The in-beam gamma energy distribution at Back-n

The back streaming white neutron beam line (Back-n) of China Spallation Neutron Source (CSNS) was built for the study of nuclear data, neutron physics, and neutron applications. Many kinds of neutron reaction cross section measurements have been performed at Back-n since the beginning of 2018. According to these measurements, there were amount of gamma rays can transmit into the experimental stations of Back-n together with the neutron beam. These gamma rays were usually called the in-beam gamma rays, which can induce non-ignorable experimental backgrounds in the neutron reaction measurements. It’s extremely necessary to study the characteristic of the in-beam gamma rays to understand the experimental background. However, it is difficult to measure the in-beam gamma rays because most gamma-ray detectors were also sensitive to neutrons, which makes it hardly to separate the neutron-induced signals from those of the in-beam gamma rays. In the present work, we proposed to use the black resonance filter (BRF) method and a CeBr3 scintillation detector to measure the characteristics of the in-beam gamma rays of Back-n. Four kinds of black resonance filters, such as 181Ta, 59Co, natAg, and natCd, were used in this measurement. The time-of-flight (TOF) technique was used to pick out the detector`s signals remaining in the absorption region in the TOF spectra, which were mainly induced by the in-beam gamma rays. By analyzing the deposited energy spectra of the CeBr3 scintillation detector and using some Monte-Carlo simulation, the energy distribution and intensity of the in-beam gamma rays of Back-n were finally determined. Based on the result of this work, the background due to the in-beam gamma rays in a measurement for neutron reaction at Back-n can be reasonable evaluated, which is helpful for improving the experimental method and data analysis.