  NMR line shift and nuclear spin-lattice relaxation rate T_1^{-1} of 11B and
25Mg were measured in superconducting MgB_2 (T_c^{ons}=38K) structurally
disordered by nuclear reactor neutrons up to the fluence of thermal neutrons
$\Phi=1 10^{19} cm^{-2}. The temperature of superconducting transition was
shifted down to T_{c,irrad}^{ons}=7K under irradiation. The change due
irradiation in the partial electron density of states (DOS) at the Fermi energy
of boron and magnesium were traced by taking into account that T_1^{-1} of 11B
and 25Mg are determined by hyperfine magnetic interactions with carriers. It
was revealed that electronic states near E_F of Mg are influenced negligibly by
irradiation whereas partial DOS of the B 2p_{x,y} states reduces greatly in
irradiated MgB_2. The NMR estimates for DOS using the McMillan formula show
that critical temperature decreases in irradiated MgB_2 mainly due to reduction
in the partial DOS of p states of boron.
