Equipment for research and development on EXO at TRIUMF

TRUMF 的 EXO 研发设备

• 批准号: SAPEQ-2014-00008
• 负责人: Retiere, Fabrice
• 金额: $ 2.62万
• 依托单位: TRIUMF
• 依托单位国家: 加拿大
• 项目类别: Subatomic Physics Envelope - Research Tools and Instruments
• 财政年份: 2014
• 资助国家: 加拿大
• 起止时间: 2014-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=567992
• 关键词: Equipment; research; development; EXO; TRIUMF

The next generation of the Enriched Xenon Observatory (nEXO) is designed to achieve unprecedented sensitivity to the possible neutrinoless double beta decay of 136Xenon. It will probe a large fraction of the parameter space allowed by theory which explicitly requires that neutrinos are Majorana particles, i.e. that neutrinos are their own anti-particles. In particular, nEXO will probe the full parameter space available if the neutrino mass hierarchy is inverted. The detection of neutrinoless double beta decay would show that neutrino are Majorana particles and would allow determining their absolute mass. nEXO will hold 50 times more xenon than EXO-200, its predecessor, which is currently running and has established one of the most competitive limits to date. nEXO design borrows heavily from EXO-200 but several key elements have to be revisited. In particular, the TRIUMF group intends to participate to the redesign of the photo-detector planes. In addition, TRIUMF intends to contribute to the development of a Barium tagging system that would dramatically enhance EXO background reduction capability. The photo-detector plane is being redesigned mainly because of issues with procurement of the large area avalanche photo-diodes (APDs) that were used in EXO-200. The leading technology envisioned to replace the APDs is Pixelated Geiger Mode Avalanche Photodiode (PPD) also called Silicon photomultiplier (SiPM, although, it is a brand name). TRIUMF has extensive expertise in the development of PPDs starting with the design, construction, and operation of the T2K Fine Grained Detector in 2007 to the development of PPD based solutions for Positron Emission Tomography or muon Spin Rotation. One of the main goals of this grant application is the construction of a setup for testing PPDs in liquid Xenon condition, at a temperature of 160K and for 178nm photons. The nEXO collaboration indeed still needs to demonstrate that PPDs fulfill the experiment requirement, especially in term of energy resolution. The proposed setup will allow the TRIUMF group to characterize PPDs from several vendors in order to identify suitable candidates. The energy resolution performance will be assessed using simulations including a precise model of the PPD response. Such a model will be developed to fully reproduce measurements following the example of the software package put together for T2K. The nEXO conceptual design is scheduled to be completed by the end of 2015, and the redesign of the photo-detector planes is one of the critical path items. Hence, it is essential that the TRIUMF group starts tackling this task in 2014. The RF funnel system for ion extraction at Stanford has been tested and a controlled Ba-ion source has been installed in the 10 bar GXe chamber. At this point, it is not clear if 100% transfer efficiency can be reached, however, this is needed in order to extract the Xe decay daughter ions. This points towards the need for clear identification of extracted species, the full understanding of loss-mechanisms and optimizing the individual components. One of the current problems, is that the number of expected ions released into the gas from the source and the number of detected ions differ by a factor of 10 000 too many. This could be due to residual gas ionization in the high pressure chamber. While a vacuum of 2-3 10^-9 mbar has been reached, there would be still sufficient residual contamination left to generate the number of detected ions. Moreover, the recirculated gas is filtered to a purity of ppb-level, but this could also be insufficient. The TRIUMF group will set up a test system for ion identification in a high pressure region, such as an RGA mass filter, where the source of unwanted ions can be identified and and reduced.

下一代浓缩氙天文台（nEXO）的设计目的是对136氙可能发生的无中微子双β衰变达到前所未有的灵敏度。它将探测理论所允许的参数空间的很大一部分，该理论明确要求中微子是马约拉纳粒子，即中微子是它们自己的反粒子。特别是，nEXO将探测全部可用的参数空间，如果中微子质量层次是颠倒的。无中微子双β衰变的探测将表明中微子是马约拉纳粒子，并将允许确定它们的绝对质量。nEXO的氙气容量将是其前身EXO-200的50倍，后者目前正在运行，并已建立了迄今为止最具竞争力的限制之一。nEXO的设计大量借鉴了EXO-200，但有几个关键元素必须重新审视。特别是，TRIUMF集团打算参与光电探测器平面的重新设计。此外，TRIUMF打算为钡标记系统的开发做出贡献，该系统将大大提高EXO背景降低能力。光电探测器平面正在重新设计，主要是因为采购EXO-200中使用的大面积雪崩光电二极管（APD）的问题。设想取代APD的领先技术是像素化盖革模式雪崩光电二极管（PPD），也称为硅光电倍增管（SiPM，尽管它是一个品牌名称）。TRIUMF在PPD开发方面拥有丰富的专业知识，从2007年T2 K细粒探测器的设计、建造和操作，到开发基于PPD的正电子发射断层扫描或μ子自旋旋转解决方案。这项资助申请的主要目标之一是建立一个测试PPD的设置在液体氙条件下，在160 K的温度和178 nm的光子。nEXO合作确实仍然需要证明PPD满足实验要求，特别是在能量分辨率方面。拟议的设置将允许TRIUMF集团对来自多个供应商的PPD进行表征，以确定合适的候选产品。将使用模拟（包括PPD响应的精确模型）评估能量分辨率性能。将开发这样一个模型，以便按照为T2 K组合的软件包的例子完全再现测量结果。nEXO的概念设计计划于2015年底完成，光电探测器平面的重新设计是关键路径项目之一。因此，TRIUMF集团必须在2014年开始处理这一任务。在斯坦福大学的离子提取的RF漏斗系统已经过测试，并已在10巴的Gestival室中安装了受控的Ba离子源。在这一点上，不清楚是否可以达到100%的转移效率，然而，这是为了提取衰变子离子所需要的。这表明需要明确识别提取的物种，充分了解损失机制和优化单个组件。目前的问题之一是，从源释放到气体中的预期离子的数量和检测到的离子的数量相差10 000倍太多。这可能是由于高压室中的残余气体电离所致。当真空达到2-3 × 10^-9毫巴时，仍然会有足够的残留污染物产生检测到的离子数。此外，再循环气体被过滤到ppb级的纯度，但这也可能是不够的。TRIUMF集团将建立一个用于高压区域离子识别的测试系统，例如RGA质量过滤器，可以识别和减少不需要的离子源。