Online ISSN: 1884-4111 Print ISSN: 0033-8303
Radioisotopes 68(8): 543-552 (2019)

特集Special Issues

7.2.1 厚い標的からの生成中性子分布(TTY)7.2.1 Secondary Neutron Production Yields from Thick Targets (TTY)

東北大学名誉教授Tohoku University, Professor Emeritus ◇ 980–8578 仙台市青葉区荒巻字青葉6–3 ◇ 6–3 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980–8578, Japan

発行日:2019年8月15日Published: August 15, 2019

重イオンによる厚い標的からの中性子生成は,加速器施設の遮蔽安全設計において放射線源となる非常に重要なデータである。様々なエネルギーのHeイオンからXeイオンに至るビームをC, Al, Cu, Pbの厚いターゲットに入射して,生成される2次中性子のエネルギースペクトルを,0, 7.5, 15, 30, 60, 90°の角度で,有機液体シンチレータを用いて飛行時間分析法(TOF)により測定した。得られたスペクトルは互いに良く似ていて,次の3種類の成分を持っている。前方方向で特に顕著な入射粒子が直接中性子をはじき出すノックオン成分,核内カスケードによる成分,複合核生成後の蒸発成分である。この実験結果はPHITSコードによる計算と比較していて,全般的にファクター3以内で良く一致している。また,Moving Source Modelという簡易評価式でもうまく表現できることがわかった。さらに,5 MeV以上のスペクトルを角度積分した結果は2つの指数関数の和で与えられ,これを0°から90°で積分した前方方向の全中性子生成量を与える近似式を得た。


Secondary neutron production yields from thick targets (TTY) are very important data concerning the radiation source for any shielding design study of an accelerator facility. By bombarding thick targets of C, Al, Cu and Pb with various heavy ions from He to Xe at various energies, the energy spectra of neutrons produced from targets were measured by the TOF method using the organic liquid scintillator at several angles (0, 7.5, 15, 30, 60, 90 degrees). The measured spectra have three components of knock-on, intra-nuclear cascade and evaporation, and are in good agreement with the PHITS Monte Carlo code within a factor 3, as a whole. The energy spectra were well expressed by the simple moving source model with fitting parameters. The angular distribution of secondary neutrons above 5 MeV was well fitted to the two terms of exponential equations; further the total neutron production yield in the forward direction was given by the simple analytical form.

These results are indispensable as source neutron data for the shielding design of high-energy heavy-ion facilities, and are widely used around the world. These TTY data are also registered in the SINBAD (Shielding Benchmark Experiments) database of OECD/NEA.

Key words: HIMAC (Heavy Ion Medical Accelerator in Chiba); secondary neutron spectra; thick target; knock-on component; intra-nuclear cascade component; evaporation component; moving source model; angular distribution; total neutron production yield

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