Effects of neutron irradiation on superconducting critical temperatures of in situ processed MgB₂ superconductors

Effects of neutron irradiation on the superconducting properties of the undoped MgB₂ and the carbon(C)-doped MgB₂ bulk superconductors, prepared by an in situ reaction process using Mg and B powder, were investigated. The prepared MgB₂ samples were neutron-irradiated at the neutron fluence of 10¹⁶-10¹⁸ n/cm² in a Hanaro nuclear reactor of KAERI involving both fast and thermal neutron. The magnetic moment-temperature (M-T) and magnetization-magnetic field (M-H) curves before/after irradiation were obtained using magnetic property measurement system (MPMS). The superconducting critical temperature (T_c) and transition width were estimated from the M-T curves and critical current density (J_c) was estimated from the M-H curves using a Bean's critical model. The T_cs of the undoped MgB₂ and C-doped MgB₂ before irradiation were 36.9-37.0 K and 36.6-36.8 K, respectively. The T_cs decreased to 33.2 K and 31.6 K, respectively after irradiation at neutron fluence of 7.16×1017 n/cm², and decreased to 22.6 K and 24.0 K, respectively, at 3.13×1018 n/cm². The J_c cross-over was observed at the high magnetic field of 5.2 T for the undoped MgB₂ irradiated at 7.16×1017 n/cm². The T_c and J_c variation after the neutron irradiation at various neutron fluences were explained in terms of the defect formation in the superconducting matrix by neutron irradiation.