  We report specific heat measurements in neutron irradiated MgB2 samples, for
which the critical temperature has been suppressed down to 8.5 K, but the
superconducting transition remains extremely sharp, indicative of a defect
structure extremely homogeneous. Our results demonstrate that the two-gap
feature is evident in the temperature range above 21 K, while the single-gap
superconductivity is well established as a bulk property not associated to
local disorder fluctuations when Tc is decreased down to 11 K.
