Ground Test And Preliminary Analysis Of Orbit Data For The Dampe BGO Calorimeter

In recent years, astronomical observation shows that dark matter occupies about 25% of the total mass-energy of the universe while the visible matter account for only 4.9%. However, in addition to the observation of the celestial movement in macroscopic scale, dark matter particle has not been detected in other methods and its nature is still unknown. Thus, dark matter is called to be one of "two clouds over 21st century physics". A breakthrough research on dark matter will greatly promote the people’s understanding of basic law of nature and the evolution of the universe. Now many U.S. and European groups pay attention to the dark matter research and carry out a series of experiments, such as AMS02, Fermi LAT, Xenon, etc. China has also incorporated dark matter research into her medium and long term scientific plan. A Chinese satellite detector named DArk Matter Particle Explorer (DAMPE), which is one of the four scientific space science missions within the framework of the Strategic Pioneer Program on Space Science of the Chinese Academy of Science, was launched in Dec.17th,2015.Currently, the weakly interacting massive particles (WIMPs) is the most popular one among the numerous dark matter cadidates. Some theoretical models predict WIMPs will decay/annihilation into high energy photons, or positive and negative particle pair (electron-positron, etc.). The DAMPE searches for the existence of dark matter particle indirectly with observing high energy cosmic photons and electrons.This thesis focuses on the BGO electromagnetic calorimeter (ECAL), which is one of the core sub-detectors of the DAMPE. The performances and research methods of the ECAL are discussed in detailed through some important ground experiments in the process of the calorimeter development and observation in orbit.In the beginning of this thesis, we introduce physical background of dark matter and detection methods. In the second chapter, we talk about the physical target of the DAMPE, detector system and its performance index, especially, the design of the BGO ECAL in detail.As a space experiment, the reliability of the DAMPE detector is essential, so the detector performance should be fully studied in the ground experiment. Before the flight model of the DAMPE, the BGO ECAL team in USTC have developed a quarter prototype (2012) and a full-size prototype (engineering qualify model, EQM,2014), and verified the design utilizing the beam test at CERN.The flight model of the BGO ECAL was developed and ground tested for nearly a year. In the beginning of chapter.3, the basic calibration methods of the ECAL is introduced through the cosmic ray test platform, including pedestal, MIPs value and dynode ratio. And then this chapter is also talk about the a serious of environment test after the ECAL assembly. The calibration performances of the ECAL are proved to be reliable in these extreme environment such as high and low temperature, high vacuum, high vibration, etc., which are important foundations for space observation after launch.In chapter 4, the temperature effect of the BGO ECAL is investigated emphatically through the thermal vacuum test of the DAMPE detector. As a space experiment, the BGO ECAL is running in a large temperature variation environment (-15℃～+20℃). BGO is an inorganic crystal, whose scintillation light yield is strongly temperature dependent. The detector unit and the electronics also depend on temperature. It is very important to well understand the temperature performance of the ECAL while the output signal is about 30% difference between the cases of high and low temperature limit. This chapter studies the temperature effect of the ECAL systematically and calibrate the temperature coefficient of each detector unit, whose mean value is -1.409%/℃. Based on the coefficient, a temperature correction algorithm has been developed. A good result shows that after correction, the nonuniformity of MIPs MPV value in different temperature is improved from 9% to 2%.In addition to high energy electrons and gamma rays, the nucleons (ions) in space are also important physical goal of the DAMPE experiment. An ion beam test of the DAMPE was performed at CERN, and chapter 5 focus on the quenching effect of BGO crystal and the response of the BGO ECAL on ion. The quenching effect of the BGO crystal with 40 GeV/c/nucleon ion beam (Z>5) is observed utilizing the ion MIPs events. We also use the ΔE detectors on the beamline to identify the ion species, and the energy fractions of ion with nucleon number from 2 to 18 are given in this chapter, which will be important references for the energy reconstruction of ion in space experiment.After the launch of the DAMPE satellite, the orbit calibration of the detectors and the experimental data analysis become the focus of our work. In chapter 6, we talk about the influence of the space environment on our detector, including solar angle and earth radiation belt. And then, the orbit calibration of the BGO ECAL is be also introduced. Thanks to the well study in ground test, the result shows that the calibration performance remains stable within six months after launch. At last, this chapter discusses the study of high energy electron in space. So far the time of observation is short, the data needs more statistic. In this thesis, physical results are not involved, and the methodological research is emphasized. The thesis gives a research process on the space electron spectrum, and some details, such as effective acceptance, event selection, electron/proton identification, trigger efficiency and exposure time, are also discussed. The preliminary shows that it can be achieved a good background rejection in 100-150 GeV energy region under the condition that the Ageomεsel of electron is about 0.3 m2Sr.