Passive neutron interrogation for fissile mass estimation, relying on neutrons coming from spontaneous fission events, is considered a standard NDT procedure in the nuclear safeguard and safety community. Since most structure materials are (relatively) transparent to neutron radiation, passive neutron interrogation is considered highly effective in the analyzing dirty, poorly characterized samples. Since a typical passive interrogation assembly is based on 3He detectors embedded in a moderating medium, neutrons from additional neutron sources (mainly (a, n) reactions and induced fissions in the tested sample) can not be separated from the main spontaneous fission source through energetic spectral analysis. There for, applying the passive interrogation methods requires the implementation of Neutron Multiplicity Counting methods (NMC) for separation between the main fission source and the additional sources. Applying NMC methods requires a well characterized system, in the sense that both system die away time and detection efficiency must be well known (and in particular, both parameters must be independent of the tested sample). Hence, the implementation of standard passive neutron interrogation methods on poorly characterized systems (say, systems with a large uncertainty on the position of the sample), or on samples containing neutron absorbers, is not practical. In the present study we introduce a new NMC method, in which the detection efficiency is computed directly through the measurement itself, without the need of a prior calibration. In particular, since we do not need to know the system efficiency in advance, the method might prove extremely useful in the above mentioned cases.