Speaker
Description
In high-intensity proton accelerator facilities such as Accelerator Driven System (ADS) and the spallation neutron source, it is crucial to evaluate the damage of beam-interception materials and accelerator components, such as a magnet coil. The displacement per atom (dpa) is used as a damage index, which is derived by integrating the particle flux and the displacement cross section based on the Norgertt-Robinson-Torrens (NRT) model. Although the dpa is employed as the standard, the experimental data of displacement cross section are scarce for a proton in the energy region above 20 MeV.
The difference exists about 8 times among the calculation models for W, so experimental data of the displacement cross-section is crucial to validate the model. To obtain the displacement cross section, we have conducted the experiment in J-PARC. So far, the cross sections of Al, Cu, Fe, and W irradiated by protons with a kinetic energy range between 0.4 and 3 GeV were obtained at J-PARC [1,2]. To obtain the data for superconducting materials for high-intensity accelerators and magnets, we measured the displacement cross section of Nb for proton irradiation with a kinetic energy range between 0.4 and 3 GeV.
Following Matthiessen's rule, the displacement cross section can be obtained with the observation of the electrical resistivity change in the sample divvied by proton fluence and the resistivity change by Franek pairs, which is known with the uncertainty of ~20% by other works. To sustain the damage in the sample, the Nb sample is required to be cooled at a cryogenic temperature (~8 K), where the recombination of Frenkel pairs is well suppressed due to thermal motion with sustaining the normal conductivity to maintain Matthiessen's rule. To obttain the proton fluence correctly, the sample was placed close to the beam profile monitor.
Calculations of displacement cross section of Nb were made using the PHITS code, which are compared with the present experimental results. The present results showed that the widely utilized Norgertt-Robinson-Torrens (NRT) model overestimates the cross section by a factor of 2, as suggested by the previous works. It is also found that the calculation with a recently proposed model of athermal recombination corrected (arc) model based on Molecular Dynamics (MD) shows good agreement with the present data.
Reference
[1] S. Meigo, H. Matsuda, Y. Iwamoto, et al., JPS Conf. Proc. 28, 061004 (2020).
[2] H. Matsuda, S. Meigo, Y. Iwamoto, et al., J. Nucl. Sci. Technol. 57, 1141-1151 (2020).