A fast neutron multiplicity counting system based on organic scintillators, i.e. EJ-309 and stilbene, has been developed and experimentally tested at the University of Michigan. The system is able to detect correlated photon and neutron multiplets emitted by the fission reaction, within a gate time of tens of nanoseconds. This counting strategy is exploited to quantify fissile mass, without the need of complex electronic circuitry and unfolding procedures, otherwise required in moderated systems. Moderated systems are traditionally based on helium-3 detectors and feature a gate time of hundreds of microseconds. Measurement precision is thus negatively affected by accidental coincidences. A prototypal version of the proposed well-shaped counter was assembled and tested in the laboratory, using a spontaneous fission and an (α, n) neutron source, i.e. 252Cf and PuBe respectively. Measured results show excellent agreement with the simulated model of the system. The viability of the system to discriminate time-correlated fission neutrons from random, uncorrelated neutrons was proved. Preliminary results of an experimental campaign, carried out at INL (Idaho National Laboratory), to characterize plutonium metal samples are also shown. Results show a monotonic increasing trend for the range of measured 240-Pu effective masses, i.e. 0.024-0.5 kg, enabling the measurement of the mass of an unknown sample.