Self-radiolysis of tritiated water. 4. The scavenging effect of azide ions (N 3 - ) on the molecular hydrogen yield in the radiolysis of water by 60 Co γ-rays and tritium β-particles at room temperature

• Published in: RSC advances 2018-01, Vol.8 (5), p.2449-2458
• Main Authors: Sanguanmith, Sunuchakan, Meesungnoen, Jintana, Stuart, Craig R, Causey, Patrick, Jay-Gerin, Jean-Paul
• Format: Article
• Language: English
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/C7RA12397C

The effect of the azide ion N on the yield of molecular hydrogen in water irradiated with Co γ-rays (∼1 MeV Compton electrons) and tritium β-electrons (mean electron energy of ∼7.8 keV) at 25 °C is investigated using Monte Carlo track chemistry simulations in conjunction with available experimental data. N is shown to interfere with the formation of H through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H yield in the diffusing radiation tracks. Chemical changes are observed in the H scavengeability depending on the particular type of radiation considered. These changes can readily be explained on the basis of differences in the initial spatial distribution of primary radiolytic species ( , the structure of the electron tracks). In the "short-track" geometry of the higher "linear energy transfer" (LET) tritium β-electrons (mean LET ∼5.9 eV nm ), radicals are formed locally in much higher initial concentration than in the isolated "spurs" of the energetic Compton electrons (LET ∼0.3 eV nm ) generated by the cobalt-60 γ-rays. As a result, the short-track geometry favors radical-radical reactions involving hydrated electrons and hydrogen atoms, leading to a clear increase in the yield of H for tritium β-electrons compared to Co γ-rays. These changes in the scavengeability of H in passing from tritium β-radiolysis to γ-radiolysis are in good agreement with experimental data, lending strong support to the picture of tritium β-radiolysis mainly driven by the chemical action of short tracks of high local LET. At high N concentrations (>1 M), our H yield results for Co γ-radiolysis are also consistent with previous Monte Carlo simulations that suggested the necessity of including the capture of the precursors to the hydrated electrons ( , the short-lived "dry" electrons prior to hydration) by N . These processes tend to reduce significantly the yields of H , as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET H β-radiolysis, leading us to conclude that the increased amount of intra-track chemistry intervening at early time under these conditions favors the recombination of these electrons with their parent water cations at the expense of their scavenging by N .