Monte Carlo simulation study of the effects of acidity and LET on the primary free-radical and molecular yields of water radiolysis--application to the Fricke dosimeter

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From: Canadian Journal of Chemistry(Vol. 85, Issue 3)
Publisher: NRC Research Press
Document Type: Article
Length: 11,007 words
Lexile Measure: 1720L

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Abstract: Monte Carlo simulations are used to investigate the effects of acidity (pH) on the primary yields of various chemical species produced in the radiolysis of de-aerated aqueous sulfuric acid solutions over the range from neutral solution to 0.4 mol/L [H.sub.2]S[O.sub.4]. The effects of the quality of radiation, measured in terms of linear energy transfer (LET), have also been studied for LET varying from ~0.3 to 15 keV/[micro]m at ambient temperature. Our results show that an increase in acidity (1 < pH < 4) leads to an increase in the yield [G.sub.[e.sub.[bar.aq]] + [H.sup.x]] of the "reducing" free radicals (hydrated electron and [H.sup.x] atom) and a slight increase in [G.sub.[sup.x]OH] and [G.sub.[H.sub.2][O.sub.2]], while there is a slight decrease in [G.sub.[H.sub.2]]. At pH < 1, [sup.x]OH radicals react with HS[O.sub.4.sup.-] anions to form S[O.sub.4.sup.x-] radicals, resulting in a steep decrease in [G.sub.[sup.x]OH]. By contrast, in the range of pH from ~4 to 7, the calculated yield values are independent of sulfuric acid concentration. In both neutral water and 0.4 mol/L [H.sub.2]S[O.sub.4] (pH 0.46) solutions, the primary molecular yields increase upon increasing LET to ~15 keV/[micro]m with a concomitant decrease in those of free radicals. As an exception, [G.sub.[H.sup.x]] at first increases with LET, reaching a maximum near 6.5 keV/[micro]m before decreasing steeply at higher LET. The results obtained are generally in good agreement with available experimental data over the whole acidity and LET ranges studied. Finally, as an application, we have simulated the radiation-induced oxidation of ferrous sulfate solutions in aerated aq. 0.4 mol/L [H.sub.2]S[O.sub.4] (Fricke dosimeter) as a function of time up to ~50 s and addressed the effects of LET on the resulting ferric ion yield at 25 [degrees]C. The production of [Fe.sup.3+] ions is highly sensitive to free-radical yields, especially [H.sup.x] atoms (via formation of H[O.sub.2.sup.x]), resulting in a marked decline of G([Fe.sup.3+]) with increasing LET. The general trend of the observed variation of G([Fe.sup.3+]) with radiation quality is well reproduced by our computed [Fe.sup.3+] ion yield values.

Key words: liquid water, acidic ([H.sub.2]S[O.sub.4]) aqueous solutions, radiolysis, free-radical and molecular yields, linear energy transfer (LET), Fricke dosimeter, Monte Carlo simulations.

Resume : A l'aide de calculs de simulation Monte Carlo, nous avons etudie l'influence de l'acidite (pH) sur les rendements primaires des diverses especes chimiques produites lors de la radiolyse de solutions aqueuses desaerees d'acide sulfurique dans la gamme de concentrations ~0-0.4 mol/L en [H.sub.2]S[O.sub.4]. L'influence de la qualite du rayonnement, decrite en termes de transfert d'energie lineique (TEL), a aussi ete etudiee pour des valeurs de TEL entre ~0,3 et 15 keV/[micron]m a la temperature ambiante. Nos resultats montrent que, pour des irradiations avec des rayonnements de faible TEL, une augmentation en acidite (1 < pH < 4) conduit a une augmentation du rendement [G.sub.[e.sub.[bar.aq]] + [H.sup.x]] - + de l'entite reductrice radicalaire (electron hydrate et atome [H.sup.x]) et a un faible accroissement de [G.sub.[sup.x]OH] et [G.sub.[H.sub.2][O.sub.2]], alors que [G.sub.[H.sub.2]] diminue legerement. A pH < 1, les radicaux [sup.x]OH reagissent avec les anions HS[O.sub.4.sup.-] pour former les...

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Gale Document Number: GALE|A163465679