Single crystals of Al were irradiated up to 480 ppm of Frenkel defects using a specially developed irradiation facility for high dose electron irradiation of large samples below 20 K. Diffuse inelastic neutron scattering measurements have been performed at these samples to study the dynamics of single self interstitial atoms. The measurements were made at 6 K on the sample before annealing and after annealing at 46 K (above stage I) and at 300 K (after complete recovery). The irradiated samples showed a significant increase of the inelastic scattering intensity at transferred energies between 0.7 and 2 THz. By comparison with the results of computer calculations the different frequencies of the energy loss spectrum could be attributed to the different vibration modes of interstitial atoms. The frequencies of the most significant modes of the self interstitial atom are determined to be υ = 0.75 THz and υ = 1.35 THz for the translation-(A2u) and libration-(Eg) mode, respectively.