A Model for Radiolysis in a Flowing-Water Target during High-Intensity Proton Irradiation

At the Facility for Rare Isotope Beams (FRIB), interactions between heavy-ion beams and beam-dump water will create a wide variety of radionuclides which can be accessed by a technique known as "isotope harvesting". However, irradiation of water is always accompanied by the creation of numerous radical, ionic, and molecular radiolysis products. Some of the radiolysis products have sufficiently long lifetimes to accumulate in the irradiated water and affect the harvesting chemistry. Here we investigate the formation of hydrogen peroxide, molecular hydrogen, and molecular oxygen during a high-intensity proton irradiation of a flowing-water isotope-harvesting target and compare the experimental results to simulations. The simulations kinetically model the chemical reactions occurring in the homogeneous phase of radiolysis in flowing water and establish an "effective yield". In both the experiment and simulations, the bulk quantities of H₂, H₂O₂, and O₂ are considerably lower than predicted by primary radiolysis yields (escape yields), meaning that in the high beam intensity regime the homogeneous phase reactions have a considerable impact on the overall chemical composition of the water. Further, it could be shown that for radiation which is characterized by a limited linear energy transfer, such as the here applied protons, the bulk outcome of the microscopic kinetic modeling could be estimated by a simplified steady-state model.

Harvesting krypton isotopes from the off-gas of an irradiated water target to generate 76Br and 77Br

A flowing-water target was irradiated with a 150 MeV/nucleon beam of 78Kr at the National Superconducting Cyclotron Laboratory to produce 77Kr and 76Kr. Real-time gamma-imaging measurements revealed the mass transport of the krypton radioisotopes through the target-water processing, or “isotope harvesting”, system. The production rates were determined to be 2.7(1) × 10–4 nuclei of 76Kr and 1.18(6) × 10–2 nuclei of 77Kr formed per incident 78Kr ion. Utilizing an off-gas processing line as part of the isotope harvesting system, a total of 7.2(1) MBq of 76Kr and 19.1(6) MBq of 77Kr were collected in cold traps. Through the decay, the daughter radionuclides 76Br and 77Br were generated and removed from the traps with an average efficiency of 77 ± 12%. Due to the differences in half-lives of 76Kr and 77Kr, it was possible to isolate a pure sample of 76Br with 99.9% radionuclidic purity. The successful collection of krypton radioisotopes to generate 76Br and 77Br demonstrates the feasibility of gas-phase isotope harvesting from irradiated accelerator cooling-water. Larger-scale collections are planned for collecting by-product radionuclides from the Facility for Rare Isotope Beams.