Study And Optimization On Electromagnetic Particle Detectors In LHAASO-KM2A

The origin,acceleration and propagation of cosmic ray are three basic topics in cosmic ray physics,which are not answered clearly until now.The high energy y-ray astronomy is an active subject in cosmic ray field.Fermi satellite experiment,H.E.S.S.and CTA Cherenkov telescope array,Milagro and Asy EAS array are some active exper-iments in high energy y-ray field.Following the development of Cherenkov technology,especially the H.E.S.S.experiment,more than 200 y-ray sources in TeV region had been detected.But the three basic questions in cosmic ray physics are still unsolved.In or-der to improve this situation,the large high altitude air shower observatory(LHAASO)project is proposed by Chinese scientists.LHAASO aim is to explore the high energy y-ray sources and measure the cosmic ray energy spectrum with precedented sensitivity,to solve the problem of origin,acceleration and propagation of cosmic ray.LHAASO consists of 3 sub-arrays:one square kilometer array(KM2A),water Cherenkov detector array(WCDA)and wide field-of-view air Cherenkov telescopes array(WFCTA).As the main array,KM2A aim is to explore the Galactic y-ray sources,covering the energy region from 10 TeV to 100 PeV.KM2A consists of 5195 electromagnetic particle detec-tors(EDs)and 1171 underground ? detectors(MDs).The gamma showers and hadronic showers should be discriminated sufficiently by the coincident measurement of ED and MD above 50 TeV.ED is used to measure the energy and position of high energy y-ray.To realize good angular resolution,good energy resolution and wide energy region,ED should get 20 photoelectrons for minimum ion particle,detect efficiency should be larg-er than 95%,time resolution should be lower than 2 ns,time synchronization between different EDs should be lower than 500 ps and dynamic range should cover 1 to 10 000 particles.Good performance of detectors is the basic of good quality of data in LHAASO project.ED must be designed and studied thoroughly.ED is plastic scintillator detector and wavelength shifting fibers(WLS fibers)are used for the readout of scintillation photons.The photomultiplier tube(PMT)is used to detect the photons from fibers and convert to electron signals,and then digitized by the front-end electronics.In order to keep ED good quality,the properties of PMT,scintillator and WLS fibers used in ED should be studied thoroughly.As the photovoltaic conversion and signal amplification device,PMT will affect the performance of ED directly.To guarantee the properties of each PMTs qualified for ED,a PMT test bench with high accuracy had been established to finish the measurement of more than 5300 PMTs in the future.With help of the test bench,two types of PMTs with low transit time spread(<2.5ns),low temperature dependence(<0.2%/?)and high quantum efficiency(>25%)had been developed by manufactures,and fulfill the require-ments of PMTs in ED.The circuit with dynode readout had been designed for each type of PMTs to reach the requirement of good linearity,which requires the maximum linear current larger than 1163 mA.The feasibility of dynode readout had been proved by the prototype array of KM2A.ED sector consists of scintillator,WLS fibers and Tyvek reflectance coating,which influence the number of photoelectrons(Npe)of minimum ion particle.There are 5 types of scintillator and 3 types of WLS fibers could be used in ED.The thickness of scintillator and number of fibers should be optimised for each ED sector to reach the requirement of 20 p.e.of ED.Geant4 package is used for the simulation of ED and the relationship between Npe and scintillator,relationship between Npe and number of WLS fibers are acquired by simulation.Optimization results of each types of ED unit had been acquired with the simulation results and measuring results of sample ED sectors.We found the time resolution would be larger than 2 ns when number of WLS fibers is less than 20,and the thickness of scintillator could not be less than 1.0 cm to fulfill the mechanical strength.With the lower limit of number of WLS fibers and scintillator thickness,the feasible ED sectors are selected.Finally,a new ED prototype had been built and properties had been measured and studied thoroughly.The optimized ED sector and candidate PMTs are used in the proto-type,and the structure of ED and polish method of WLS fibers are improved to simplify the manufacture of ED.Npe of new ED prototype reach 19.4,time resolution is 1.64 ns and detect efficiency is 98.3%,all the properties of this new ED prototype reach the re-quirements.Meanwhile,the influence to time resolution from each part of ED and the source of ED noise are studied thoroughly,which could provide a reference for the study of other plastic scintillator detectors.Detectors with good performance is the basis of high quality data of array.LHAASO will take big breakthrough in cosmic ray physics.