Abstract A neutron counting (NC) technique for in-beam active neutron interrogation has been developed and deployed at the National Research Universal (NRU) reactor of the Canadian Nuclear Laboratories (CNL). A distinguishing feature of the method is that the sample to be interrogated remains stationary during the irradiation and counting periods for the detection of prompt and delayed neutrons. The NC system permits measurements of prompt and delayed neutrons using a mono-energetic or broad-spectrum (white) thermal neutron beam. An MCNPX computational study of an NC system employing nine 3He tubes showed that a delayed neutron detection efficiency of 22% could be achieved. The presence of a sub-milligram mass of 235U could be revealed in less than ten minutes in the in-beam delayed neutron experiment incorporating a white thermal neutron beam. The technique readily differentiated between 235U and 233U isotopes by analysis of delayed neutron count rates. The lower flux of the in-beam DN experiment does not permit the trace analysis that is possible with in-core irradiation, but does permit non-destructive analysis of large samples and could prove invaluable for the initial survey of materials of unknown content and origin. The study also demonstrated that in-beam prompt neutron analysis could be the ultimate solution when the neutron source is weak, the sample is shielded, the fissile mass is small, or interrogation time is limited. Detection time was reduced to a few seconds in the examination of prompt neutrons either in single or coincidence method. This paper presents the application of an in-beam NC system using a mono-energetic and white thermal neutron beam at the NRU reactor and assesses the performance of a newly constructed portable NC system with a large neutron detection array.

中文翻译：

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用于特殊核材料检测和表征的束内瞬发和延迟中子计数技术研究

摘要 在加拿大核实验室 (CNL) 的国家研究通用 (NRU) 反应堆中开发并部署了一种用于束内主动中子询问的中子计数 (NC) 技术。该方法的一个显着特点是在辐照和计数期间，待测样品保持静止，以检测瞬发和延迟中子。NC 系统允许使用单能或广谱（白色）热中子束测量瞬发和延迟中子。对采用九个 3He 管的 NC 系统进行的 MCNPX 计算研究表明，可以实现 22% 的延迟中子探测效率。在结合了白色热中子束的射束内延迟中子实验中，可以在不到 10 分钟的时间内揭示出 235U 亚毫克质量的存在。通过分析延迟中子计数率，该技术很容易区分 235U 和 233U 同位素。束内 DN 实验的较低通量不允许进行堆芯内照射可能进行的痕量分析，但允许对大样本进行无损分析，并且对于未知含量和来源的材料的初步调查证明是非常宝贵的. 该研究还表明，当中子源较弱、样品被屏蔽、裂变质量较小或询问时间有限时，束内瞬发中子分析可能是最终解决方案。在以单一或符合方法检查瞬发中子时，检测时间减少到几秒钟。