We studied the influence of the disorder introduced in polycrystalline MgB 2 samples by neutron irradiation. To circumvent self-shielding effects due to the strong interaction between thermal neutrons and 10 B we employed isotopically enriched 11 B which contains 40 times less 10 B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10 B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1 -2 ϫ 10 18 cm −2 . With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behavior has been carried out to identify what scattering and pinning mechanisms come into play. Finally, the correlation between some characteristic lengths and the transition widths is analyzed.

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We study the effect of neutron irradiation on the critical current density J c of isotopically pure polycrystalline Mg 11 B 2 samples. For fluences in the range 10 17 -10 18 cm -2 , J c is enhanced and its dependence on magnetic field is significantly improved: we demonstrate that, in this regime, pointlike pinning centers are effectively introduced in the system proportionally to the neutron fluence.Instead, for larger fluences, a strong suppression of the critical temperature accompanied by a decrease of both the upper critical field B c2 and J c is found.

The effects of neutron irradiation on normal state and superconducting properties of epitaxial magnesium diboride thin films are studied up to fluences of 10 20 cm −2 . All the properties of the films change systematically upon irradiation. Critical temperature is suppressed and, at the highest fluence, no superconducting transition is observed down to 1.8 K. Residual resistivity progressively increases from 1 to 190 ⍀ cm; c axis expands and then saturates at the highest damage level. We discuss the mechanism of damage through the comparison with other damage procedures. The normal state magnetoresistivity of selected samples measured up to high fields ͑28 and 45 T͒ allows to determine unambiguously the scattering rates in each band; the crossover between the clean and dirty limit in each sample can be monitored. This set of samples, with controlled amount of disorder, is suitable to study the puzzling problem of critical field in magnesium diboride thin films. The measured critical field values are extremely high ͑of the order of 50 T in the parallel direction at low fluences͒ and turns out to be rather independent of the experimental resistivity, at least at low fluences. A simple model to explain this phenomenology is presented.

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