Self-radiolysis of tritiated water. 4. The scavenging effect of azide ions (N3 -) on the molecular hydrogen yield in the radiolysis of water by 60Co γ-rays and tritium β-particles at room temperature

Abstract

The effect of the azide ion N₃ ^- on the yield of molecular hydrogen in water irradiated with ⁶⁰Co 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. N₃ ^- is shown to interfere with the formation of H₂ through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H₂ yield in the diffusing radiation tracks. Chemical changes are observed in the H₂ 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 H₂ for tritium β-electrons compared to ⁶⁰Co γ-rays. These changes in the scavengeability of H₂ 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 N₃ ^- concentrations (>1 M), our H₂ yield results for ⁶⁰Co 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 N₃ ^-. These processes tend to reduce significantly the yields of H₂, as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET ³H 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 N₃ ^-.

Fig. 1. Simulated track histories (projected into the XY plane of figure) of a 7.8 keV tritium β-electron (complete track; mean LET ∼ 5.9 eV nm⁻¹) (panel a) and a 300 MeV proton (track segment; LET ∼ 0.3 eV nm⁻¹) (panel b) incident in liquid water at 25 °C. The two irradiating particles are generated at the origin and start traveling along the Y axis. Dots represent the energy deposited at points where an interaction occurred.

Fig. 2. Time evolution of the H₂ yield (in molecule per 100 eV) for the radiolysis of air-saturated aqueous sodium azide (NaN₃) solutions by 300 MeV incident protons (which mimic irradiation with ⁶⁰Co γ-rays or fast electrons, LET ∼0.3 eV nm⁻¹) (panel a) and by 7.8 keV ³H β-particles (LET ∼5.9 eV nm⁻¹) (panel b) at neutral pH and 25 °C. Calculations were carried out using our Monte-Carlo track chemistry simulations over the time interval 1 ps to 10 μs. The blue, green, red, orange, cyan, and magenta lines correspond to six different concentrations of N₃⁻ anions: 10⁻⁴, 10⁻³, 10⁻², 0.1, 1, and 5 M, respectively. For both types of radiation, the limiting plateau values of G(H₂) continuously decrease with increasing the concentration of N₃⁻ ions. For ⁶⁰Co γ/fast electron irradiation, the arrow pointing downwards indicates the time τ_s ∼0.2 μs required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solutions, at 25 °C. The black solid line in panels a and b show the kinetics of H₂ formation in azide-free aerated solutions (shown here for the sake of reference). Finally, the concentration of dissolved oxygen used in the simulations was 2.5 × 10⁻⁴ M.

Fig. 3. Decrease in the molecular hydrogen yield (in molecule per 100 eV) with concentration of N₃⁻ ions for 300 MeV incident protons (LET ∼ 0.3 eV nm⁻¹) (panel a) and for 7.8 keV ³H β-particles (LET ∼ 5.9 eV nm⁻¹) (panel b) in the radiolysis of air-saturated aqueous azide (NaN₃) solutions (neutral pH, 25 °C), calculated from our Monte Carlo simulations over the range of 10⁻⁴ to 5 M. The blue solid lines show our simulated results (see text). Experimental data for γ and tritium β-particle irradiations: (●), ref. 22; (□), ref. 25; (○), ref. 60. For the sake of comparison, the H₂ yields calculated from ref. 26 for both types of radiation, assuming that N₃⁻ scavenges the short-lived precursor to H₂ with a rate constant of 10¹² M⁻¹ s⁻¹ (dashed line) and does not scavenge the short-lived precursor to H₂ (dotted line), are also shown in the figure.

Fig. 4. Time dependence of the extents ΔG(H₂) (in molecule/100 eV) of the reactions (e_aq⁻ + H˙) (panel a) and (e_aq⁻ + e_aq⁻) (panel b) that contribute to the formation of molecular hydrogen, calculated from our Monte Carlo simulations of the radiolysis of air-saturated aqueous azide (NaN₃) solutions (pH neutral, 25 °C) by 7.8 keV ³H β-particles (LET ∼ 5.9 eV nm⁻¹) in the time interval 1 ps to 10 μs. The blue, green, red, orange, and cyan lines correspond to the five different concentrations of azide anions studied: 10⁻⁴, 10⁻³, 10⁻², 0.1, and 1 M, respectively (see text). For the sake of reference, the black lines in panels a and b show the cumulative yield variations ΔG(H₂) of the two reactions (e_aq⁻ + H˙) and (e_aq⁻ + e_aq⁻) that contribute to the formation of H₂ in azide-free solutions. Finally, the concentration of dissolved oxygen used in the simulations was 2.5 × 10⁻⁴ M.