The structural relaxation dynamics in pure (unhydrogenated) amorphous silicon (a-Si) following ion-irradiation-induced defect injection is investigated by measuring the changes in electrical conductivity. It is found that the conductivity of a-Si decreases over three orders of magnitude upon relaxation by defect annihilation. Subsequent ion irradiation reverses the conductivity changes, confirming that the observed conductivity change is solely due to structural relaxation. Through a combined analysis of both in situ and ex situ measurements of dynamics of change in conductivity upon structural relaxation, the density of relaxation states with activation energy Q associated with localized electron states near the Fermi level is measured. It is found to be a continuous, monotonically decreasing function extending from Q < 0. 9 eV to Q > 2.5 eV, and in situ measurement of conductivity suggests a lower limit of Q < 0. 23 eV. Knowledge of this quantity in turn enables a quantitative estimate of the activation energy spectrum for structural relaxation of a-Si. Results are compared with previous investigations of structural relaxation and solid-phase epitaxy of a-Si, and the relevance of defect injection and structural relaxation in a-Si to irradiation-enhanced nucleation of crystal silicon is discussed.