Measurement of intrinsic radioactive backgrounds from the 137Cs and U/Th chains in CsI(Tl) crystals

The inorganic CsI(Tl) crystal scintillator is a candidate anti-compton detector for the China Dark matter Experiment. Studying the intrinsic radiopurity of the CsI(Tl) crystal is an issue of major importance. The timing, energy and spatial correlations, as well as the capability of pulse shape discrimination provide powerful methods for the measurement of intrinsic radiopurities. The experimental design, detector performance and event-selection algorithms are described. A total of 359×3 kg-days data from three prototypes of CsI(Tl) crystals were taken at China Jinping Underground Laboratory (CJPL), which offers a good shielding environment. The contamination levels of internal isotopes from ¹³⁷Cs, ²³²Th and ²³⁸U series, as well as the upper bounds of ²³⁵U series are reported. Identification of the whole α peaks from U/Th decay chains and derivation of those corresponding quenching factors are achieved.     

掺钆液闪中子探测器的研制

中子在探测器中能产生与暗物质粒子(WIMPs)类似的核反冲信号,对于低背景的暗物质直接探测实验,精确测量和排除中子的干扰尤其重要。为测量中国锦屏地下实验室(CJPL)中子本底的通量,研制了高效的、具有很强"中子-伽马"分辨的掺钆液闪快中子探测器。介绍了掺钆液闪中子探测器的研制和性能,包括探测器形状和尺寸的设计,液体闪烁体类型、光电倍增管型号以及液闪容器材料的选择,探测器的伽马能量刻度以及中子与伽马信号的甄别。用Am-Be中子源对探测器进行探测效率刻度,得到阈值为0.2 MeV等效电子能量的中子探测效率为(6.30±0.30)%,满足中国锦屏地下实验室对中子通量测量的要求。     

氦对铜钨纳米多层膜界面稳定性的影响

采用射频磁控溅射方法,分别在纯Ar和Ar, He混合气氛下制备了多个不同调制周期的Cu/W纳米多层膜. 利用增强质子背散射(EPBS)、扫描电子显微镜(SEM)、X射线衍射(XRD)分别对Cu/W多层膜中He含量、 截面形貌和相结构进行了分析.结果表明:多层膜的界面稳定性是耐氦损伤的前提和保证. 在适当的调制周期下,纳米多层膜能有抑制氦泡成核及长大的能力.     

10-25 keV电子致厚W,Au靶韧致辐射谱的测量

测量了10-25 keV电子碰撞厚W, Au靶产生的韧致辐射谱, 并与Monte Carlo程序PENELOPE模拟的X射线谱进行了比较, 除在3 keV前实验谱略低于理论谱外, 整体上两者符合得很好. 在模拟电子与靶材料相互作用产生韧致辐射时, PENELOPE程序中只包含有普通韧制辐射的截面数据. 我们的实验结果表明, 在电子与固体靶相互作用时, 没有明显的极化韧致辐射产生, PENELOPE程序能够可靠地描述电子与固体厚靶相互作用产生的韧致辐射.     

Tritium and helium analyses in thin films by enhanced proton backscattering

In order to study how to reliably perform quantitative tritium and helium analyses in thin film samples using enhanced proton backscattering(EPBS),several EPBS spectra for some samples consisting of non-RBS light elements(i.e.,T,4He,12C,16O,natSi),medium and heavy elements have been measured and analyzed using analytical SIMNRA and Monte Carlo-based CORTEO codes.The non-RBS cross sections needed in the CORTEO code are taken from the ENDF/B-Ⅶ.1 database and the calculations of SigmaCalc code and are incorporated into the CORTEO code.All non-RBS cross section data over the entire proton incident energy-scattering angle plane are obtained by interpolation.It is quantitatively observed that in EPBS analysis the multiple and plural scattering effects have little impact on the energy spectra for light elements and the RBS cross sections of light elements can be used in the SIMNRA code for dual scattering calculations.It is also observed that the results given by the CORTEO code are higher than the results of the SIMNRA code in the low energy part of EPBS spectra,and are in better agreement with the experimental data.Tritium and helium analyses in thin film samples using EPBS can be performed reliably when the multiple and plural scattering contributions are completely accounted.     

Limits on light WIMPs with a 1 kg-scale germanium detector at 160 eVee physics threshold at the China Jinping Underground Laboratory

We report results of a search for light weakly interacting massive particle(WIMP) dark matter from the CDEX-1 experiment at the China Jinping Underground Laboratory(CJPL). Constraints on WIMP-nucleon spin-independent(SI) and spin-dependent(SD) couplings are derived with a physics threshold of 160 eVee, from an exposure of 737.1 kg-days. The SI and SD limits extend the lower reach of light WIMPs to 2 GeV and improve over our earlier bounds at WIMP mass less than 6 GeV.     

Study of the background in the experiments of inner-shell ionization of atoms by positron impact

One of main difficulties in the experiments of inner-shell ionization of atoms by positron impact near threshold energy region is the relatively high low-energy background, which is caused by the deposited part of energy in semi-conductor X-ray detectors of 0.511 MeV γ rays that are produced by positron annihilations in targets and target chamber. In this paper, by using the Monte Carlo method, we simulated the backgrounds for the X-ray detectors with the sensitive layer thickness of 0.3 mm and 3 mm in the case of 0.511 MeV γ rays impacting vertically on a Ti plate of 0.2 mm in thickness, and compared the simulation results with the experimental observations of the other research group and our own. Moreover, we also simulated the backgrounds for a simplified experimental setup in the case of 20 keV positrons impacting vertically on a thick Ti target and observed that the backgrounds for the X-ray detectors with the sensitive layer thickness of 0.3 mm and 3 mm, are very similar.     

Simulation of large-scale fast neutron liquid scintillation detector

Neutron background measurement is always very important for dark matter detection due to almost the same effect for the recoiled nucleus scattered off by the incident neutron and dark matter particle. For deep under-ground experiments, the flux of neutron background is so low that large-scale detection is usually necessary. In this paper, by using Geant4, the relationship between detection efficiency and volume is investigated, meanwhile, two geometrical schemes for this detection including a single large-sized detector and arrayed multi-detector are compared under the condition of the same volume. The geometrical parameters of detectors are filtrated and detection efficiencies obtained under the similar background condition of China Jingping Underground Laboratory (CJPL). The results show that for a large-scale Gd-doped liquid scintillation detector, the detection efficiency increases with the size of detector at the beginning and then trends toward a constant. Under the condition of the same length and cross section, the arrayed multi-detector has almost similar detection performance as the single large-sized detector, while too much detector number could cause degeneration of detection performance. Considering engineering factors, such as testing, assembling and production, the 4 × 4 arrayed detector scheme is flexible and more suitable. Furthermore, the conditions for using fast and slow signal coincidence detection and the detectable lower limit of neutron energy are evaluated by simulating the light process.     

界面对ZrN/TaN纳米多层膜固氦性能的影响

采用射频磁控溅射方法, 在混合气氛下制备了ZrN/TaN多层膜. 利用X射线衍射、慢正电子束分析、增强质子背散射、扫描电子显微镜, 分别对ZrN/TaN多层膜中相结构、氦相关缺陷、氦含量、截面形貌等进行了分析. 结果表明, 调制周期为30 nm的ZrN/TaN多层膜在600℃退火后, 氦的保持率仍能达到45.6%. 在适当的调制周期下, ZrN/TaN多层膜能够耐氦损伤并且其界面具有一定的固氦性能. 关键词： ZrN/TaN 纳米多层膜 界面 固氦     

KIMS地下实验中μ子本底的研究

KIMS是寻找和研究暗物质WIMP的实验组. 在WIMP寻找实验中，μ子是十分重要的一种本底. 我们对地下700m深的Yangyang实验室内的μ子通量进行了测量. 介绍了μ子探测器的结构和性能测试. 描述了对μ子入射位置和角度分布的分析，并进行了蒙特卡罗模拟. 实验室内的μ子通量测量值为(7.0±0.4))×10^-8/s/cm²/sr.