The k0-INRIM is a software implemented at the Italian Metrology Institute (INRIM) to aid users elaborating data to obtain results including uncertainty budgets for Neutron Activation Analysis (NAA). The code relies on the efficiency evaluation based on an experimental characterization of the gamma detection system to calculate efficiency-based corrections. In order to validate the models adopted for these corrections in most demanding experimental conditions, an experiment is performed to collect data from point-like and extended-sources counted far from and near to the end-cap of two gamma detectors having different relative efficiency. The results comparing the experimentally evaluated and the calculated corrections are reported with detailed uncertainty estimation.