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Water radiolysis

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Water radiolysis is a cornerstone in radiation chemistry. The radiolysis of pure water is quite well understood, though the finest details of the physico-chemical stage regarding molecular reorganization of the ionized medium at the picosecond time scale are not known. After more than 60 years of research, water radiolysis is now a reference for radiation chemistry of other organic materials.
The SIMUL team has a long standing collaboration in this field with M. Beuve from the IPN Lyon and with the University of Rosario in Argentina. Our simulation is based on a microscopic description of the three temporal stages that take place during water radiolysis. We consider explicitly the penetration of the incoming radiation and the whole electronic cascade induced by ionization of the medium until thermalization of the electrons. This physical stage is followed by a physic-chemical stage during which the ionized and excited molecules rearrange with their closest neighbors and the electrons get solvated. The outcome of the two stages is a 3-dimensional map of the radicals created by irradiation. This map is used to initiate a Kinetic Monte Carlo (KMC) simulation of the chemical reactions that take place up to the microsecond range of time. Our simulation gives thus the yield of each chemical species produced by irradiation.
We have improved the KMC simulation to take into account the presence of a uniform concentration of solutes (O₂, H₃O⁺, Br⁻…) that react with the products of water radiolysis and change the chemical yields. The simulation has been used to analyze the radiolysis of HBr acid solutions by high TEL ions during the thesis of D. Saffré (Laboratoire de Radiolyse - Saclay).
We have more recently adapted our simulation to study the radiolysis solutions made of Gold nanoparticles (GNP) embedded in water. It has been shown recently that the delivery of such solution combined with X-rays irradiation reduces significantly the tumor development for mice. However the underlying mechanisms are unknown, despite numerous studies undertaken so far. In this framework, we have extended our existing transport code to simulate the penetration of X-rays and the transport of electrons in a composite medium made of Gold nanoparticles embedded in liquid water. We take into account most of the significant processes including inner-shell ionization, Auger electrons and radiative cascades, plasmon and vibrational excitations in both media. This work is a part of the BIOHYDRA project funded by the “plan physicancer” in collaboration between CIMAP Caen and IPN Lyon. It follows a previous study of the radiolysis of water confined in silica. Our preliminary results indicate that the role of neutral gold nanoparticles is manly to act as an electron scavenger, creating a small depletion of solvated electrons around each gold nanoparticle. This is a rather weak effect at the concentration used for experiments. In particular, the primary yields are barely modified by the presence of the GNP. Nevertheless, by the time the chemical reactions take place, some specific reactions with the gold surface can take place to modify the yields observed at the microsecond and beyond. This work will continue in collaboration with IPN Lyon and the “laboratoire de Physico-chimie” in Orsay.