  We studied the influence of the disorder introduced in polycrystalline MgB2
samples by neutron irradiation. To circumvent self shielding effects due to the
strong interaction between thermal neutrons and 10B we employed isotopically
enriched 11B which contains 40 times less 10B 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 10B 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 1-2x1018 cm-2. With increasing fluence the
superconducting properties are depressed. An in-depth analysis of the critical
field and current density behaviour 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 analysed.
