Yuanjing Li

Measurement of the intrinsic radiopurity of 137Cs/235U/238U/232Th in CsI(Tl) crystal scintillators

Abstract The inorganic crystal scintillator CsI(Tl) has been used for low energy neutrino and Dark Matter experiments, where the intrinsic radiopurity is an issue of major importance. Low-background data were taken with a CsI(Tl) crystal array at the Kuo-Sheng Reactor Neutrino Laboratory. The pulse shape discrimination capabilities of the crystal, as well as the temporal and spatial correlations of the events, provide powerful means of measuring the intrinsic radiopurity of 137 Cs as well as the 235 U, 238 U and 232 Th series. The event selection algorithms are described, with which the decay half-lives of 218 Po, 214 Po, 220 Rn, 216 Po and 212 Po were derived. The measurements of the contamination levels, their concentration gradients with the crystal growth axis, and the uniformity among different crystal samples, are reported. The radiopurity in the 238 U and 232 Th series are comparable to those of the best reported in other crystal scintillators. Significant improvements in measurement sensitivities were achieved, similar to those from dedicated massive liquid scintillator detector. This analysis also provides in situ measurements of the detector performance parameters, such as spatial resolution, quenching factors, and data acquisition dead time.

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.

The China Jinping Underground Laboratory and its Early Science

The China Jinping Underground Laboratory, inaugurated in 2010, is an underground research facility with the deepest rock overburden and largest space by volume in the world. The first-generation science programs include dark matter searches conducted by the CDEX and PandaX experiments. These activities are complemented by measurements of ambient radioactivity and the installation of low-background counting systems. Phase II of the facility is being constructed, and its potential research projects are being formulated. In this review, we discuss the history, key features, results, and status of this facility and its experimental programs, as well as their future evolution and plans.

Measurement of the dead layer thickness in a p-type point contact germanium detector

A 994 g mass p-type PCGe detector has been deployed during the first phase of the China Dark matter EXperiment, aiming at direct searches for light weakly interacting massive particles. Measuring the thickness of the dead layer of a p-type germanium detector is an issue of major importance since it determines the fiducial mass of the detector. This work reports a method using an uncollimated 133Ba source to determine the dead layer thickness. The experimental design, data analysis and Monte Carlo simulation processes, as well as the statistical and systematic uncertainties are described. A dead layer thickness of 1.02 mm was obtained based on a comparison between the experimental data and the simulated results.

Fusion of X-ray Imaging and Photoneutron Induced Gamma Analysis for Contrabands Detection

A 7 MV LINAC based photoneutron interrogating system has been setup to fulfill the demand of contrabands detection in homeland security. Both X-ray imaging and photoneutron induced γ-ray analysis are used to extract the information of inspected materials. 480 CsI detectors of 5 mm (height) x 10 mm (width) x 20 mm (length) are used to form the detector array to measure the attenuation information of penetrating X-rays. 16 NaI(Tl) detectors of 127 mm (height) x 127 mm (diameter) are used to register the photoneutron induced γ-ray spectra of inspected materials. 2-dimensional elemental distributions of H, N, Fe and Cl are extracted by calculating the area of 2.223 MeV, 10.829 MeV, 7.64/9.298 MeV and 1.165/1.951/1.959 MeV γ-ray peaks in the spectra measured by NaI(Tl) arrays respectively. Mixed materials like iron, salt, water, melamine and sugar are scanned to test the contrabands detection capability. The images of X-ray attenuation and 4 elemental 2-dimensional distributions are fused together to separate suspicious materials. Areas with high concentrations of nitrogen and chlorine are easily identified and can indicate the potential existence of illicit substances.

Aging test of high rate MRPC

A new kind of low resistivity glass has been developed. Its volume resistivity is on the order of 10 10 Ωcm and MRPCs assembled with it show a very promising rate capability (>25 kHz/cm 2 ). This new detector has a very important range of application in high energy physics experiments such as FAIR-CBM, LHC-ATLAS or Jlab-SOLID, to mention some. In this paper we report new results related to its long-term behavior (ageing). The module has been irradiated by X-rays at a mips-equivalent flux of 15kHz/cm 2 , for 300 hours and for a released charge totaling 0.22C. No noticeable degradation could be observed. Compared to common glass MRPCs, the newly developed high rate counter responds faster to sudden irradiation.

Materials identification by X-ray and photoneutron transmission

In this paper, we present the method research of materials identification based on X-ray and photoneutron transmission. X-ray is produced by 7MeV LINAC and photoneutron is emitted as the byproduct of X-ray when beryllium convertor is used to convert X-ray to neutron. The different attenuation coefficients of X-ray and neutron in materials are used to form a factor, named V, to discriminate materials. The main problems of MIXPT (materials identification by X-ray and photoneutron transmission) come from (1) the interference of X-ray pulses to the detection of photoneutrons and (2) varied V value of materials originated by continuous spectra of X-rays and photoneutrons when penetrating materials; To the first problem, a thermal neutron detector is designed to detect the fast penetrating photoneutron. With the aid of neutron moderation substances surrounding BF3 detector, fast photoneutron is thermalized and timely delayed thus could be detected free of X-ray pulses interference and with high detection efficiency. A time window of 950μs width after each X-ray pulse is carefully determined according to both simulation and experimental research to measure the penetrating photoneutron. To the second problem, the result of simulation with MCNP5 shows that each material has a definite curve of V versus mass per unit area and could be discriminated from other materials effectively. The experimental results of polyethylene, aluminium, iron, copper, lead and explosive simulant also confirm these curves;

Studies on RPC position resolution with different surface resistivity of high voltage provider

The effect of surface resistivity of high voltage provider on the space dispersion of induced charge of RPC has been studied experimentally. The result of experimental measurement shows that two-Gaussian function can be used to fit the dispersion of induced charge of RPC. The Gaussian function with narrower width is mainly due to the expansion of avalanche charge in the gas gap of RPC and the Gaussian function with larger width is due to the charge dispersion when it passes through the resistive carbon film.

Study on the performance of high rating MRPC

Six-gap resistive plate chamber prototypes with semiconductive glass electrodes (bulk resistivity 1010Ω.cm) were studied for suitability in time-of-flight (TOF) applications at high rates, such as CBM-TOF. These studies were performed using a continuous electron beam of 800 MeV at IHEP and an X-ray machine. Time resolutions of about 100 ps and eficiencies larger than 90% were obtained for flux densities up to 28 kHz/cm2.

The development and evaluation of solid-state detector arrays for high energy x-ray imaging

Several detector modules which consist of linear CdWO/sub 4/ crystal arrays optically coupled to photodiode arrays were developed for a high energy X-ray NDT system. An electron linear accelerator (LINAC) which produces pulsive X rays with maximum energy of 9 MeV is used for X ray source. 16 channels were assembled in each module, with crystal size of 0.8 mm /spl times/ 5 mm /spl times/ 30 mm and pitch size of 1.3 mm. Lead spacers were placed between crystals to reduce the cross-talk. Based on M-C simulation, the detection efficiency of the detector can reach 70%, energy deposit rate 30%, while crosstalk can be reduced to 5%. After fabrication, we tested their performances under the radiation sources of /sup 60/Co, 6 MeV LINAC and 9 MeV LINAC. The results show that the arrays satisfy the meet of the NDT system.