Scavenging of 'dry' electrons prior to hydration by azide ions: effect on the formation of [H.sub.2] in the radiolysis of water by [.sup.60]Co [gamma]-rays and tritium [beta]-electrons

In this study, we use Monte Carlo track chemistry simulations to show that 'dry' secondary electrons, precursors of the 'hydrated' electron ([e.sup.-.sub.aq]), can be scavenged on the sub-picosecond time scale prior to hydration, by a high concentration (>0.1-1M) of azide ions ([N.sub.3.sup.-])in water irradiated with [.sup.60]Co [gamma]-rays and tritium [beta]-electrons at 25 [degrees]C. This is a striking result, as [N.sub.3.sup.-] is known to react very slowly with [e.sup.-.sub.aq]. These processes tend to significantly reduce the yields of [H.sub.2] as observed experimentally. For both energetic Compton electrons ('linear energy transfer', LET ~ 0.3 keV/[micro]m), which are generated by the cobalt-60 [gamma]-rays, and [.sup.3]H [beta]-electrons (LET ~ 6keV/[micro]m), our [H.sub.2] yield results confirm previous Monte Carlo simulations, which indicated the necessity of including the capture of the precursors to [e.sup.-.sub.aq]. Interestingly, our calculations show no significant changes in the scavenging of 'dry' electrons at high azide concentrations in passing from [gamma]-radiolysis to tritium [beta]-radiolysis (i.e., with LET). This led us to the conclusion that the higher [H.sub.2] yield observed experimentally for [.sup.3]H [beta]-electrons compared with [.sup.60]Co [gamma]-rays is mainly explained by the difference in the radiation track structures during the chemical stage (>1 ps). The higher LET of tritium [beta]-electrons leads to more molecular products ([H.sub.2] in this case) in tritium radiolysis than in [gamma]-radiolysis. Finally, a value of ~0.5 nm was derived for the reaction distance between [N.sub.3.sup.-] and the 'dry' electron from the [H.sub.2] yields observed in [.sup.60]Co [gamma]-radiolysis at high [N.sub.3.sup.-] concentrations. Key words: liquid water, azide ion ([N.sub.3.sup.-]) solutions, radiolysis, linear energy transfer (LET), cobalt [gamma]-rays, tritium [beta]-electrons, secondary electrons, 'dry' electron scavenger, mobility, [H.sub.2] yield, Monte Carlo track chemistry simulations. Dans le cadre de la presente etude, nous avons utilise des simulations Monte Carlo de la trajectoire des particules pour montrer que les electrons secondaires dits << secs >>, precurseurs des electrons << hydrates >> ([e.sup.-.sub.aq]), peuvent etre pieges, a une echelle de tempssousla barriere de la picoseconde avant l'hydratation, par des ions azoture ([N.sub.3.sup.-]) a forte concentration (>0,1 a 1 M) dans l'eau soumise a l'irradiation par des rayons [gamma] de [.sup.60]Co et des electrons [beta] de tritium a 25 [degrees]C. Ce resultat est etonnant, puisque l'on sait que [N.sub.3.sup.-] reagit tres lentement avec les [e.sup.-.sub.aq]. Ces processus ont tendance a reduire de facon considerable les rendements en [H.sub.2], comme cela est observe experimentalement. Pour les electrons Compton de haute energie (transfert d'energie lineaire [TEL] ~ 0,3 keV/[micro]m), qui sont generes par les rayons [gamma] du cobalt 60, et les electrons [beta] du [.sup.3]H (TEL ~ 6keV/[micro]m), nos resultats du rendement en [H.sub.2] confirment les simulations Monte Carlo precedentes, qui ont indique la necessite d'inclure le piegeage des precurseurs des [e.sup.-.sub.aq]. Fait interessant, lorsque l'on passe de la radiolyse [gamma] a la radiolyse [beta] par le tritium, nos calculs ne montrent aucun changement substantiel quant au piegeage des electrons << secs >> a des concentrations elevees d'azoture. Ces resultats nous amenent a conclure que le rendement plus eleve en [H.sub.2] observe experimentalement pour les electrons [beta] du [.sup.3]H par rapport aux rayons [gamma] du [.sup.60]Co s'explique principalement par la difference entre la structure des trajectoires de rayonnement pendant le stade chimique (>1 ps). Le TEL plus eleve des electrons [beta] du tritium donne lieu a la formation d'un plus grand nombre de produits moleculaires ([H.sub.2] dans ce cas) lors de la radiolyse par le tritium que lors de la radiolyse [gamma]. Enfin, nous avons calcule une valeur d'environ 0,5 nm pour ce qui est de la distance de reaction entre [N.sub.3.sup.-] et l'electron << sec >> a partir des rendements en [H.sub.2] observes lors de la radiolyse [gamma] par le [.sup.60]Co a des concentrations elevees de [N.sub.3.sup.-]. [Traduit par la Redaction] Mots-cles : eau liquide, solutions d'ions azoture ([N.sub.3.sup.-]), radiolyse, transfert d'energie lineaire (TEL), rayons [gamma] de cobalt, electrons [beta] de tritium, electrons secondaires, piegeur d'electrons << secs >>, mobilite, rendement en [H.sub.2], simulations Monte Carlo de la trajectoire des particules.
1. Introduction The radiation chemistry of liquid water (1-3) is of fundamental importance for a variety of practical applications, particularly in the nuclear power industry, where radiolysis products cause corrosion [...]