表征用于弹头核查的 Excalibur（铀校准实验）中子源实验
Characterizing the Experiment for Calibration with Uranium (Excalibur) Neutron Source for Use in Warhead Verification
AI总结：
该研究表征了用于弹头核查的 Excalibur 氘氚中子源，通过准直与慢化两种配置的中子角分布、能谱及伽马测量验证其性能，并与 MCNP6 模拟吻合。
摘要：
中子源可在弹头核查中发挥多种作用。对于透射射线照相，需要定向高能中子源；而在探测裂变同位素的应用中，亚 MeV 中子更受青睐。Excalibur（Experiment for Calibration with Uranium，铀校准实验）中子源已经建成，并被用于多种与核查相关的实验。Excalibur 基于一台商用氘氚中子发生器，其经规定和测量能够以高达 8.2×10^8 neutrons/s 的速率产生 14 MeV 中子。该发生器封闭在一个直径 32 英寸、高 23.62 英寸的碳钢圆柱体内，该圆柱体将平均中子能量慢化至 500 keV 以下。该圆柱又被包裹在含硼 5% 的聚乙烯中，使整个装置成为一个 48 英寸×48 英寸、高 30 英寸的箱体。对于射线照相应用，钢和聚乙烯中的狭窄锥形通道允许 14 MeV 中子直接从发生器流向测试对象。通过测量中子通量剖面，证明了其准直能力。在慢化运行模式下，发生器完全被钢封闭，但聚乙烯的一大部分被移除，从而可在很宽的角度范围内提供亚 MeV 中子通量。使用嵌套式中子谱仪以及与 ^3He 探测器耦合的商用液体闪烁体进行的中子角分布和能谱测量表明，慢化模式下出现了预期的较软中子能谱，并与 MCNP6 计算结果高度一致。Excalibur 的伽马射线能谱也与 MCNP 建模结果高度一致。基于这些发现，本文讨论了 Excalibur 在其两种配置下的未来应用。
英文摘要：
Neutron sources can play a variety of roles in warhead verification. For transmission radiography, a source of directed high energy neutrons is required, while for applications to detect fissile isotopes, sub-MeV neutrons are preferred. The Excalibur (Experiment for Calibration with Uranium) neutron source has been built and used in a variety of verification-related experiments. Excalibur is based on a commercial deuterium-tritium neutron generator specified and measured to be capable of producing 14 MeV neutrons at rates of up to 8.2$\times$10$^8$ neutrons/s. The generator is enclosed in a carbon-steel 32$^{\prime\prime}$ diameter, 23.62$^{\prime\prime}$ high carbon-steel cylinder that moderates the mean neutron energy to under 500 keV. This, in turn, is encased in 5\%-borated polyethylene such that the entire assembly is a 48$^{\prime\prime}$$\times$48$^{\prime\prime}$ box that is 30$^{\prime\prime}$ tall. For radiographic applications, a narrow, tapered channel in the steel and polyethylene allows 14 MeV neutrons to stream directly from the generator to a test object. Its collimating capability is demonstrated by measuring the neutron flux profile. In the moderated mode of operation, the generator is fully enclosed in the steel, but a large section of the polyethylene is removed, providing a flux of sub-MeV neutrons from a wide range of angles. Neutron angular and spectral measurements using both a nested neutron spectrometer and a commercial liquid scintillator coupled with a $^3$He detector show the expected softer neutron spectrum in moderated mode in good agreement with MCNP6 calculations. The gamma-ray spectrum from Excalibur is also in good agreement with MCNP modeling. Based on these findings, the future application of Excalibur in its two configurations is discussed.
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