Geometrical effect of asymmetric target in two-step shielding calculation for 70 MeV cyclotron facility

Currently, radiation shielding analyses of the cyclotron to be installed into the Institute for Basic Science of Korea are carried out as a way of construction of heavy ion accelerator facility. In the course of radiation shielding calculation for cyclotron, it is known to be easy to have neutron source information in advance, being generated from the reaction between proton and target material. In the case of asymmetric target geometry against the axis of the proton beam direction, it is, however, difficult to accurately generate source terms in two-step calculation as such above. In this work, geometrical effect of target was investigated in the first-step of two-step calculation. As the target geometry, cylinder target was employed because it is widely used in the symmetric condition between beam direction and target. Neutron source term from proton interaction with the small cylindrical aluminum target employed in this work was firstly calculated as functions of solid angle and energy in order to save the computation time. The calculated source term is used in the second-step calculation (Case 2). The results were compared with those from the direct calculation using proton source (Case 1) in the 70 MeV-cyclotron room. Geometrical effect of target was studied in the source term calculation of two-step calculation. In this work, cylindrical target was employed and the results from the two-step calculation were compared with the reference calculation. It is found that two-step method can give large error at the specific points according to target geometry in the source term calculation