An intense 14 MeV neutron source facility named OKTAVIAN was installed in the A15 building, Osaka University in 1981. Along the operation period, new radioisotopes with various half-life have been produced as neutron activation products in its concrete wall shield. In this work, we investigated the concrete wall in the heavy irradiation room of OKTAVIAN using gamma spectrometry method to discover the presence of radioisotope having large half-life value (long-lived radioisotope) as neutron activation products. Computational simulations were performed prior to measurement to predict the presence of long-lived radioisotopes by employing MCNP5 and FISPACT codes. A pre-calibrated Germanium detector with high energy resolution was employed to measure the concrete. Several long-lived activation products have been observed such as 152Eu, 54Mn, 65Zn, 22Na and 60Co. The activity of each radioisotope was derived after estimating the detector efficiency using MCNP5. As a result of the measurement and analysis, the followings are concluded: (1) Though presence of activation products represents radiological risk to everyone who performs an experimental activity in the irradiation room of the OKTAVIAN facility, the present result shows that past experiments were carried out safely without any significant additional exposure dose coming from the wall for the last 38 years. (2) The approximated total fluence of D-T neutrons to the wall was successfully estimated from the produced radioisotope, 152Eu, because it has the longest half-life of 13.5 years among the observed radioisotopes. (3) From the results of (1) and (2), it could be possible to estimate the total activity of the concrete wall in the OKTAVIAN facility, which is very essential and important information, because this would be very valuable for decommissioning or disposal of the facility in the future.