Li Yu-Lan

Measurement of cosmic ray flux in the China JinPing underground laboratory

The China JinPing underground Laboratory (CJPL) is the deepest underground laboratory running in the world at present. In such a deep underground laboratory, the cosmic ray flux is a very important and necessary parameter for rare-event experiments. A plastic scintillator telescope system has been set up to measure the cosmic ray flux. The performance of the telescope system has been studied using the cosmic rays on the ground laboratory near the CJPL. Based on the underground experimental data taken from November 2010 to December 2011 in the CJPL, which has an effective live time of 171 days, the cosmic ray muon flux in the CJPL is measured to be (2.0±0.4)×10-10/(cm2 ·s). The ultra-low cosmic ray background guarantees an ideal environment for dark matter experiments at the CJPL.

A novel CZT detector using strengthened electric field line anode

In this paper, we report on the design, simulation and testing of a novel CZT detector with an electrode named the Strengthened Electric Field Line Anode (SEFLA). The Strengthened Electric Field (SEF) technique and Single Polarity Charge Sensing (SPCS) technique are implemented. It could achieve the same performance as Coplanar Grid, Pixel Array CZT detectors but requires only a simple readout system. Geant4, Ansoft Maxwell and a self-developed Induced Current Calculator (ICC) package are used to develop an understanding of how the energy spectrum is formed, and the parameters of the detector are optimized. A prototype is fabricated. Experimental results demonstrate the effectiveness of this design. The test shows that the SEFLA detector achieves a FWHM of 6.0% @59.5 keV and 1.6% @662 keV, which matches well with the simulations.

Development of a multi-channel readout ASIC for a fast neutron spectrometer based on GEM-TPC

Fast neutron spectrometers with high energy resolution are required in some research fields such as nuclear physics and thermo-nuclear fusion plasma diagnostics. In this study, a fast neutron spectrometer based on GEM-TPC has been proposed, which uses the energy of recoiled protons and the recoil angles from reconstructed proton tracks to derive the neutron energy. Compared to the traditional spectrometers, this new method has merits such as high detection efficiency and excellent discrimination ability for neutron from gamma.

Study of the material photon and electron background and the liquid argon detector veto efficiency of the CDEX-10 experiment

The China Dark Matter Experiment (CDEX) is located at the China Jinping Underground Laboratory (CJPL) and aims to directly detect the weakly interacting massive particles (WIMP) flux with high sensitivity in the low mass region. Here we present a study of the predicted photon and electron backgrounds including the background contribution of the structure materials of the germanium detector, the passive shielding materials, and the intrinsic radioactivity of the liquid argon that serves as an anti-Compton active shielding detector. A detailed geometry is modeled and the background contribution has been simulated based on the measured radioactivities of all possible components within the GEANT4 program. Then the photon and electron background level in the energy region of interest (<10−2eventskg1day−1keV−1 (cpkkd)) is predicted based on Monte Carlo simulations. The simulated result is consistent with the design goal of the CDEX-10 experiment, 0.1cpkkd, which shows that the active and passive shield design of CDEX-10 is effective and feasible.

High efficiency event-counting thermal neutron imaging using a Gd-doped micro-channel plate

An event-counting thermal neutron imaging detector based on 3 mol % natGd2O3-doped micro-channel plate (MCP) has been developed and tested. A thermal neutron imaging experiment was carried out with a low flux neutron beam. Detection efficiency of 33% was achieved with only one doped MCP. The spatial resolution of 72 μm RMS is currently limited by the readout anode. A detector with larger area and improved readout method is now being developed.

Study of VUV detector based on thinner THGEM for CDEX

In the China Dark matter EXperiments (CD EX), HPGe is the main target detector while liquid Ar is used to provide the low temperature working environment for HPGe detector. In the meantime, as a scintillation detector, it is also served as a veto detector. Gaseous avalanche Photomultipliers (GPMT) is one of the most promising candidates for the readout of scintillation light from liquid Ar, that is, 128nm Vacuum Ultraviolet light (VUV). Compared with PMT, the GPMT has many advantages, such as lower radioactivity. A kind of GPMT is researched in this study based on THGEM coated with CsI. The influence of different parameters on the THGEM is studied and finally an optimized thinner THGEM (thickness 0.2mm) is selected. CsI is deposited successfully on the top layer of the thinner THGEM by vacuum evaporation. The electric field near the photocathode and gas flowing through the chamber are investigated to get the best working conditions. Simulations and experiment verification are performed and the first detection of VUV using thinner THGEM is carried out successfully.

Drift field improvement and test in a GEM-TPC prototype

With the help of Maxwell, Ansys and Garfield, a simulation of the electric field and the deviation of electron drift in the drift volume of a GEM-TPC prototype has been accomplished under the following conditions: Field Cages with one-side and double-side strips, with and without a guard ring. The advantage and necessity of a Field Cage with mirror strips and a guard ring were foreseen. According to the simulation results, TU-TPC was modified and tested; a larger effective area and better resolution were achieved.

Application of a wedge strip anode in micro-pattern gaseous detectors

The wedge strip anode (WSA) has been widely used in 2-D position-sensitive detectors. A circular WSA with an effective diameter of 52 mm is successfully coupled to a tripe gas electron multiplier (GEM) detector through a simple resistive layer. A spatial resolution of 440 μm (FWHM) is achieved for a 10 kVp X-ray using 1 atm Ar:CO2=70:30 gas. The simple electronics of only three channels makes it very useful in applications strongly requiring simple interface design, e.g. sealed tubes and high pressure detectors.

A numerical model of a coated capillary-plate thermal neutron collimator

A novel thermal neutron collimator was successfully fabricated by coating the inner surface of the capillary plate (CP) with gadolinium oxide using atomic layer deposition (ALD) technology. This CP-based collimator is efficient and compact. A numerical model is presented in the paper to estimate the main performance characteristics of the collimator and to optimize the design for specific applications. According to the results of the calculation based on currently available CPs, the FWHM of the collimators rocking curve can be smaller than 0.15° while suppressing more than 99.9% of the incident thermal neutrons on the double wings of the curve. Such a coated CP is as thin as 1.25 mm or even thinner, providing high angular resolution with good transmission in a very limited space.

A Monte Carlo study for the shielding of γ backgrounds induced by radionuclides for CDEX

The CDEX (China Dark matter EXperiment) Collaboration will carry out a direct search for WIMPs (Weakly Interacting Massive Particles) using an Ultra-Low Energy Threshold High Purity Germanium (ULE-HPGe) detector at the CJPL (China JinPing deep underground Laboratory). A complex shielding system was designed to reduce backgrounds and a detailed GEANT4 Monte Carlo simulation was performed to study the achievable reduction of γ rays induced by radionuclides and neutron backgrounds by D(γ,n)p reaction. Furthermore, the upper level of allowed radiopurity of shielding materials was estimated under the constraint of the expected goal. Compared with the radiopurity reported by other low-background rare-event experiments, it indicates that the shielding used in the CDEX can be made out of materials with obtainable radiopurity.