Oxygen-18 Research Articles

Hillslope hydrograph separation: The effects of variable isotopic signatures and hydrodynamic mixing in macroporous soil

The prevailing opinion on the temporal origin of water in a hillslope stormflow hydrograph is that the pre-event water represents a dominant fraction. Such conclusion is usually based on hydrograph separation techniques using stable water isotopes (or other conservative tracers) in conjunction with a mass balance approach. In this study, a two-dimensional dual-continuum model was used to study preferential flow of water and transport of Oxygen-18 (O-18) in a vertical cross-section of a hillslope located in a temperate spruce forest. The effects of hydrodynamic mixing and the spatiotemporal variability of isotopic signatures on estimated pre-event/event water fractions in the hillslope discharge were studied by means of numerical simulation experiments. Pre-event and event water contributions to hillslope stormflow were evaluated using a two-component mass balance approach combined with the 2D flow and transport simulations involving real as well as synthetic O-18 signatures. Long-term simulations of O-18 transport in the hillslope segment were compared with the observed O-18 content in soil water and in the hillslope effluent. The results of the long-term simulations indicated significant mixing of pre-event and event water occurring near the subsurface trench and in the soil above the soil–bedrock interface where the transfer of O-18 from the soil matrix to the preferential pathways takes place. Despite the dominant role of preferential flow in the generation of hillslope stormflow, the pre-event water formed 52–84% of total subsurface stormflow. The mass balance approach failed in partitioning the hillslope discharge into the pre-event/event water components for two thirds of the selected rainfall–runoff episodes due to similar natural isotopic signatures of pre-event and event water. The analysis showed that spatially and temporally variable exchange of O-18 between the soil matrix and preferential pathways exerted a primary control on the estimates of the temporal origin of water in the hillslope runoff. It was demonstrated that the degree of hydrodynamic mixing in the flow domain played an important role in the interpretation of the isotope-based hydrograph separation.

Stable isotopes of the water vapour in the Atmospheric boundary layer from five Atlantic Ocean cruises in 2012-2015

The marine boundary layer atmospheric water vapour isotopic composition (oxygen 18 and deuterium) has been measured above the Atlantic Ocean from 5 research vessels between 2012 and 2015. Using laser spectroscopy analysers, measurements have been carried out semi-continuously on samples collected 10-20 meter above sea level.

SU‐FF‐J‐143: A Feasibility Study of Using O‐18 Enriched Water to Develop a Method of In‐Vivo Range Verification in Proton Therapy

Purpose: To report a feasibility study of using oxygen‐18 (O‐18) enriched water in combination with PET to develop a method of in vivo range verification in proton therapy. O‐18 enriched water is non‐toxic and non‐radioactive and can be safely administered to patients before proton treatment. When protons interact with O‐18, it is converted to F‐18 which decays back to O‐18 by positron emission with a half‐life of 110min. For the O18(p,n)F18 reaction, the interaction cross‐section is larger, the energy threshold lower, and the half‐life longer compared to most other inelastic proton interactions in patients leading to positron decay. Method and Materials: O‐16 water, 98% O‐18 enriched water, and 10% O‐18 enriched water samples were placed inside a polystyrene phantom and irradiated with a modulated 149 MeV proton beam with a 6 cm wide Spread Out Bragg Peak (SOBP) at three depths (mid‐entrance region, proximal, and distal third of the SOBP). The activity decay of each water sample was measured using a NaI detector for 1–2 hours. Additional information was obtained by scanning the irradiated water samples with a high‐resolution animal PET scanner. Results: As expected O‐16 irradiated water had a higher initial activity than 10% O‐ 18 and 98% O‐18 enriched water at each depth. However, at later times, the activity in the O‐18 samples was larger due to the short half‐times of positron‐emitting products resulting from O‐16 irradiation. Furthermore, there was a larger reduction of activity of irradiated O‐16 across the SOBP as compared to O‐18 irradiated water. Conclusion: Application of O‐18 enriched water has features that make it an attractive method for in‐vivo range verification of proton beams. It is likely that this method could also be useful for in vivo dose verification.

Oxygen vacancy kinetics in ferroelectric PbZr0.4Ti0.6O3

Oxygen vacancy kinetics in ferroelectric PbZr0.4Ti0.6O3 were studied by oxygen18 (O18) tracer self-diffusion in epitaxial thin films as well as bulk polycrystalline samples. O18 exchange annealing was carried out at an oxygen partial pressure of 250mbar and temperatures between 250 and 400°C. Isotope depth profiling was performed by secondary ion mass spectrometry as well as neutral secondary mass spectrometry. The observed concentration depth profiles of the polycrystalline samples show two distinct diffusion paths, namely, bulk diffusion and grain boundary (GB) diffusion. It appears to be of type B-kinetics in the investigated temperature range, with DGB∕Dbulk⪢100. Donor doped samples with different levels of Nb5+ (1–4mol.%) were also investigated. The effect on the diffusion depth profiles, however, were negligible and can solely be attributed due to the change in the samples microstructure as induced by the dopants. A diffusion coefficient for the bulk diffusion of the O18 isotope, Dbulk=10±5×10−8cm2∕sexp(−0.87±0.1eV∕kT) was found. The faster GB diffusion process shows an activation enthalpy of only EA=0.66±0.2eV.

Mechanism of autoxidation of 5,7-dihydroxytryptamine: singlet oxygen-18 is incorporated on C-4 during oxidation with molecular oxygen-18

Mechanism of autoxidation of 5,7-dihydroxytryptamine: singlet oxygen-18 is incorporated on C-4 during oxidation with molecular oxygen-18

Oxygen 18 investigation of the prostaglandin synthetase-dependent co-oxidation of diphenylisobenzofuran.

Oxygen 18 investigation of the prostaglandin synthetase-dependent co-oxidation of diphenylisobenzofuran.

Cyclizations to lactones. Oxygen-18 mechanism study

Cyclizations to lactones. Oxygen-18 mechanism study

Acetolysis of trans-2-(tosyloxy)cyclohexyl substituted benzoates. Kinetics and oxygen-18 studies

Acetolysis of trans-2-(tosyloxy)cyclohexyl substituted benzoates. Kinetics and oxygen-18 studies

A study of photophosphorylation with oxygen-18

A study of photophosphorylation with oxygen-18