The use of accelerator-driven systems for incineration of nuclear waste and energy production requires monitoring of different parameters that govern reactor safety. One of the most important parameters is the multiplication factor keff . The present paper describes the results of experiments carried out on a subcritical system (BRAHMMA) using a pulsed neutron source. The value of the multiplication factor keff obtained from time responses of the core that were measured in situ using neutron detectors after insertion of a neutron pulse matches well with the calculated value.