In-beam γ-rays of back-streaming white neutron source at China Spallation Neutron Source
Authors: Ren Jie , Ruan Xi-Chao , Chen Yong-Hao , Jiang Wei , Bao Jie , Luan Guang-Yuan , Zhang Qi-Wei , Huang Han-Xiong , Wang Zhao-Hui , An Qi , Bai Huai-Yong , Bao Yu , Cao Ping , Chen Hao-Lei , Chen Qi-Ping , Chen Yu-Kai , Chen Zhen , Cui Zeng-Qi , Fan Rui-Rui , Feng Chang-Qing , Gao Ke-Qing , Gu Min-Hao , Han Chang-Cai , Han Zi-Jie , He Guo-Zhu , He Yong-Cheng , Hong Yang , Huang Wei-Ling , Huang Xi-Ru , Ji Xiao-Lu , Ji Xu-Yang , Jiang Hao-Yu , Jiang Zhi-Jie , Jing Han-Tao , Kang Ling , Kang Ming-Tao , Li Bo , Li Chao , Li Jia-Wen , Li Lun , Li Qiang , Li Xiao , Li Yang , Liu Rong , Liu Shu-Bin , Liu Xing-Yan , Mu Qi-Li , Ning Chang-Jun , Qi Bin-Bin , Ren Zhi-Zhou , Song Ying-Peng , Song Zhao-Hui , Sun Hong , Sun Kang , Sun Xiao-Yang , Sun Zhi-Jia , Tan Zhi-Xin , Tang Hong-Qing , Tang Jing-Yu , Tang Xin-Yi , Tian Bin-Bin , Wang Li-Jiao , Wang Peng-Cheng , Wang Qi , Wang Tao-Feng , Wen Jie , Wen Zhong-Wei , Wu Qing-Biao , Wu Xiao-Guang , Wu Xuan , Xie Li-Kun , Yang Yi-Wei , Yi Han , Yu Li , Yu Tao , Yu Yong-Ji , Zhang Guo-Hui , Zhang Lin-Hao , Zhang Xian-Peng , Zhang Yu-Liang , Zhang Zhi-Yong , Zhao Yu-Bin , Zhou Lu-Ping , Zhou Zu-Ying , Zhu Dan-Yang , Zhu Ke-Jun , Zhu Peng ,   Key Laboratory of Nuclear Data, China Institute of Atomic Energy, Beijing 102413, China,   Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China,   Spallation Neutron Source Science Center, Dongguan 523803, China,   State Key Laboratory of Particle Detection and Electronics, Beijing 100049, China,   Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China,   State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China,   Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China,   Northwest Institute of Nuclear Technology, Xi’an 710024, China,   University of Chinese Academy of Sciences, Beijing 100049, China,   Department of Engineering and Applied Physics, University of Science and Technology of China, Hefei 230026, China,   School of Physics, Beihang University, Beijing 100083, China
Published: December 2020

The back-streaming neutron beam line (Back-n) was built in the beginning of 2018, which is part of the China Spallation Neutron Source (CSNS). The Back-n is the first white neutron beam line in China, and its main application is for nuclear data measurement. For most of neutron-induced nuclear reaction measurements based on white neutron facilities, the beam of gamma rays accompanied with neutron beam is one of the most important experimental backgrounds. The back streaming neutron beam is transported directly from the spallation target to the experimental station without any moderator or shielding, the flux of the in-beam gamma rays in the experimental station is much larger than those of these facilities with neutron moderator and shielding. Therefore, it is necessary to consider the influence of in-beam gamma rays on the experimental results. Studies of the in-beam gamma rays are carried out at the back-n. Monte-Carlo simulation is employed to obtain the energy distribution and the time structure of the in-beam gamma rays. According to the simulation results, when the neutron flight time is longer than 1.0 μs the energy distribution of the in-beam gamma rays does not vary with flight time. Therefore, the time structure of these gamma rays can be measured without the correction of the detection efficiency. In this work, the time structure of the in-beam gamma rays in the low neutron energy region is measured by both direct and indirect methods. In the direct measurement, a <sup>6</sup>Li loaded ZnS(Ag) scintillator is located on the neutron beam line and the time of flight method is used to determine the time structure of neutrons and gamma rays. The gamma rays are separated from neutrons with pulse-shape discrimination. The black filter method is used to verify the particle discrimination results. In the indirect measurement, the C<sub>6</sub>D<sub>6</sub> scintillation detectors are used to measure the gamma rays scattered off a Pb sample on the way of the neutron beam. The time structure of the in-beam gamma rays is derived from that of the scattered gamma rays. The experimental results are in good agreement with the simulations with the time-of-flight between 12 μs and 2.0 ms. Besides, according to the simulation results, the intensity of the in-beam gamma rays is 1.21 × 10<sup>6</sup> s<sup>–1</sup>·cm<sup>–2</sup> in the center of the experimental station 2 of Back-n, which is 76.5 m away from the spallation target of CSNS.