Yang Tian

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.

High detection efficiency neutron sensitive microchannel plate

Direct addition of neutron-absorbing atoms into MCP glass, such as 10B and 155,157Gd, without changing the remainder of MCP fabrication process, can make MCP sensitive to neutrons. Since the much larger cross section of 157Gd, the MCP doped with natGd2O3 is more efficient than one doped with enriched 10B2O3, theoretical indicates that the detection efficiency of a MCP doped with 3 mole% of natGd2O3 is equivalent even superior to a MCP doped with 20 mole% of 10B2O3. In this paper a 50 mm diameter rimless format 10 μm pore diameter MCP doped with 3 mole% natGd2O3 was fabricated, but the MCP glass doped with 20 mol% of natB2O3 failed to pass through the corrosive chemical processing necessary in MCPs manufacture. A primary experimental tests proved the MCP doped with 3 mole% natGd2O3 is capable of imaging thermal neutrons with 35% detection efficiency. A potential of enhancement on detection efficiency should be achieved for a optimized geometry MCP with an appropriate selection between bias angle, open area ratio and length to diameter ratio.

Nuclear material identification by photoneutron and X-Ray radiography

Transport of illicit materials, especially nuclear materials, poses a threat to homeland security. Previous studies about Photoneutron and X-Ray Radiography (PNXR) had achieved identification of common materials so that PNXR can be used to detect illicit materials like explosives and drugs [1]. Recent studies have shown that the PNXR can also be used to detect nuclear materials. In our experiment, a 7MeV LINAC working at 160Hz repetition frequency and 11Gy/min@1m dose rate is used to provide X-rays and photoneutrons for active interrogation. Produced by bremsstrahlung and photonuclear reaction, MeV photons and neutrons subsequently penetrate objects or induce fissions in nuclear materials. A V value defined by attenuation information of neutrons versus photons is used for preliminary screening. Beta-delayed fission neutrons are detected within the time window of 2ms to 6ms after each electron pulse by 3He counters to provide further corroboration. When a 4cm×4cm×4cm depleted uranium sample was inspected, 1.74±0.28 cps count rate of delayed neutron was acquired and the sample was confirmed to be nuclear material with probability of approximate unity.

Detection efficiency evaluation for a large area neutron sensitive microchannel plate detector

In this paper, the detection efficiency of a large area neutron sensitive microchannel plate detector has been evaluated. A 6LiF/ZnS scintillator detector 65 mm in diameter and 0.32 mm in thickness, with product code, EJ426HD2, produced by Eljen Technology, was employed as the benchmark detector. The TOF spectra of these two detectors were simultaneously measured and the energy spectra were then deduced to calculate the detection efficiency curve of the nMCP detector. Tests show the detection efficiency@25.3 meV thermal neutrons is 34% for this nMCP detector.

Realization of neutron sensitive MCP with ALD technique

We present the research of thermal neutron detector based on Micro Channel Plate (MCP). natGd2O3 was successfully coated on the inner surface of MCP using Atomic Layer Deposition (ALD) technology. Neutron beam test results clearly reflect its sensitivity to thermal neutrons. The results were compared with boron doped MCP. One can simply use it as a compact thermal neutron collimator (several mm thick). It is also encouraging to build a neutron imaging detector based on this new coated MCP to achieve both high detection efficiency and good spatial resolution.

Detection of high-Z materials using 7MeV X-rays scattering

By detecting and analyzing positron annihilation photons, bremsstrahlung photons and Compton scattered photons that are produced after the interaction of X-rays and matter, atomic number may be identified to find concealed SNM (special nuclear material). A code of reading and analyzing data from PTRAC card from MCNP5 simulation has been designed to explore the birth history of each photon. The simulation results indicate that the optimum detection angle is 145 degree and show that the information of Z (especially for high Z) of inspected materials can be extracted effectively. An experimental facility based on 7 MeV LINAC and LaBr3(Ce) detector has been set up for experimental study. Preliminary results demonstrate the possibility of high Z materials identification.

Performances of a prototype point-contact germanium detector immersed in liquid nitrogen for light dark matter search

The CDEX-10 experiment searches for light weakly interacting massive particles, a form of dark matter, at the China Jinping Underground Laboratory, where approximately 10 kg of germanium detectors are arranged in an array and immersed in liquid nitrogen. Herein, we report on the experimental apparatus, detector characterization, and spectrum analysis of one prototype detector. Owing to the higher rise-time resolution of the CDEX-10 prototype detector as compared with CDEX-1B, we identified the origin of an observed category of extremely fast events. For data analysis of the CDEX-10 prototype detector, we introduced and applied an improved bulk/surface event discrimination method. The results of the new method were compared to those of the CDEX-1B spectrum. Both sets of results showed good consistency in the 0–12 keVee energy range, except for the 8.0 keV K-shell X-ray peak from the external copper.

Analysis and optimization of spatial resolution for a neutron sensitive microchannel plate detector

Neutron imaging is widely used in scientific experiments and industrial applications. The neutron sensitive microchannel plate (nMCP) detector plays the role of registering penetrating neutrons and realizing high spatial resolution. The delay-line readout anode can achieve a high spatial resolution with an acceptable throughput. To optimize working parameters of the nMCP detector with the delay-line readout anode, simulations are conducted to understand the signal formation process. In order to realize the best temporal resolution (and hence the best spatial resolution), the differential current output by the delay line anode is calculated to find the largest slope of the rising edge of the current.

The study of saturation effect of GEM detector for X-ray imaging

In our lab, an imaging detector prototype based on gas electron multiplier (GEM) was successfully designed and constructed. During the systematic response measurement of this device, an unexpected saturation phenomenon was highlighted. When the effective gas gain is increased, apparent saturation effect is observed. At high pressure, such as 2 atm or 3 atm, there is a similar phenomenon. Taking into account drift voltage, effective gas gain, area effect and resistive network, we now significantly diminish the saturation of the detectors response and get some useful results.