Novel Techniques for Neutrinoless Double Beta Decay Search - bridging grant

像SNO和Super-K这样的实验已经观察到了中微子的振荡，并令人信服地证明了这些难以捉摸的粒子具有有限的质量。但中微子振荡实验无法确定绝对质量尺度，也无法探测中微子是否是Majorana粒子，这与已知的带电费米子是狄拉克粒子形成了鲜明对比。无中微子双β衰变的观测可能会对中微子质量的产生有更深刻的认识。对有效中微子质量的测量，与衰变的半衰期有关，将有助于确定中微子质量等级。 下一代无中微子双β衰变实验将需要非常大的活动质量和超低的本底。拟议中的Nexo是一个5吨重的液体氙气时间投影室(TPC)，旨在对Xe-136衰变的半衰期具有大约10^28年的灵敏度。所有下一代探测器都面临着降低残留天然放射性本底的巨大挑战。当前一代的探测器，如EXO-200，已经对Xe-136的无中微子双β衰变半衰期提供了大约10^25年的限制，它是用经过仔细筛选的材料建造的，使用了特殊的设计和建造技术。为了达到Nexo的物理目标，必须进一步减少本底。标准技术已经达到极限，因此必须开发一种新的技术。 Xe-136的双β衰变产生了一种Ba-136离子，这是唯一一种实验证明具有单离子探测和识别能力的元素，也称为标记。因此，可以预见，由于残留的放射性杂质或宇宙射线引起的本底，钡离子标记可以导致全部消除。然而，将Ba2+标记技术有效地应用于大规模低温TPC是一个挑战。 我们提出了一项基本的研发计划，旨在开发、优化和表征一种用于使用放射性离子从液体氙气TPC收集离子的探针置换装置。这个项目是与Nexo合作完成的，将为将Ba离子标记技术应用于无中微子双β衰变搜索的可行性提供一个明确的答案。这项工作将主要集中在三个方面：准确的液体氙离子跟踪，包括研究离子在TPC中漂移过程中的电荷状态，静电离子捕获、引出和随后移位到TPC外的效率评估，以及与TPC阴极和离子收集探头周围的高电场有关的问题。 这项建议，除了EXO-200数据采集和分析，将为硕士和博士水平的研究生提供一个有趣的研究计划。 只要求提供一年的支持，因为此后申请者将在接下来的几年内加入加拿大EXO的主要SAP-项目申请。

Experiments like SNO and Super-K have observed neutrino oscillations and convincingly demonstrated that these elusive particles have finite masses. But neutrino oscillation experiments cannot determine the absolute mass scale nor probe if the neutrino is a Majorana particle, in contrast to charged fermions known to be Dirac particles. The observation of neutrinoless double beta decay may bring insight into the neutrino mass generation. The measurement of the effective neutrino mass, related to the half-life of the decay, will help determine the neutrino mass hierarchy. The next generation neutrinoless double beta decay experiments will require a very large active mass and ultra low background. The proposed nEXO, a 5-tonnes liquid xenon time projection chamber (TPC), is designed to have a sensitivity to the half-life of Xe-136 decay of the order of 10^28 years. All next-generation detectors face tremendous challenges for reducing the background induced by the residual natural radioactivity. Current generation detectors, like EXO-200 which has provided limits on the Xe-136 neutrinoless double beta decay half-life of the order 10^25 years, are built with carefully screened materials using special design and construction techniques. To reach the nEXO physics goal, further background reduction is mandatory. Standard techniques have reached a limit and therefore a novel one must be developed. The double beta decay of Xe-136 produces a Ba-136 ion that is the only element for which there is experimentally demonstrated single-ion detection and identification capability aka tagging. Therefore it is envisaged that Ba ion tagging can lead to the total elimination of the background due to residual radioactive impurities or induced by cosmic rays. However, applying efficiently the Ba ion tagging technique to a massive cryogenic TPC is a challenge. We are presenting an essential R&D program aimed at developing, optimizing and characterizing a probe displacement device for ion collection from a liquid xenon TPC using radioactive ions. This project is done in coordination with the nEXO collaboration and will provide a definitive answer regarding the feasibility of applying the Ba ion tagging technique to neutrinoless double beta decay search. The work will focus on three main aspects: accurate ion tracking in liquid xenon including studying the ion charge state during its drift in the TPC, efficiency evaluation of electrostatic ion grabbing, extraction and subsequent displacement outside of the TPC and HV issues related to the high electric fields surrounding the TPC cathode and the ion collection probe. This proposal, in addition to EXO-200 data taking and analysis, will provide an interesting research program for graduate students at both Master's and PhD levels. Only support for one year is asked since afterwards the applicant will join the main SAP - Project application for EXO Canada for following years.