新型HPGe探测器开展0υ2β实验的关键技术问题研究

What are the masses of the neutrinos? Are neutrinos their own anti-particles? the total lepton number is not conserved? These key physical problems in particle physics domain can be answered by the results from 0υ2β experiment. So it is a very important and front research topic for double beta decay experiment to be persued in particle physics and cosmology. .HPGe detector has some advantages for the detection of double beta decay event of Ge-76 nucleus with good energy resolution and ultra-low internal background for Ge crystal. The lowest neutrino mass limit now was given with HPGe detector by Heidelberg -Moscow group in 2004. Now 0υ2β experiment has gone into the stage of the detector development and background discriminaton for ton-mass scale double beta decay project.Due the many technical challenges and expensive cost,the progress goes not smoothly. .This proposal will try to study the background levels, types and sources of 1kg mass scale Point-Contact Germanium detector(PCGe) and Broad Energy Germanium detecter(BEGe) which we will run and test at our ground laboratory at Tsinghua university and in China JinPing underground Laboratory(CJPL). .CJPL has the deepest rock overburden in the world and will provide a very good experimental place for double beta decay experiment with ultra-low muon flux.We have also built passive shielding system with 1m-thick polyethylene,20cm lead and 20cm high purity oxygen-free high-conductivity copper. Our group has prepared a well-type NaI(Tl) detector as the active shielding for the PCGe and BEGe detector. A new low temperature and active shielding system with liquid argon(LAr) as medium will be finished to construct by the end of 2012 and will also served as a new kind of anti-compton detector system for PCGe detector which has no LN2 dewar cooling system and can directly immerged into the LAr low temperature and active shielding system. All of the conditions and instruments will be used for the measurement and understanding of Ge detector background contributed by ambient materials, Ge detector crystal itself and muon-induced radioactive nuclei..The effective pulse shape discrimination methods for PCGe and BEGe detector will also be developed for discriminating single-scatter event(SSE) and multiple-scatter event(MSE), surface events and bulk events. This will be very helpful for us to understand the background sources and find the powerful methods to get rid of the backgrounds for the future double beta decay experiment with Ge detector. Based on the wonderful condition of CJPL,we wish to build a very good and solid technical basis for our country to promote a world-level Ge-76 0υ2β experiment in the future with our studies on the key background understanding and the development of pulse shape discrimination method.

0υ2β实验可测量中微子质量、确定中微子是否是其自身反粒子、验证总轻子数不守恒等一些关键物理学问题，是粒子物理学、宇宙学等领域重要前沿研究课题。HPGe探测器测量76Ge 0υ2β衰变过程具有诸多优势，当前中微子质量最低下限就是由HPGe探测器测得。国际上已经进入吨量级76Ge 0υ2β的前期研究阶段，面临诸多挑战，研究进展较慢。.本项目利用锦屏地下实验室低宇宙线环境和配套的被动和主动屏蔽体，对公斤量级PCGe和BEGe探测器开展实验测量和模拟研究，理解和掌握76Ge 0υ2β实验2.039MeV目标能区的本底强度、种类和来源等，发展基于PCGe和BEGe探测器的脉冲波形甄别技术，对Ge探测器单点和多点事例、表面和内部事例进行有效甄别。通过深入研究HPGe探测器的几个关键技术问题，依托锦屏地下实验室优势，为我国未来开展国际水平的76Ge 0υ2β实验奠定良好的技术基础。

高纯锗探测器开展锗-76双贝塔衰变实验具有诸多技术优势。本项目主要基于创新发展的极低能量阈值的点电极高纯锗探测器良好的低阈值、低本底、高能量分辨率和宽的动态范围的特点，对锗-76双贝塔衰变的一些关键技术问题进行研究，主要包括：公斤量级的点电极高纯锗系统的搭建和性能研究、理解和掌握2MeV附近锗-76双贝塔衰变能区的本底来源、种类和强度进行研究，并发展单点事例和多点事例的甄别能力。通过本项目的研究，在如下几个方面取得了非常好的进展：建立了两个基于公斤量级点电极高纯锗探测器的实验系统，配套建设了铅、铜等材料组成的被动屏蔽体以及仅靠高纯锗探测器探头的高纯碘化钠探测器组成的主动反符合探测器，并开展探测器系统刻度研究和实验运行，获得了双贝塔能区报告的实验数据；通过对实验数据进行分析，得到了双贝塔衰变能区的本底能谱，并利用探测器自身的非常微弱的放射性杂质核素的特征峰对探测器进行了能量刻度；对探测器双贝塔衰变能区的本底来源进行了实验研究，结合模拟研究，给出了探测器内部宇生放射性、探测器附近电子学和结构材料放射性、探测器外部屏蔽材料放射性等不同来源的本底贡献和相对强度关系；得到了探测器系统锗-76双贝塔衰变能区的本底水平，约为当前国际最灵敏的高纯锗双贝塔衰变实验GERDA实验组二期2016年实验本底水平的1000倍。这是目前我国高纯锗探测器双贝塔衰变能区得到的最低水平的计数率，并且了解高能区的本底来源，为下一步采取有效措施降低探测器本底水平，开展更高灵敏度的双贝塔衰变实验奠定了很好的技术基础。项目组发表了多篇高水平研究论文。