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Fast neutron multiplicity measurement technology is an important non-destructive testing technology in the field of arms control verification. The technique uses the liquid scintillation detector to detect the fission neutron, combined with the time correlation analysis method to extract multiplicity counting rates in the pulse signals. It is commonly used to measure the mass of nuclear materials. However, this technique is based on the point model assumption, which holds that the neutron multiplication coefficient remains constant in the whole space volume, which will lead to overestimate the multiplication coefficient, resulting in the system deviation. To correct the deviation and improve the measurement accuracy, the fast neutron multiplicity simulation measurement was carried out on the samples of spherical and cylindrical shapes in this paper. The relationship between the position of neutron generation, absorption and net growth in the space volume of the material was obtained. According to the definition of the leakage multiplication coefficient, the leakage multiplication coefficient at different position in the space volume of the material was calculated. On this basis, according to the functional relationship between the neutron multiplicity factorial moment and the unknown parameters, a method based on the spatial multiplication coefficient correction was proposed. In this method, the n-order multiplication coefficient was modified by introducing a weight factor gn, and the fast neutron multiplicity weighted point model equation was derived. To verify the accuracy of this method, a set of fast neutron multiplicity detection model was built by Geant4, and the fast neutron multiplicity simulation measurement was carried out on the samples of spherical and cylindrical shapes. The result shown that the solution accuracy of the weighted point model equation is higher than that of the standard point model equation, and the measurement deviation was reduced to less than 6 %. This paper provides an optimization method for solving the mass of kilograms-level plutonium samples, and promotes the development of the fast neutron multiplicity measurement technology.
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