Background Shielding Design for TMSR Photoneutron Source

In order to meet the nuclear data requirement of Thorium Molten Salt Reactor (TMSR), a compact photoneutron source (PNS) driven by a 15 MeV electron LINAC was designed and built up by Shanghai Institute of Applied Physics. All devices including the LINAC, the neutron production target and the detector systems were all arranged in a shared hall, causing high backgrounds of neutron and γ-ray. The existing shields could not meet the requirements of low neutron background for the measurement in the thermal neutron energy regions. Therefore, more shields are needed to further reduce the neutron and γ-ray backgrounds. According to the analysis of neutron background source terms and the simulation of shielding effects of lead, concrete and boron polyethylene, the new local shields was designed. The MCNP5 simulation results show that, the new local shields can reduce the thermal neutron background by three orders of magnitude and the γ-ray background by two orders of magnitude. The experimental results with the new local shields show that, the ratio of effective thermal neutron to background thermal neutron is up to 100:1, which is of great significance for launching the foreseen physics program in the thermal neutron energy regions.