The magnetic-field-induced variations of the microwave Surface resistance have been investigated in a heavily neutron-irradiated MgB2 sample, in which the irradiation has Caused the merging of the two gaps into a single value. The experimental results have been analyzed in the framework of the Coffey and Clem model. By fitting the experimental data, we have determined the field dependence of the depinning frequency, omega(0), at different Values of the temperature. Although the pinning is not particularly effective, the value of omega(0) obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors. (C) 2008 Elsevier B.V. All rights reserved.

"We report on the microwave surface resistance of two polycrystalline (MgB2)-B-11 samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near T-c and for magnetic fields greater than 0.5H(c2)(T) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account."