Shielding design of a neutron beam line 'NOBORU' at JSNS/J-PARC

NOBORU, one of 23 instruments in the Materials and Life science experimental Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC), has been designed to evaluate characteristics of Japan Spallation Neutron Source (JSNS) and also used for a variety of applications. A heavy shielding structure was required for NOBORU to accept neutrons in very wide energy range up to 1 GeV with a large neutron beam cross section of 10 x 10 cm² at 1 MW. Neutronics calculations were fully applied to examine the shielding structure. In the shielding calculation, we considered various loss points of the neutron beam caused by several experimental setups. As a result, a shielding structure satisfied the radiation dose limits adopted at J-PARC for all the loss point conditions. Effectiveness of the neutron beam shutter was evaluated for radiation safety point of view. In addition, we evaluated radioactivity of a T₀ chopper, which needs a periodic hands-on maintenance, for the radiation safety. To confirm the shielding calculation, we have carried out preliminary dose rate measurements in the experimental room of NOBORU. The measured values were in good agreement with the calculated ones within a factor of 2. (author)