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快中子多重性测量技术是军控核查领域一项重要的无损检测技术，可用于核材料的质量衡算。但该方法是基于点模型假设建立的，会造成系统偏差。为修正偏差提升测量精度，本文对两种不同形状的样品进行了快中子多重性模拟测量，得到了材料空间体积内中子产生、吸收和净增长随位置的变化关系，发现了中子泄露增殖系数的空间变化规律。根据中子多重性阶乘矩与待测参数间的函数关系，提出了一种基于空间增殖系数修正的方法，通过引入修正因子gn，推导了快中子多重性加权点模型方程。为验证该方法的准确性，本文通过Geant4搭建了一套测量模型，对球体和圆柱体两种形状的公斤级钚样品进行了模拟测量。结果表明，快中子多重性加权点模型方程的测量精度高于点模型方程，测量偏差缩小至6%以内，提供了一种求解公斤级钚样品质量的优化方法，推动了快中子多重性测量技术向前发展。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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