We have investigated the irreversibility line (IRL) of a ceramic YBa2Cu3O7 sample by probing the onset of irreversibility in four DC and AC techniques. All techniques yield a line which obeys a power law H=A( 1-T Tc)n but A and n depend on the technique. Also, the location of the peak in the AC signal obeys this power-law but it does not scale with the onset of irreversibility. We have also investigated the effect of two doses of 2.5 MeV electron irradiation on the IRL and find that after irradiation both the transition temperature Tc and the irreversibility temperature Tirr decrease. The decrease in Tirr scales with the decrease in Tc. We propose that this scaling indicates that electron-irradiation does not induce pinning centers stronger than those present in the unirradiated sample.
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It is a pleasure to thank H. Pascard for providing the ceramic samples. The authors thank L. Klein for suggestions and helpful discussions. The work in Israel is partially supported by the Fund for Research and Development administered by the Ministry of Science and Technology.