Critical currents and Flux Creep in MgB2

We report detailed measurements on small (~ 100 micron) polycrystalline fragments of MgB2 of critical current densities Jc and flux creep rate S. The natural grain boundaries within the fragments appear fully transparent to supercurrent. Within the framework of collective pinning theory that has been applied to HTS materials, the dependences of Jc and S on field suggest a catastrophic decline in pinning energy of pure MgB2 with increasing field