Measurement of displacement cross-sections of Nb irradiated by protons with kinetic energy range between 0.4 and 3 GeV

0.43GeVの陽子におけるNbの弾き出し断面積の測定

明午 伸一郎   ; 中野 敬太*; 松田 洋樹  ; 岩元 洋介   ; 吉田 誠*

Meigo, Shinichiro; Nakano, Keita*; Matsuda, Hiroki; Iwamoto, Yosuke; Yoshida, Makoto*

加速器駆動未臨界システム(ADS)や核破砕中性子源などの大強度陽子加速器施設では、電磁石コイルなどの加速器構成機器の放射線損傷を評価することが重要である。損傷指標としては、NRT(Norgertt-Robinson-Torrens)モデルに基づき、粒子束と弾き出し断面積を積分して得られるdpaが用いられてきた。20MeV以上のエネルギー領域の陽子に対する弾き出し断面積の実験データは乏しいため、J-PARCにおいてNbの弾き出し断面積の測定を行った。Nb試料を極低温(8K)で冷却し、熱運動によるフレンケル対の再結合を抑制するとともに、マッティセン則を維持するために常伝導度を維持した。PHITSコードを用いてNbの弾き出し断面積の計算を行い、今回の実験結果と比較した。その結果、広く用いられるNRTモデルは、これまでの研究で示唆されているように、断面積を2倍程度過大評価することが明らかになった。また、最近提案された分子動力学(MD)に基づく熱中性子再結合補正モデル(arc)を用いた計算が、今回のデータと良い一致を示すことも明らかにした。

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 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. To obtain the data for superconducting materials for high-intensity accelerators and magnets, the displacement cross section of Nb for proton irradiation with a kinetic energy range between 0.4 and 3 GeV was measured. For sustaining damage in the sample, the Nb sample was cooled at a cryogenic temperature (8 K), where the recombination of Frenkel pairs due to thermal motion was well suppressed with maintaining the normal conductivity to maintain Matthiessen's rule. The displacement cross section of Nb was calculated using the PHITS code, and was compared with the present experimental results. It was found that the widely utilized NRT model overestimates the cross section by a factor of 2, as suggested by the previous works. It was also found that the calculation with a recently proposed athermal recombination corrected (arc) model based on Molecular Dynamics (MD) shows good agreement with the present data.