Simulation of the Nondestructive Assay of ²³⁷Np Using Active Neutron Multiplicity Counting

In this work, an epithermal neutron multiplicity counter (ENMC) and an organic scintillator multiplicity counter (OSMC) are compared in the assay of ²³⁷Np, a potentially weapons-usable isotope, using active neutron multiplicity counting. In active neutron multiplicity counting, the neutron doubles and triples multiplicity rates are used to analytically calculate sample mass. To distinguish the masses of two different samples, the measured triples and doubles rates of each sample must be separated by 1σ. The time it takes each system to separate the measured triples multiplicity rates was compared using 20 metal samples of ²³⁷Np with masses logarithmically distributed between 10 and 1000 g. The results show that the OSMC can distinguish the smallest masses (10.0 and 12.7 g) beyond 1σ in 20 min and that the ENMC requires more than 400 times the measurement time to obtain the same precision. Similarly, the OSMC is at least 4.5 times faster than the ENMC in separating the doubles multiplicity rates.