超低阈值（sub-keV）气液两相TPC探测器的关键技术研究

Coherent elastic neutrino-nucleus scattering (CEvNS) has evaded experimental demonstration for forty-three years following its first theoretical description. The interest in CEvNS detection goes beyond completing the picture of neutrino couplings predicted by the Standard Model of particle interactions. In the time since its description, CEvNS has been suggested as a tool to expand our knowledge of neutrino properties. These studies include searches for sterile neutrinos, a neutrino magnetic moment, non-standard interactions mediated by new particles, probes of nuclear structure, and improved constraints on the value of the weak nuclear charge. In addition to these, the reduction in neutrino detector mass may lead to a number of technological applications, such as non-intrusive nuclear reactor monitoring. CEvNS is also expected to dominate neutrino transport in neutron stars, and during stellar collapse. Direct searches for Weakly Interacting Massive Particles (WIMPs), presently favored dark matter candidates, rely on the same untested coherent enhancement to the WIMP-nucleus scattering cross-section, and will soon be limited by an irreducible CEνNS background from solar and atmospheric neutrinos. .It is very difficult to detect the low-energy (sub keV to few keV) nuclear recoil produced as the single outcome of the interaction. Compared to a minimum ionizing particle of the same energy, a recoiling nucleus has a diminished ability to generate measurable scintillation or ionization in common radiation detector materials. This is exacerbated by a trade-off between the enhancement to the CEvNS cross-section brought about by a large nuclear mass, and the smaller maximum recoil energy of a heavy target nucleus. And also, the development of detectors having a low-energy threshold and low noise is of paramount importance for present-day and future low-background experiments. In particular, in experiments searching for dark matter and coherent neutrino-nucleus scattering. Cryogenic dual-phase detectors in general and specifically dual-phase Ar detectors are currently favoured for these applications due to their capability of efficient background rejection and higher threshold..In this project, the performance of a two-phase Ar detector in gas phase photons counting mode was studied, with regard to potential application in coherent neutrino-nucleus scattering and dark matter search experiments. The detector comprised a thick liquid Ar layer and a SiPM operated in the saturated vapour above the liquid phase, to reach the sub keV low threshold detection.

中微子-核子弹性相干散射过程（CEvSN）的意义不仅是完善了标准模型中粒子相互作用，还是一个很有用的手段研究中微子属性、核物理、天体物理及暗物质物理等。探测CEvSN过程的核心在于超低能量阈值探测技术，这项技术不仅对CEvSN意义重大，对暗物质直接探测、双β衰变等其他低本底实验的成功开展也起着非常关键的作用。但其研发难点在于低事例率和低反冲核动能、低的光收集效率、以及大本底中提取弱信号等技术。本项目拟利用气液两相时间投影室TPC和锦屏地下实验室低本底环境的优势，选择液氩做靶提高事例率和反冲核动能；采用先进的SiPM提高闪烁光的收集效率；单一气相读出技术提高弱信号幅度等手段，最终实现超低能量阈值（sub-keV）的探测技术，解决当今CEvSN探测中的关键科学问题，并为高精度的暗物质探测奠定基础。本项目突出科学问题的原创性，促进与技术创新的结合，不断提升我国在核技术探测研究领域的国际地位。

中微子-核子弹性相干散射过程（CEvSN）的意义不仅是完善了标准模型中粒子相互作用，还是一个很有用的手段研究其他物理和应用目标。探测CEvSN过程的核心在于超低能量阈值探测技术, 这项技术不仅对CEvSN意义重大，对暗物质直接探测、双β衰变等其他低本底实验的成功开展也起着非常关键的作用。本项目完成如下工作：（1）研发了100-kg容积的气液两相低温探测器，测试了制冷能力和内容器的温度变化，经分析，制冷机能有效到达足够液氩的制冷温度，波纹管泵不停机的情况下进行充氩操作实验过程更为平稳。（2）针对低温SiPM读出系统的研制和测试，测试了S14160和VUV4两种型号SiPM低温下的性能，随着温度降低，SiPM暗计数有明显增减，但单光电子分辨率随温度变化并未发生明显变化；击穿电压随温度降低而降低，但是线性变化不明显；平均波形上升时间与下降时间均未观测到明显随温度的变化趋势。（3）进行了小型两相氩TPC的设计和制作，两相氩TPC探测器由1-cm厚的PTFE套筒构成灵敏区，灵敏区直径8-cm，高10.5-cm，含液氩0.74-kg。套筒内表面贴有镜面反射膜(ESR)，以增加反射率。（4）基于小型两相氩TPC进行了大面积SiPM的低温测试，每个SiPM的通道进行了低温测试，显示了良好的性能，提高光的收集效率。（5）理论计算了反应堆中微子相干散射测量的可行性，完成了不同阈值、不同距离等条件下的灵敏度分析工作；着重分析了相干散射反应在测量裂变比例并应用到核反应堆监管中的可行性。研究结果表明，Xe和CsI探测器的有效阈值相近，均为0.7-keV，NaI探测器的有效阈值为1-keV，Ge探测器的有效阈值为1.1-keV，Ar探测器的有效阈值为1.7-keV。相对分子质量越大的核子构成的探测器有效阈值越小。当前的技术条件有希望将探测器的能量阈值制成1-keV，这意味着可选择的探测器种类为NaI、Ge和Ar，而相对分子质量较大的和探测器在1-keV的能量阈值条件下是失效的。本项目从理论估算和探测技术两方面分析验证反应堆相干散射过程的测量，突出科学问题的原创性，促进了理论与技术创新的结合。