Background Characterisation for the Water and Scintillator Phases of SNOPLUS

SNO+是一个多国合作项目，致力于探测被称为无中微子双β衰变的放射性衰变过程。该实验基于安大略萨德伯里的Creighton镍矿，并建立在SNO探测器上进行的先前工作的基础上（SNO+实验的全面概述可参见[1]）。实验动机观测无中微子双β衰变的主要目的是检验标准模型之外的物理。如果这个衰变过程被观察到，它将证明中微子实际上是马约拉纳费米子，也就是说它们是自己的反粒子。这将导致轻子数守恒被打破，并对轻子生成机制产生更广泛的影响。此外，如果中微子是马约拉纳粒子，这将使人们能够识别中微子质量等级并计算3种类型中微子的绝对质量。由于探测器的灵敏度，SNO+可能无法完全限制马约拉纳中微子的质量。然而，它将有能力设定55 - 133 meV之间的有效质量的上限。SNO+还将能够在其运行期间进行其他物理研究。在实验期间，探测器将对高能太阳中微子、超新星中微子和不可见的核子衰变敏感。此外，SNO+将能够在特定阶段探测低能太阳能、反应堆和地中微子。SNO+探测器在其寿命期间将有3个不同的阶段。最初，SNO+用作水切伦科夫探测器，并装有约900吨超纯水（UPW）。在第二阶段，探测器将充满液体闪烁体（LS），使探测器对反中微子相互作用敏感。最后，将碲酸加载到闪烁体中，以开始最终阶段。博士研究的背景在这个博士期间进行的研究将主要集中在探测器内的放射性背景的表征和监测。氡同位素通过子体同位素的放射性衰变作为几项研究的背景;由于矿井空气中氡的含量很高，这一点特别重要。太阳中微子和核子衰变研究易受这些背景影响（在水相期间），以及反应堆、地中微子和无中微子双β衰变研究（在LS和碲加载阶段期间），在处理探测器数据之后，将采用识别铋214衰变的技术来消除切伦科夫阶段期间的氡背景。此外，还将发展一种鉴别α-中子反应的时间延迟鉴别技术，以抑制由钋210引起的背景信号。由于探测器内装有闪烁体，因此将在第二和第三探测器阶段测试、加强和采用这一技术来分析这一背景，还将在探测器现场花费时间，以帮助将探测器推进到LS阶段。在此期间，将协助准备和组装探测器硬件。这将侧重于脐带缆回收机制的安装（用于部署校准源）。此外，将进行调查，以优化将在闪烁器内部署的脐带缆的清洁技术。参考文献Andringa等人，“SNO+实验的现状和未来前景”，《高能物理进展》，2016年第一卷，第10页。1-21，2016.可用：10.1155/2016/6194250 [访问15/11/19].

Introduction SNO+ is a multinational collaboration working on the detection of the radioactive decay process known as the neutrinoless double beta decay. The experiment is based at the Creighton nickel mine in Sudbury, Ontario and builds upon previous work carried out at the location on the SNO detector (a comprehensive overview of the SNO+ experiment is available at [1]). Experimental Motivations The principal aim, of observing the neutrinoless double beta decay, is to test for physics beyond the Standard Model. If this decay process is observed, it will demonstrate that neutrinos are in fact Majorana fermions i.e. that they are their own anti-particles. This would therefore result in lepton number conservation being broken and would have wider reaching implications for mechanisms of leptogenesis.Furthermore, if neutrinos are Majorana particles, this would enable one to identify the neutrino mass hierarchy and calculate the absolute mass of the 3 neutrino types. SNO+ may not completely constrain the Majorana neutrino mass due to the detector's sensitivity. However, it will have the ability to set an upper limit of between 55 - 133 meV on this effective mass.SNO+ will also be able to conduct other physical studies during its operation. The detector will be sensitive to high energy solar neutrinos, supernovae neutrinos and invisible nucleon decays for the duration of the experiment. Moreover, SNO+ will be able to detect low-energy solar, reactor and geoneutrinos during specific phases. Experimental Context The SNO+ detector will have 3 distinct phases during its lifetime. Initially, SNO+ functioned as a water Cherenkov detector and was filled with approximately 900 tonnes of Ultra Pure Water (UPW). During the second phase, the detector will be filled with liquid scintillator (LS) making the detector sensitive to antineutrino interactions. Finally, Telluric acid will be loaded into the scintillator to begin the final phase. Context of PhD ResearchThe research carried out during this PhD will predominantly focus on the characterisation and monitoring of radioactive backgrounds within the detector. Radon isotopes act as a background to several of the studies through the radioactive decays of daughter isotopes; this is of particular importance due to the high presence of radon in the mine air. Solar neutrino and nucleon decay studies are susceptible to these backgrounds (during the water phase) as well as reactor, geoneutrino and neutrinoless double beta decay studies (during the LS and Tellurium loaded stages).Techniques to identify bismuth 214 decays will be employed to characterise radon backgrounds during the Cherenkov phase following the processing of the detector data. Furthermore, a prompt-delay discrimination technique to identify alpha-neutron reactions will be developed to supress the background signal caused by polonium 210. As the detector is filled with scintillator, this technique will be tested, enhanced and employed to analyse this background during the second and third detector phases.Time will also be spent at the detector site to aid in advancing the detector into the LS stage. During this time, contributions will be made to preparing and assembling detector hardware. This will focus on the installation of an umbilical retrieval mechanism (for the deployment of calibration sources). Furthermore, investigations will be preformed to optimise cleaning techniques for the umbilicals that will deployed within the scintillator. ReferenceS. Andringa et al., "Current Status and Future Prospects of the SNO+ Experiment", Advances in High Energy Physics, vol. 2016, pp. 1-21, 2016. Available: 10.1155/2016/6194250 [Accessed 15/11/19].