[en] Neutron source play essential roles in various industrial and scientific fields. In particular, the fast neutron can be used to study of irradiation effect of neutrons on single event upset of semiconductor devices, materials, and bio-sciences. There are several approaches to obtain neutron beams with the desired energy spectrum such as a nuclear research reactor, a neutron-emitting radio isotopes ("2"5"2Cf and Be-couple "2"4"1Am), and a bombarding proton beams on a target. In the bombarding proton beams case, neutron beams are generated mainly through the (p, n) charge exchange reaction. By adjusting the beam energy, the target material and the thickness of the target, the resulting neutron spectrum can be controlled to a certain degree. Beryllium is widely used as the target material because of its high melting point, good thermal conductivity and many other advantaged features. The purpose of present work is to characterize the neutron flux energy spectrum which produced by a beryllium target with 0.5 cm thickness at 35 MeV energy providing by the MC-50 cyclotron of the KIRAMS (KOREA). Neutron spectrum at the irradiation position of the MC-50 cyclotron was measured by using activation method and been unfolded with the both interactive codes as SAND and STAYSL. Measurements made with foil-activation technique provided valuable information about the neutron spectrum in the irradiation position of the MC-50 cyclotron in KIRAMS. The group cross sections of a certain thickness of foil were processed before unfolding. The results of spectrum unfolding are generally consistent