Solar neutrino background in high-pressure gaseous ^(82)SeF_(6) TPC neutrinoless double beta decay experiments

In this study,the possibility of observing a solar neutrino background in a future neutrinoless double beta decay experiment using a high-pressure gaseous ^(82)SeF_(6) TPC is investigated.Various contributions are simulated,and possible features that could be used for event classification are discussed;two types of backgrounds are identified.The rate of multi-site background events is approximately 0.63 events/(ton·yr)in a 30 keV ROI window.This background could be effectively reduced to less than 0.0001 events/(ton·yr)(95%C.L.)while maintaining a high signal efficiency of 93%by applying a selection based on the number of clusters and energy of the leading cluster.The rate of the single-electron background events is approximately 0.01 events/(ton·yr)in the ROI.Assuming a reduction factor of 10 for the single-electron background events obtained via the algorithms developed for radioactive background rejection,the total background induced by the solar neutrino would be 0.001 events/(ton·yr),which is sufficiently small for conducting ton-level experiments.

Searching for^(76)Ge neutrinoless double beta decay with the CDEX-1B experiment

We operated a p-type point contact high purity germanium(PPCGe)detector(CDEX-1B,1.008 kg)in the China Jinping Underground Laboratory(CJPL)for 500.3 days to search for neutrinoless double beta(0νββ)decay of^(76)Ge.A total of 504.3 kg⋅day effective exposure data was accumulated.The anti-coincidence and the multi/single-site event(MSE/SSE)discrimination methods were used to suppress the background in the energy region of interest(ROI,1989–2089 keV for this work)with a factor of 23.A background level of 0.33 counts/(keV⋅kg⋅yr)was realized.The lower limit on the half life of^(76)Ge 0νββdecay was constrained as T_(1/2)^(0ν)>1.0×10^(23)yr(90%C.L.),corresponding to the upper limits on the effective Majorana neutrino mass:<mββ><3.2–7.5 eV.

NνDEx-100 conceptual design report

Observing nuclear neutrinoless double beta (0vββ) decay would be a revolutionary result in particle physics.Observing such a decay would prove that the neutrinos are their own antiparticles,help to study the absolute mass of neutrinos,explore the origin of their mass,and may explain the matter-antimatter asymmetry in our universe by lepton number violation.We propose developing a time projection chamber (TPC) using high-pressure ^(82)SeF_(6) gas and Topmetal silicon sensors for readout in the China Jinping Underground Laboratory (CJPL) to search for neutrinoless double beta decay of82Se,called the NvDEx experiment.Besides being located at CJPL with the world’s thickest rock shielding,NvDEx combines the advantages of the high Qββ(2.996 MeV) of82Se and the TPC’s ability to distinguish signal and background events using their different topological characteristics.This makes NvDEx unique,with great potential for low-background and high-sensitivity 0 vββsearches.NvDEx-100,a NvDEx experiment phase with 100 kg of SeF_(6)gas,is being built,with plans to complete installation at CJPL by 2025.This report introduces 0 vββ physics,the NvDEx concept and its advantages,and the schematic design of NvDEx-100,its subsystems,and background and sensitivity estimation.

无中微子双贝塔衰变实验发展沿革与未来展望

无中微子双贝塔衰变是一种重要的超出标准模型的新物理。在数十年理论和实验的发展与探索的基础上,人们对其可能的物理机制以及实验的技术需求已有较为深刻的理解。国际上有多个实验组通过不同的探测器技术尝试寻找无中微子双贝塔衰变事件,并对半衰期下限给出了1026年量级的限制。目前各实验组正在积极进行下一代实验装置的预研和建设,致力于将半衰期灵敏度提高到1027年以上。中国正依托锦屏地下实验室国际领先的实验环境,开发多个不同路线的探测技术。文章将概述国际上主要的大型无中微子双贝塔衰变实验的现状,并展示在锦屏地下实验室中探索这一前沿物理领域的前景以及基于不同探测器技术的实验方案。

First results on ^(76)Ge neutrinoless double beta decay from CDEX-1 experiment

We report the first results on 76Ge neutrinoless double beta decay from stage one of the China dark-matter experiment （CDEX）. A p-type point-contact high-purity germanium detector with a mass of 994g has been installed to detect neutrinoless double beta decay events, as well as to directly detect dark matter particles. An exposure of 304kgd has been analyzed over a wide spectral band from 500keV to 3MeV. The average event rate obtained was about 0.012 counts per keV per kg per day over the 2.039MeV energy range. The half-life of76Ge neutrinoless double beta decay derived based on this result is 70v2〉6.4× 1022 yr （90%C.L.）. An upper limit on the effective Majorana-neutrino mass of 5.0eV has been achieved.

无中微子双贝塔衰变:新物理探索和宇宙学观测

近二十多年来,太阳、大气、反应堆和加速器的中微子振荡实验提供了大量确凿的证据表明中微子有质量。然而,粒子物理学标准模型却预言中微子质量为零。这意味着中微子质量的产生必然要求引入超出标准模型的新物理。文章将介绍无中微子双贝塔衰变对新物理探索和宇宙学观测的重要影响,尤其是该稀有衰变过程的实验观测在突破粒子物理学和宇宙学标准模型的前沿研究中扮演的独一无二的角色。

Study of CdMoO4 crystal for a neutrinoless double beta decay experiment with ^116Cd and ^100Mo nuclides

The scintillation properties of a CdMoO4 crystal have been investigated experimentally.The fluorescence yields and decay times measured from 22 K to 300 K demonstrate that CdMoO4 crystal is a good candidate for an absorber for a bolometer readout,for both heat and scintillation signals.The results from Monte Carlo studies,taking the backgrounds from 2ν2β of^100 42Mo ^116 48Cd） and internal trace nuclides ^214Bi and ^208Tl into account,show that the expected sensitivity of a CdMoO4 bolometer for neutrinoless double beta decay experiments with an exposure of 100kg·years is one order of magnitude higher than those of the current sets of the limT1/2^0νββ of ^100 42Mo and ^116 48Cd.

Panda X-III： Searching for neutrinoless double beta decay with high pressure ^(136)Xe gas time projection chambers

Searching for the neutrinoless double beta decay(NLDBD)is now regarded as the topmost promising technique to explore the nature of neutrinos after the discovery of neutrino masses in oscillation experiments.Panda X-III(particle and astrophysical xenon experiment III)will search for the NLDBD of136Xe at the China Jin Ping Underground Laboratory(CJPL).In the first phase of the experiment,a high pressure gas Time Projection Chamber(TPC)will contain 200 kg,90%136Xe enriched gas operated at10 bar.Fine pitch micro-pattern gas detector(Microbulk Micromegas)will be used at both ends of the TPC for the charge readout with a cathode in the middle.Charge signals can be used to reconstruct the electron tracks of the NLDBD events and provide good energy and spatial resolution.The detector will be immersed in a large water tank to ensure～5 m of water shielding in all directions.The second phase,a ton-scale experiment,will consist of five TPCs in the same water tank,with improved energy resolution and better control over backgrounds.

Towards the meV limit of the effective neutrino mass in neutrinoless double-beta decays

We emphasize that it is extremely important for future neutrinoless double-beta(0νββ)decay experiments to reach the sensitivity to the effective neutrino mass|mββ|≈1 meV.With such a sensitivity,it is highly possible to discover the signals of 0νββ decays.If no signal is observed at this sensitivity level,then either neutrinos are Dirac particles or stringent constraints can be placed on their Majorana masses.In this paper,assuming the sensitivity of|mββ|≈1 meV for future 0νββ decay experiments and the precisions on neutrion oscillation parameters after the JUNO experiment,we fully explore the constrained regions of the lightest neutrino mass m1 and two Majorana-type CP-violating phases{ρ,σ}.Several important conclusions in the case of normal neutrino mass ordering can be made.First,the lightest neutrino mass is severely constrained to a narrow range m1∈[0.7,8]meV,which together with the precision measurements of neutrino mass-squared differences from oscillation experiments completely determines the neutrino mass spectrum m2∈[8.6,11.7]meV ing phases is limited to ρ∈[130°,230°],which cannot be obtained from any other realistic experiments.Third,the sum of three neutrino masses is found to beΣ≡m1+m2+m3∈[59.2,72.6]meV,while the effective neutrino mass for beta decays turns out to be mβ≡(|Ue1|2m1^2+|Ue2|2m2^2+|Ue3|2m3^2)1/2∈[8.9,12.6]meV.These observations clearly set up the roadmap for future non-oscillation neutrino experiments aiming to solve the fundamental problems in neutrino physics.

Hiding neutrinoless double beta decay in the minimal seesaw model with the TM1 orμ-τreflection symmetry

In Asaka et al(2021 Phys.Rev.D 103,015014),Asaka,Ishida and Tanaka put forward an interesting possibility that the neutrinoless double beta decay can be hidden in the minimal seesaw model with the two right-handed neutrinos having a hierarchical mass structure:the lighter one is lighter enough than the typical Fermi-momentum scale of nuclei while the heavier one is sufficiently heavy to decouple from the neutrinoless double beta decay.Then,in the basis where the mass matrices of the charged leptons and right-handed neutrinos are diagonal,for some particular texture of the Dirac neutrino mass matrix M_(D),the neutrinoless double beta decay can be hidden.In this paper,on top of this specified model,we study the interesting scenario that M_(D)further obeys the TM1 symmetry orμ-τreflection symmetry which are well motivated by the experimental results for the neutrino mixing parameters.

地下实验室μBq/kg量级辐射本底测量装置研究进展

中国锦屏地下实验室开展暗物质直接探测、无中微子双贝塔衰变等稀有事例探测实验,需要在地下实验室建立极低本底测试平台,对大部分材料实现毫贝可每千克(m Bq/kg)量级的放射性水平测量;对铜等材料实现微贝可每千克(μBq/kg)量级的放射性水平测量。正在建设中的极低本底分析测试平台,针对材料放射性筛选需求建立地下放射性测量分析系统和μBq/kg量级辐射本底测量装置,使用低本底高纯锗伽马谱仪作为探测器。μBq/kg量级辐射本底测量装置的设计目标是实现对10μBq/kg量级的材料放射性的测量。

Gamma-,neutron-,and muon-induced environmental background simulations for 100Mo-based bolometric double-beta decay experiment at Jinping Underground Laboratory

The sensitivity of an experiment to detect the Majorana neutrino mass via neutrinoless double-beta decay(0νββ) strongly depends on the rate of background events that can mimic this decay. One major source of this background is the radioactive emissions from the laboratory environment. In our study, we focused on assessing the background contributions from environmental gamma rays, neutrons, and underground muons to the Jinping bolometric demonstration experiment. This experiment uses an array of lithium molybdate crystal bolometers to probe the potential 0νββ decay of the100Mo isotope at the China Jinping Underground Laboratory. We also evaluated the shielding effectiveness of the experimental setup through an attenuation study. Our simulations indicate that the combined background from environmental gamma rays, neutrons, and muons in the relevant100Mo 0νββ Q-value region can be reduced to approximately 0.003 cts/kg/keV/yr.

中国锦屏地下实验室与稀有物理实验

中国锦屏地下实验室是目前世界上岩石覆盖最深的深地实验室,对宇宙线μ子具有良好的屏蔽作用,具有极低的宇宙线通量,因此适用于开展对稀有物理事例的探测实验。目前,包括中国暗物质实验(China Dark matter EXperiment,CDEX)、粒子和天体物理氙实验(Particle and Astrophysical Xenon Experiments,PandaX)、锦屏深地核天体物理实验装置(Jinping Underground Nuclear Astrophysics Experiment,JUNA)等实验组已经入驻锦屏实验室,对暗物质直接探测、无中微子双β衰变、天体核反应等一系列稀有物理事例进行实验探测,并给出了一系列国际先进水平的研究成果。本文总结了中国锦屏地下实验室自成立以来的发展过程和本底控制情况,并对实验室中各个实验组所开展实验的物理意义、实验原理、关键技术、物理成果等进行介绍,同时给出实验组接下来的实验计划。

TeO_2晶体材料的纯度和低本底γ测量

CUORE是测量中微子质量和性质的实验装置,本文介绍我们在其探测单元生长过程中,控制其放射性污染的测量方法和技术,为控制污染所采取的具体方法和技术,包括本底的γ射线测量,原料所含的微量元素的纯度测量,γ射线的检测,以及为γ射线检测所进行的探测器的探测效率的实验和模拟计算。

地下实验室^(96)Zr无中微子双贝塔衰变实验研究进展

无中微子双贝塔衰变实验是为了找寻超出标准模型的新物理(new physics beyond the standard model)而在地下实验室开展的最具潜力的低本底前沿物理实验之一。符合无中微子双贝塔衰变实验要求的核素有^(76)Ge、^(96)Zr、^(100)Mo、^(130)Te、^(136)Xe等,其中^(96)Zr具有独特的理论优势,但相关实验较少。该文从理论上分析^(96)Zr无中微子双贝塔衰变的高事例发生率、强中微子质量限制能力和潜在发生无中微子四贝塔衰变的特点及实验本底要求,阐述地下实验室^(96)Zr无中微子双贝塔衰变实验研究现状,展望未来在中国锦屏地下实验室开展^(96)Zr无中微子双贝塔衰变实验的愿景。

高纯锗探测器应用于稀有事例探测的研究进展

高纯锗探测器(HPGe)对于低本底稀有事例探测的发展具有重要的意义.本文介绍了HPGe的工作原理及其制备工艺,分析了对探测器起关键作用的钝化层和死层部分,并讨论了降低本底的关键技术.对于HPGe应用于包括暗物质探测和无中微子双贝塔衰变的稀有事例探测实验的进展,重点介绍了应用HPGe的CDEX、SuperCDMS、GERDA、MAJORANA等国际领先的稀有事例探测合作组相关实验技术和物理进展,进一步分析了HPGe用于下阶段国际稀有事例探测的发展方向.

基于低温量热器的无中微子双贝塔衰变探测实验国内外研究进展

无中微子双贝塔衰变过程能够揭示中微子的粒子属性,打破标准模型的轻子数守恒,并且是目前获得中微子有效质量的重要途径.本文通过对国内外无中微子双贝塔衰变探测实验的最新研究进展进行详细调研,重点对基于低温量热器的无中微子双贝塔衰变实验设计方案进行深入研究,并以意大利格兰萨索地下实验室(cryogenic underground observatory for rare events,CUORE)实验方案为例,详细地阐述了低温量热器技术的系统组成、工作原理以及现阶段所面临的关键难点,将要解决的问题主要包括极低温环境的搭建、核素种类的选取与探测塔的搭建、本底的甄别和微弱信号的前置放大.

低放射性本底金属钛冶炼工艺研究

以暗物质、无中微子双β衰变探测为代表的稀有事例实验所期望的信号极其稀少与微弱,从而对探测器所用材料的放射性本底要求十分苛刻。低本底控制是此类实验的核心工作之一。^(238)U、^(232)Th衰变链前端的^(226)Ra和^(228)Ra具有低沸点高蒸气压的特点,去除^(226)Ra与^(228)Ra就可以截断^(238)U衰变链,或者在一定时间内控制^(232)Th衰变链后端活度,从而降低对稀有事例实验有明显负面影响的核素含量,为探测器运行创造低本底环境。比较不同的冶炼设备,发现高温、高真空环境有助于K、Cs、Ra、Pb、Po、Rn等低沸点高蒸气压放射性杂质的挥发,冶炼试验结果说明,高温高真空冶炼有去除放射性核素的迹象。在真空电子束炉中冶炼的纯钛,^(232)Th-^(228)Ac可小于1.26 mBq∙kg^(−1),^(238)U-^(222)Rn可小于0.55 mBq∙kg^(−1),性能较为稳定,可用于下一代PandaX探测器。

Search for two-neutrino double-beta decay of^(136)Xe to the 0^(+)_(1)excited state of 136Ba with the complete EXO-200 dataset

A new search for two-neutrino double-beta(2νββ)decay of^(136)Xe to the 0+1 excited state of 136Ba is performed with the full EXO-200 dataset.A deep learning-based convolutional neural network is used to discriminate signal from background events.Signal detection efficiency is increased relative to previous searches by EXO-200 by more than a factor of two.With the addition of the Phase II dataset taken with an upgraded detector,the median 90%confidence level half-life sensitivity of 2νββdecay to the 0+1 state of 136Ba is 2.9×10^(24)yr using a total^(136)Xe exposure of 234.1 kg yr.No statistically significant evidence for 2νββdecay to the 0^(+)_(1)state is observed,leading to a lower limit of T2ν1/2(0^(+)→0^(+)_(1))>1.4×10^(24)yr at 90%confidence level,improved by 70%relative to the current world's best constraint.

单晶体多电极读出高纯锗探测器^(76)Ge的0νββ甄别研究

^(76)Ge的无中微子双贝塔(0νββ)衰变探测实验中本底抑制十分关键,单电子(本底事例)和双电子(0νββ事例)事例甄别因其信号特征区分度极小而十分具有挑战性.本文针对单晶体多电极读出的高纯锗探测器建立了波形模拟研究方法,对双面交叉条型多电极高纯锗探测器的位置分辨能力进行了研究.同时通过搭建lightGBM模型,对多电极高纯锗探测器各个电极所收集的模拟波形进行分类训练.结果表明,该结构的多电极高纯锗探测器在电极分布方向具有良好的位置敏感特性,同时可以通过两端电极过中时间差确定粒子作用深度.基于所搭建的lightGBM模型对0νββ双电子事例的识别效果为68.4%.经验证,所训练lightGBM模型在保证0νββ信号事例没有被明显抑制的基础上,对本底中^(68)Ge和^(60)Co抑制效果相对显著,甄别前后分别可以被抑制3.33倍和2.66倍.结果表明,单晶体多电极读出的高纯锗探测器对于^(76)Ge的0νββ衰变探测的应用具备可行性,为将来在中国锦屏地下实验室对^(76)Ge的0νββ衰变实现event-by-event搜寻提供了重要参考.