Research Of Front-end Electronics For The WCDA In LHAASO

The Large High Altitude Air Shower Observatory(LHAASO)is a large scientific complex instrument to be built in the 12th Five-Year Plan of the National Development and Reform Commission(NDRC).LHAASO consists of five kinds of detector arrays,to continuously measure the Extensive Air Shower(EAS)with the energy from 1011 eV to 1015 eV.The Water Cherenkov Detector Array(WCDA)is one of the key components in LHAASO,and it is composed of 3120 Photomultiplier Tubes(PMTs)scared in 3 water pools.The PMTs receive the Cherenkov light generated by the secondary particles of the EAS in the water,and output signals to the Front-end Electronics(FEE).FEE are responsible for the time and charge measurement,and the results are used to identify the primary particle types and the injection angle of the cosmic ray.The readout electronics is designed based on a distributed electronics structure:the FEE modules are placed nearby the PMTs and finish digitization of time and charge information in them,and then transferred the data results to the DAQ(Data Acquisition)over long distances through optical fibers.The work in this dissertation focuses on the front-end analog electronics design,and efforts were devoted to the techniques for time and charge measurement of PMT signals over a large dynamic range.As for charge measurement,an optimized circuit scheme was proposed through analysis,simulations as well as actual circuits design and tests,and in the same way,the key parameters in circuit design were finally determined.As for time measurement,the work focuses to enhance the time resolution of previously designed FPGA(Field Program Gate Array)based TDC(Time-to-Digital Converter).Considering future work in massive production of the FEEs,automatic calibration circuits were also designed.After all necessary technical preparation was done through the above work,an engineering prototype was finally designed and fabricated.Tests were also conducted to systematically evaluate its performance.The laboratory test results indicate that the charge measurement resolution is better than 8%@ Single P.E.(S.P.E.)and 1%@4000 P.E.;the time resolution is better than 300 ps RMS over the whole dynamic range.Furthermore,this prototype was also tested with two types of PMTs,which are candidates to be employed in WCDA,and the test results are good as expected.Besides,this paper also presents an improved leading edge discrimination circuit based on the baseline restoring method.This circuit decreases the dead time while other performance remains pretty good.The work is expected to provide design reference for similar applications dealing with large dynamic range signals;The work in this paper also includes the design of another FEE prototype based on an ASIC named PASC(Pre-Amplifier and Shaping Circuit),designed by our laboratory.Complexity of the analog circuits are reduced based on the ASICs.Initial tests were also conducted to evaluate the system performance.This dissertation is organized as follows.Chapter 1 introduces the LHAASO WCDA experiment,and the electronics design requirements are presented;Chapter 2 reviews the mainstream charge and time measurement methods.Through analysis of typical examples in actual applications,these methods are categorized and summarized,which serve as the design reference of the FEE design;Chapter 3 presents the design scheme and the key techniques of the charge and time measurement circuit,including the parameter optimization through calculation and simulation;The design details of the FEE engineering prototype are presented in Chapter 4,including implementation of the amplifier&shaping,A/D conversion,and time discrimination circuit,as well as the digital processing logic integrated in the FPGA device;In Chapter 5,the tests results of the FEE are presented.The test results indicate that this FEE can complete the charge and time measurement in the dynamic range from S.P.E.to 4000 P.E.,with performance all beyond the application requirements;Tests were also conducted by connecting the FEE with PMTs,and two kinds of PMTs are used.The test results presented in Chapter 6 also concord well with the physics expected;Chapter 7 concludes this dissertation and outlines plans of future work.