SYSTEMATIC EVALUATION OF NEUTRON SHIELDING EFFECTS FOR MATERIALS

Three types of experiments with a Cf-252 neutron source are proposed t o evaluate systematically the neutron shielding effects of a material. The type I experiment deals with each shielding material alone, the t ype 2 experiment combines a shielding material and a structural materi al, and the type 3 experiment constructs the optimization with The mat erials used in the type 2 experiment. In the stainless steel (SS) + po lyethylene shielding system, because of the location of the SS slabs a t the source side, the tenth layer of the polyethylene becomes approxi mately one-half the value as when the polyethylene is employed alone. This is the enhancement effect of the SS. In the type 3 experiment, th e total thickness of the SS + polyethylene shielding system is 40 cm, and the total thicknesses of the SS and the polyethylene slabs are fix ed at 25 and 15 cm thick, respectively. The minimum total dose-equival ent rate (neutron + secondary gamma rays) is observed when the polyeth ylene slabs are located at a 20-cm depth from the source side, with an arrangement of 20-cm-thick SS + 15-cm-thick polyethylene + 5-cm-thick and SS, and with a ratio of the maximum to the minimum dose-equivalen t rate of 2.5. The shielding optimization can be constructed by combin ing the materials having different shielding characteristics. The expe rimental results of the three types of experiments are reproduced fair ly well by using the continuous-energy Monte Carlo code MCNP 4A with a next-event surface crossing estimator.