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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
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 experi...
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Published in: | RSC advances 2018-01, Vol.8 (5), p.2449-2458 |
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Main Authors: | , , , , |
Format: | Article |
Language: | English |
Subjects: | |
Online Access: | Get full text |
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Summary: | 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
−
.
The effect of the azide ion on the yield of molecular hydrogen in water irradiate |
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ISSN: | 2046-2069 |
DOI: | 10.1039/c7ra12397c |