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

The effect of the azide ion N(3)(−) on the yield of molecular hydrogen in water irradiated with (60)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(3)(−) is shown to interfere with the formation of H(2) through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H(2) yield in the diffusing radiation tracks. Chemical changes are observed in the H(2) 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 (i.e., 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(−1)), radicals are formed locally in much higher initial concentration than in the isolated “spurs” of the energetic Compton electrons (LET ∼0.3 eV nm(−1)) 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(2) for tritium β-electrons compared to (60)Co γ-rays. These changes in the scavengeability of H(2) 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(3)(−) concentrations (>1 M), our H(2) yield results for (60)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 (i.e., the short-lived “dry” electrons prior to hydration) by N(3)(−). These processes tend to reduce significantly the yields of H(2), as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET (3)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(3)(−).