistry occurring within the PB of ion beams. Further studies should be conducted using devices with improved spatial resolutions and by combining experiments under continuous and pulsed beam conditions.

https://cnrs.zoom.us/j/93722733367?pwd=dWYzZEFIT1FNY3NSL0FuWU0rZTY2dz09

ID de réunion: 937 2273 3367
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