Xilei Sun

Neutron–gamma discrimination of CsI(Na) crystals for dark matter searches

Abstract The luminescent properties of CsI(Na) crystals are studied in this report. Using a TDS3054C oscilloscope with a sampling frequency of 5xa0GS/s, we find out that nuclear recoil signals are dominated by a very fast light pulse with a decay time of ∼20xa0ns, while γ-ray signals have a decay time of ∼600xa0ns. The study of n / γ separation shows that the rejection factor can reach an order of 10 −7 with a signal efficiency of more than 80% at an equivalent electron recoil energy of 20xa0keV or more, and the quality factors are ∼10 times smaller than that for CsI(Tl) crystals in the corresponding energy region. It is suggested that the wavelength of nuclear recoil and γ-ray signals is also different. Such a property makes CsI(Na) an ideal candidate for dark matter searches.

Experimental study of a THGEM detector with mini-rims

The gas gain and energy resolution of single and double THGEM detectors (5{times}5cm2 effective area) with mini-rims (rim is less than 10{mu}m) were studied. The maximum gain can reach 5{times}103 and 2{times}105 for single and double THGEM respectively, while the energy resolution of 5.9 keV X-ray varied from 18% to 28% for both single and double THGEM detectors of different hole sizes and thicknesses.All the experiments were investigated in mixture of noble gases(argon,neon) and small content of other gases(iso-butane,methane) at atmospheric pressure.

Fast light of CsI(Na) crystals

The responds of different common alkali halide crystals to alpha-rays and gamma-rays are tested in our research. It is found that only CsI(Na) crystals have significantly different waveforms between alpha and gamma scintillations, while others have not this phenomena. It is suggested that the fast light of CsI(Na) crystals arises from the recombination of free electrons with self-trapped holes of the host crystal CsI. Self-absorption limits the emission of fast light of CsI(Tl) and NaI(Tl) crystals.

Luminosity monitoring and calibration of BLM

The BEPC II Luminosity Monitor (BLM) monitors relative luminosity per bunch. The counting rates of gamma photons, which are proportional to the luminosities from the BLM at the center of mass system energy of the psi(3770) resonance, are obtained with a statistical error of 0.01% and a systematic error of 4.1%. Absolute luminosities are also determined by the BESIII End-cap Electro-Magnetic Calorimeter (EEMC) using Bhabha events with a statistical error of 2.3% and a systematic error of 3.5%. The calibration constant between the luminosities obtained with the EEMC and the counting rates of the BLM are found to be 0.84 +/- 0.03 (x10(26) cm(-2).count(-1)). With the calibration constant, the counting rates of the BLM can be scaled up to absolute luminosities.

Temperature dependence of the light yield of the LAB-based and mesitylene-based liquid scintillators

We studied the temperature dependence of the light yield of linear alkyl benzene (LAB)-based and mesitylene-based liquid scintillators. The light yield increases by 23% for both liquid scintillators when the temperature is lowered from 26 degrees C to 40 degrees C, correcting for the temperature response of the photomultiplier tube. The measurements help to understand the energy response of liquid scintillator detectors. Especially, the next generation reactor neutrino experiments for neutrino mass hierarchy, such as the Jiangmen Underground Neutrino Observatory (JUNO), require very high energy resolution. As no apparent degradation on the liquid scintillator transparency was observed, lowering the operation temperature of the detector to similar to 4 degrees C will increase the photoelectron yield of the detector by 13%, combining the light yield increase of the liquid scintillator and the quantum efficiency increase of the photomultiplier tubes.

Simulation of background reduction and Compton suppression in a low-background HPGe spectrometer at a surface laboratory

High-purity germanium (HPGe) detectors are well suited to analyse the radioactivity of samples. In order to reduce the environmental background for an ultra-low background HPGe spectrometer, low-activity lead and oxygen free copper are installed outside the probe to shield from gamma radiation, with an outer plastic scintillator to veto cosmic rays, and an anti-Compton detector to improve the peak-to-Compton ratio. Using Geant4 tools and taking into account a detailed description of the detector, we optimize the sizes of these detectors to reach the design requirements. A set of experimental data from an existing HPGe spectrometer was used to compare with the simulation. For the future low-background HPGe detector simulation, considering different thicknesses of BGO crystals and anti-coincidence efficiency, the simulation results show that the optimal BGO thickness is 5.5 cm, and the peak-to-Compton ratio of (40).K is raised to 1000 when the anti-coincidence efficiency is 0.85. In the background simulation, 15 cm oxygen-free copper plus 10 cm lead can reduce the environmental gamma rays to 0.0024 cps/100 cm(3) Ge (50 keV-2.8 MeV), which is about 10(-5) of the environmental background.

A novel 2-dimensional cosmic ray position detector based on a CsI(Na) pixel array and an ICCD camera

A novel 2-D cosmic ray position detector has been built and studied. It is integrated from a CsI(Na) crystal pixel array, an optical fiber array, an image intensifier and an ICCD camera. The 2-D positions of one cosmic ray track is determined by the location of a fired CsI(Na) pixel. The scintillation light of these 1.0×1.0 mm CsI(Na) pixels is delivered to the image intensifier through fibers. The light information is recorded in the ICCD camera in the form of images, from which the 2-D positions can be reconstructed. The background noise and cosmic ray images have been studied. The study shows that the cosmic ray detection efficiency can reach up to 11.4%, while the false accept rate is less than 1%.