Neutron and γ background measurements of the experimental halls at the CSNS back-streaming white neutron source

Accurate nuclear data measurements require a good background control of the experimental environment. At the back-streaming white neutron source (the so-called Back-n White Neutron Source) at China Spallation Neutron Source that is dedicated for nuclear data measurements, comprehensive shielding measures have been employed around the neutron beam line to reduce the neutron and γ backgrounds in the experimental end-stations. The basic background generated by the neutron beam without samples was simulated by the Monte Carlo method and correspondingly measured with a Bonner sphere spectrometer. The results show that the experimental measurements and simulations are generally in good agreement, and the neutron background level meets the requirements of the nuclear data experiments. Moreover, the γ background that was measured by using liquid BC501A and NaI scintillator detectors is mainly from the natural radioactivity of environment, and the beam induced γ background is very weak, whereas the natural γ background can be removed by a coincidence in time with beam bunch or be deducted by background measurement without samples. While the present measurements prove that the neutron/γ basic background has been well controlled, the experimentalists can devote the effort to solve the background problem caused by the samples.