To understand the damage of metal exposed by a high-intensity proton beam
—Contributing to the safety of accelerator-driven systems using high-energy proton beam—

R&D is currently moving forward on accelerator-driven systems (ADS) for reducing the volume and hazard of high-level radioactive wastes produced at the nuclear reactors. In this system, damage evaluation is crucial to understand the damage of materials serving, such as beam windows exposed by a high-intensity proton beam. As the damage index of the materials caused by the radiation, a unit of displacement per atom is widely used, which is determined by the particle fluence and “displacement cross-section”. However, there is little experimental data of the displacement cross-section, which makes difficult to validate the evaluation calculations for the radiation damage.

At the proton accelerator facility of J-PARC, measurements were conducted of the displacement cross-sections of iron and copper, which are the materials of ADS equipment. Following Matthiessen's rule, the displacement induced by the radiation is proportional to the change of electrical resistance of the irradiated sample. The cross section can be obtained from the growth of the resistance of the sample irradiated by the proton beam of the accelerator. In order to sustain the displacement without the elimination due to the thermal diffusion, the iron and copper samples were cooled less than -265⁰C using cryocooler. For the first time in the world, the displacement cross-section was successfully obtained in the energy region of proton beams used for ADS.

The findings obtained in this research have enabled more precise damage evaluation of materials used not only in ADS, but also in high-intensity of high-energy accelerator facilities like J-PARC.