The Research On Large Dynamic Range Readout For The PMT Of LHAASO WCDA

Since discovered, the cosmic ray has been the important medium of cosmic detecting. During the last100years, some information of cosmic ray, such as the component and energy spectrum, was gradually understood by the scientists, but more about cosmic ray was still unsolved mysteries.The core problem of present cosmic ray physics is to look for the origin of cosmic ray. Aiming to solve this problem, a large cosmic ray observatory——Large High Altitude Air Shower Observatory (LHAASO), is to be built in high altitude area. The essential scientific goal of LHAASO project is to discover the origins of cosmic rays and the cosmological evolution, as well as the research on high energy astro evolution and dark matter. The LHAASO project consists of four detector arrays, which are the Kilometer-square Array (KM2A), Water Cherenkov Detector Array (WCDA), Wide Field of View Cherenkov Telescope Array (WFCTA) and Shower Core Detector Array (SCDA).WCDA is one of the important components of LHAASO. The scientific goal of WCDA is to survey the northern sky for the VHE gamma ray sources. The total area of WCDA is90000m2, consisting of four ponds with same size. Each pond is divided into900cells sized5m x5m, partitioned by black plastic curtains to prevent the penetration of light yielded in neighboring cells. An8-in hemispheric photomultiplier tube (Hamamatsu R5912) resides in the bottom of each cell, looking upward to collect Cherenkov lights, recording the arrival time and the charge of pulses. The simulation indicates that the PMT used in WCDA should fulfill the following requirements:gain is about2.5×106; peak-valley ratio of SPE spectrum is more than2.0; dark pulse rate is less than5kHz; rise time is less than4ns; transit time spread (TTS) is less than4ns and linear dynamic range is from SPE to4000PE.The key point and difficult point of PMT requirements is the large linear dynamic range with the high gain, as well as good SPE spectrum and time performance. Since the normal readout circuit is not contributing, the redesign and improvement of the new readout circuit is needed.The thesis introduces the detailed information of cosmic ray, including the component, energy spectrum, origin, mechanism of acceleration and propagation, detecting methods. The thesis also introduces the readout precept and PMT performance index of several experiments using water Cherenkov detector technology. The detailed information of LHAASO and WCDA is present, including the working mechanism and experimental requirements.The main work of the thesis is the design and improvement of base for the large area PMT (Hamamatsu R5912) base on the WCDA requirements, and to present a program of PMT high voltage dividing and signal readout which meet WCDA requirements through a number of experiments. The test results show that the PMT gain is2.5×106at1000V, the peak-valley ratio of SPE spectrum is2.4, the rise time of pulse is about4ns, the TTS is3.67ns, dark pulse rate is about1kHz, the linear dynamic range of anode and10th dynode is3500PE while8th dynode is more than6000PE. This is the first time that R5912could achieve this dynamic range at the gain of2.5×106, which solves the key problem of WCDA development and PMT readout. The thesis also introduces the test methods of PMT properties and the test system built for the performance test.The other work of thesis is the constructing of batch test system. There are3600cell detectors, so the performance test will be taken for3600PMTs to select eligible PMTs. Then a batch test system is needed. According to the requirement of the test, a batch test system has been built which consist of dark box, optical splitter, fibers, electronics and DAQ. Some properties of the system are measured, comprising the property of photophobism, electromagnetic shielding and light split. The results indicates that the performance of the system meets the requirements.The research results provide important data for the project approval of LHAASO and engineering design of WCDA, as well as the necessary foundation for the following research.