Modelling the electron anti-neutrino flux from nuclear reactors

自从20世纪50年代考恩-莱因斯实验首次在实验上探测到中微子以来，核试验堆作为电子反中微子的来源，在中微子物理学领域发挥了核心作用。这些电子反中微子可以通过逆β衰变反应来探测：[1]，它们是由核内的大放线裂变形成的富中子核的β衰变引起的。因此，这种电子反中微子通量本质上与反应堆的运行有关，并与反应堆的功率及其核心的裂变库存有关[2]。由于依赖于反应堆的运行参数和裂变库存，利用中微子监测反应堆的概念已经存在了一段时间，但直到最近对中微子的理解才使这种前景变得可行[3]。与原子能机构目前的程序形成对比的是，基于反中微子的保障措施可确保核燃料的批量核算，从而估算出核燃料所生产的裂变材料的质量，而不需要对物件的质量作出假设。此外，反中微子方法是非侵入性的，通过使用与反应堆保持一定距离的探测器来执行，并允许实时测量堆芯。在不久的将来，使用这项技术的最合理预期是通过在10米左右的距离部署吨级便携式反中微子探测器。在这些距离下，中微子振荡实验证明，与在更远的距离运行的探测器相比，中微子消失对通量的影响大大减少，例如在卡姆兰实验[4][5]中。虽然通过中微子探测对核反应堆进行远场监测是很有前景的，但中微子混合的贡献和谱背景的增加大大增加了所需建模的复杂性。用于反应堆监测的电子反中微子谱依赖于与预期通量和能谱的偏差。因此，需要建立预期通量和能谱的模型，与测量的速率相比较，通过统计方法，可以建立与模型显著偏差的阈值，以及检测这种偏差所需的时间框架[6]。这种反中微子模型很可能构成反中微子物理论文的主要基础。这将包括基于从核反应堆(如哈特勒普尔)提供的运行数据以及乏燃料库存生成反应堆的反中微子源项。这些源项将被量化，以包括对在不同距离入射到探测器的通量的估计，考虑这种通量的中微子混合以及预期的实时核核演化等现象，这是现有的应用程序，如地球中微子[7]没有考虑的，因此可能被用来通知这样的软件。在这之后，还有几个途径可以进一步研究。其中包括一些现象，如在5 MeV左右探测到的原因不明的反中微子出乎意料地消失，一些人推测是由于地形敏感性，如中微子诱导的氦核解体或中微子与原子核的相干弹性散射[8]。另一个可能有助于研究的进一步异常是在短基线实验中预期的反应堆反中微子事件的下降[9].参考文献[1]Cowan，C.，Reines，F.，Harison，F.，Kruse，H.和McGuire，A.，1956.自由中微子的探测：一个确认。《科学》，124(3212)，第103-104页[2]Bemporad，C.，Gratta，G.和Vogel，P.基于反应堆的中微子振荡实验。《现代物理学评论》，74(2)，第297-328页。反应堆监控

Since the first experimental detection of the neutrino, with the Cowan-Reines experiment in the 1950s,nuclear test reactors have played a central role within the field of neutrino physics as a source of electronanti-neutrinos. These electron anti-neutrinos can be detected through an inverse beta decay reaction:[1] and arise from the beta decay of neutron rich nuclei formed by the fission of majoractinides within the core. As a result, this electron anti-neutrino flux is inherently tied to the operation ofa reactor, and related to the power of the reactor and the fissile inventory of its core [2].Given the dependence on the operational parameters and fissile inventory of a reactor, the concept of utilisingneutrinos to monitor reactors has existed for some time, but it is only recently that the understanding of thehas made such prospects feasible [3]. In contrast to current IAEA procedures, anti-neutrino based safeguardsallow for bulk accountancy of nuclear fuel, yielding an estimate of the masses of fissile material producedwithout necessitating assumptions regarding the mass of items. Additionally, anti-neutrino methods arenon-invasive, being carried out through use of a detector at a standoff distance from the reactor and allowfor real time measurement of the core.In the near future, the most reasonable expectation for the utilisation of this technology is through thedeployment of tonne-scale portable anti-neutrino detectors at a standoff distance of the order of 10 metres.At these distances, neutrino oscillation experiments have demonstrated the vastly reduced effects of neutrinodisappearance on the flux compared to detectors operating at far greater standoff distances, such as in theKamland experiment [4] [5]. Whilst far field monitoring of nuclear reactors through neutrino detection is adesirable prospect, the contributions of neutrino mixing and the increased background of the spectra greatlyincrease the complexity of modelling required.Use of the electron anti-neutrino spectra for the purposes of reactor monitoring is reliant on deviationsfrom an expected flux and energy spectra. Thus, models of an expected flux and energy spectra are neededin comparison with the measured rate, where through statistical methods, a threshold for a significant deviationfrom the model, and the time frame needed to detect this deviation, can be established [6].This anti-neutrino modelling is likely to form the main underpinning of a thesis in anti-neutrino physics.This will include generation of anti-neutrino source terms for reactors based on operational data providedfrom nuclear reactors, such as Hartlepool, along with spent fuel inventories. These source terms will bequantified to include to provide an estimation of the flux incident to a detector at varying distances, consid-1ering phenomena such the neutrino mixing of this flux along with the expected evolution of a nuclear core ona real time basis, something that existing applications, such as geo-neutrinos [7], do not consider and mighttherefore be used to inform such software.Beyond this, several avenues exist for further pursuit. These include phenomena such as the unexpectedexcess of anti-neutrinos detected at around 5 MeV with an unknown cause, speculated by some to be due topossibilities such as neutrino induced deuteron disintegration or coherent elastic neutrino-nucleus scattering[8]. Another, further, anomaly that may lend itself to investigation is that of the drop in expected reactoranti-neutrino events in short baseline experiments [9].2 References[1] Cowan, C., Reines, F., Harrison, F., Kruse, H. and McGuire, A., 1956. Detection of the Free Neutrino:a Confirmation. Science, 124(3212), pp.103- 104.[2]Bemporad, C., Gratta, G. and Vogel, P., 2002. Reactor-based neutrino oscillation experiments. Reviewsof Modern Physics, 74(2), pp.297-328.[3] Bowden, N., 2008. Reactor monitori