中国散裂中子源反角白光中子源束内伽马射线研究

In-beam γ-rays of back-streaming white neutron source at China Spallation Neutron Source

在基于白光中子源的中子核反应测量中,伴随中子束的伽马射线是重要的实验本底之一.本文对中国散裂中子源反角白光中子源的束内伽马射线进行了研究.通过蒙特卡罗模拟,得到了伽马射线的能量分布和时间结构.通过直接测量和间接测量两种方法测得低能中子区的束内伽马射线的时间结构.直接测量实验中,将载6Li的ZnS(Ag)闪烁体探测器置于束流线上,通过飞行时间法直接测量束内的中子和伽马射线的时间结构,并利用波形甄别技术进行粒子鉴别.间接测量法是将铅样品置于束流线上,利用C6D6闪烁体探测器测量样品上的散射伽马射线,从而得到入射伽马射线的时间结构.实验测量结果与模拟结果在12μs—2.0 ms的时间区间内具有较好的一致性.

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 1.0μs 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 6Li 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 C6D6 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 timeof-flight between 12μs and 2.0 ms.Besides,according to the simulation results,the intensity of the in-beam gamma rays is 1.21×10^6 s^–1·cm^–2 in the center of the experimental station 2 of Back-n,which is 76.5 m away from the spallation target of CSNS.