Effects of neutron irradiation on the superconducting properties of the undoped MgB 2 and the carbon(C)-doped MgB 2 bulk superconductors, prepared by an in situ reaction process using Mg and B powder, were investigated. The prepared MgB 2 samples were neutron-irradiated at the neutron fluence of 10 16 -10 18 n/cm 2 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 c s of the undoped MgB 2 and C-doped MgB 2 before irradiation were 36.9-37.0 K and 36.6-36.8 K, respectively. The T c s decreased to 33.2 K and 31.6 K, respectively after irradiation at neutron fluence of 7.16×10 17 n/cm 2 , and decreased to 22.6 K and 24.0 K, respectively, at 3.13×10 18 n/cm 2 . The J c cross-over was observed at the high magnetic field of 5.2 T for the undoped MgB 2 irradiated at 7.16×10 17 n/cm 2 . 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.

Temperature dependence of magnetic moment (m−T) and the magnetization (M− ) at 5 K and 20 K of the in situ processed MgB 2 bulk pellets with/without carbon (C) doping were examined. The superconducting critical temperature (T c ), the superconducting transition width (∆T) and the critical current density (J c ) were estimated for ten test samples taken from the MgB 2 bulk pellets. The reliable m−T characteristics associated with the uniform MgB 2 formation were obtained for both MgB 2 pellets. The T c s and ∆Ts of all test samples of the undoped MgB 2 were the same each other as 37.5 K and 1.5 K, respectively. The T c s and ∆Ts of the C-doped MgB 2 were 36.5 K and 2.5 K, respectively. Unlike the m−T characteristics, there existed the difference among the M−H curves of the test samples, which might be caused by the microstructure variation. In spite of the slight T c decrease, the C doping was effective in enhancing the J c at 5 K.

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