Shaoyong Wu

Limits on light weakly interacting massive particles from the CDEX-1 experiment with ap-type point-contact germanium detector at the China Jinping Underground Laboratory

We report results of a search for light Dark Matter WIMPs with CDEX-1 experiment at the China Jinping Underground Laboratory, based on 53.9 kg-days of data from a p-type point-contact germanium detector enclosed by a NaI(Tl) crystal scintillator as anti-Compton detector. The event rate and spectrum above the analysis threshold of 475 eVee are consistent with the understood background model. Part of the allowed regions for WIMP-nucleus coherent elastic scattering at WIMP mass of 6-20 GeV are probed and excluded. Independent of interaction channels, this result contradicts the interpretation that the anomalous excesses of the CoGeNT experiment are induced by Dark Matter, since identical detector techniques are used in both experiments.

A Search of Low-Mass WIMPs with p-type Point Contact Germanium Detector in the CDEX-1 Experiment

The CDEX-1 experiment conducted a search of low-mass (< 10 GeV/c2) Weakly Interacting Massive Particles (WIMPs) dark matter at the China Jinping Underground Laboratory using a p-type point-contact germanium detector with a fiducial mass of 915 g at a physics analysis threshold of 475 eVee. We report the hardware set-up, detector characterization, data acquisition and analysis procedures of this experiment. No excess of unidentified events are observed after subtraction of known background. Using 335.6 kg-days of data, exclusion constraints on the WIMP-nucleon spin-independent and spin-dependent couplings are derived.

Measurement of the half-life of 79Se with PX-AMS

Abstract The half-life of 79Se has been re-measured with projectile X-rays detection (PXD) in accelerator mass spectrometry after a new PXD system had been set up in China Institute of Atomic Energy (CIAE). The PXD technique has been used for separation of 79Se from its isobar, 79Br. The detection efficiency of 79Se Kα X-rays is obtained from that of 80Se after energy correction. The atom number of 79Se in each sample is deduced from the measured ratios of 79Se/Se. From the measured decay rates of each sample, the half-life is deduced to be (2.80±0.36)×105 a. In order to check the reliability of the result of 79Se half-life, the 75Se half-life is also measured with PX-AMS method. The measured half-life of 75Se is in good agreement with well-known literature value.

99Tc measurements with accelerator mass spectrometry at CIAE

Abstract 236 U is a long-lived radioactive isotope which is produced principally by thermal neutron capture on 235 U. 236 U may be potentially applied in geological research and nuclear safeguards. Accelerator mass spectrometry is presently the most sensitive technique for the measurement of 236 U and a measurement method for long-lived heavy ion 236 U has been developed. The set-up uses a dedicated injector and the newly proposed 208 Pb 16 O 2 - molecular ions for the simulation of 236 U ion transport. A sensitivity of lower than 10 −10 has been achieved for the isotopic ratio 236 U/ 238 U in present work.

Development of AMS measurements and applications at the CIAE

Abstract The HI-13 tandem accelerator in the China Institute of Atomic Energy (CIAE) is a multi-user facility. Only a small part of the beam time can be used for AMS. So, it is impossible to measure very many samples and to carry out wider applications by using the HI-13 tandem AMS system. Therefore, we focus our studies on the development of measurement and detection methods and several interesting applications utilizing the long-lived radionuclides 26Al, 41Ca, 36Cl and 79Se.

Measurement of and with PXAMS

Abstract A projectile X-ray accelerator mass spectrometer (PXAMS) system has been set up at the China Institute of Atomic Energy (CIAE). The first applications were the measurement of the 79 Se half-life and the analysis of a radioactive 64 Cu beam and its contaminating components. The improved detection limit for 79 Se / Se is of the order of 10−9.

A gas-filled time-of-flight detector system and a new method for particle identification

In order to increase the power of isobaric identification in accelerator mass spectrometry (AMS) measurements, we built a new gas-filled time-of-flight (GF-TOF) detector system (Jiang et al.). The principle, construction and results of first on line tests with a Cu sample are described. Based on the GF-TOF detector system, we also propose a new particle identification method called energy-loss and time-of-flight (ΔE–T) parabola. The principle of the method and the first test for the method with the 36Cl AMS measurement are described.

Measurement of ultra-low potassium contaminations with Accelerator Mass Spectrometry

Abstract Levels of trace radiopurity in active detector materials is a subject of major concern in low-background experiments. Among the radio-isotopes, 40 K is one of the most abundant and yet whose signatures are difficult to reject. Procedures were devised to measure trace potassium concentrations in the inorganic salt CsI as well as in organic liquid scintillator (LS) with Accelerator Mass Spectrometry (AMS), giving, respectively, the 40 K-contamination levels of ∼ 10 - 10 and ∼ 10 - 13 g / g . Measurement flexibilities and sensitivities are improved over conventional methods. The projected limiting sensitivities if no excess of potassium signals had been observed over background are 5 × 10 - 14 and 3 × 10 - 17 g / g for the CsI and LS, respectively. Studies of the LS samples indicate that the radioactive contaminations come mainly in the dye solutes, while the base solvents are orders of magnitude cleaner. This work demonstrates the possibilities of measuring naturally occurring isotopes with the AMS techniques.

Study on UF - 2 Extraction in Accelerator Mass Spectrometry Measurement of 236 U

This study extracts UF-2 ions from UF4 sample material for the first time so as to improve the measurement accuracy and sensitivity for accelerator mass spectrometry (AMS) measurement of 236U. Compared to the commonly used UO-/UO2 (or U3O8) combination, the UF-2/UF4 approach brings a higher beam current of extracted U-containing ions and lower interference from U isotopes (235U in particular). The UF4 prepared with the procedures developed in this work can provide a higher ratio of F- /O- and therefore lower interference from O-containing 235U and 238U molecular ions, compared with that from the UF4 made by conventional liquid-phase reaction. The AMS experiment was carried out on the AMS system at China Institute of Atomic Energy (CIAE), where only a simple surface barrier detector was used to record ions and a reference 236U sample with a 236U/238U ratio of 10-10 was analyzed. The result shows that the measurement sensitivity of the UF-2/UF4 approach is lower than 10-10 and that the reference 236U sample result is in agreement with the reference value within the uncertainty limits, with the relative uncertainty only 4%. In comparison, the measurement sensitivity of the UO- /U3O8 combination approach is 10-9, and it cannot give a concrete value for the same reference sample using the same AMS system. If the sophisticated 500-ps-resolution time-of-flight detection system is used in combination with the UF-2/UF4 approach, a sensitivity of 10-13 (or lower) is expected.

AMS Measurement of 237 Np at CIAE

237 Np (≃2.14 × 10 6 yr half-life) is potentially applicable in studies on nuclear safeguards and radioactive waste migration. The atomic ratio of 237 Np/U in nature is 10 −12 or even lower, depending strongly on the integral neutron flux received by the material. As an ultra-sensitive technique, accelerator mass spectrometry (AMS) is the best for measuring ultratrace 237 Np. By extracting negative molecular ions NpO – from the oxide sample using 238 UO – and 208 Pb 16 O 2 – pilot beams for the simulation of 237 Np ion transport, identifying the interference isotopes by high-resolution dedicated injector, electrostatic analyzer, and time of flight (TOF) detector, a method for AMS measurement of 237 Np was set up on the HI-13 accelerator at the China Institute of Atomic Energy (CIAE). A sensitivity of –11 has been achieved for the isotopic ratio 237 Np/ 238 U.