Preliminary Research On Prototype Of A Si-W Imaging Electromagnetic Calorimeter For The CEPC

The standard model,a theory proposed in the middle of the 20th century to describe elementary particles and basic interactions,unified the strong interactions and the weak interactions.As an important component of the standard model theory,the Higgs field as well as the Higgs particle is the key to the formation of the inertial mass.The research of the characteristics of Higgs particle has become one of the most cutting-edge topics in physics,ever since the first discovery of the Higgs particle from 2012.The collision of positron and electron is an effective way to generate Higgs particles,since the Higgs particle has a low mass(about 125 GeV).Based on this,the Institude of High Energy Physics(IHEP)of the Chinese Academy of Science(CAS)proposed the Circular Electron Positron Collider(CEPC)project,aiming to build a circular collider as Higgs factory in China with the technology of circular electron accelerator becoming mature.The poor stability of the Higgs particles means it decays to other particles quickly.So the CEPC adopted Particle Flow Algorithm(PFA)to accurately measure the jet's energy and to reconstruct the incident particle in the case of high brightness and high stack.The PFA,of which the basic principle is to use the accurate position to make up for the lack of energy measurement,can accurately reconstruct the collision vertex and collision products.Based on the algorithm,CEPC put forward the need for an imaging Electromagnetic CALorimeter(ECAL)with fine granularity and high energy resolution.Although a few imaging ECAL prototypes have been manufactured in recent years,the prototyps are all for the linear collider.There is not yet one available for the circular collider.This paper explores the technical route of Silicon-Tungsten ECAL(Si-W ECAL)for CEPC,proposing a prototype and designing a readout electronics system after referring the CALICE Si-W ECAL prototype for the International Linear Collider(ILC).In first part of the paper,the background and requirements of the CEPC are introduced.The ECAL should have the ability of distinguishing adjacent incident particles,as well as an energy resolution up t0 16%/?E.The Si-W ECAL scheme put forward by the CEPC Pre-Concept Design Report(Pre-CDR)is simulated with Geant4 software and the ECAL parameters are optimized according to the simulation results.The specific requirements for the electronics are proposed based on the simulation.In order to study the feasibility of the Si-W ECAL scheme,the paper proposes the system architecture of the ECAL prototype.The prototype consists of 38 layers of sensitive layers,where the size of detector cell is 5×5mm2 and the thickness of depletion layer is 460?m,and a total thickness of 84mm of tungsten plate.Since the sensitive layer cosists of an 8×8 silicon PIN detector array,the total detector channel reaches 2432.The paper also puts forward and implements a scalable electronics system,which is divided into the front-end part and the back-end part.The front-end part includes 38 Front-End Boards(FEB),each of which is followed by a Data-Interface board(DIF).The FEB,which has a core of 64-channel ASIC named SKIROC2a,is responsible for mounting the detector array and reading out the detector signals as well as digitizing them.The DIF,howerer,is in charge of controlling the FEB,packaging the digitized data and hit signals,transmitting them to the back-end electronics via an optical fiber.The back-end part includes a Data Collection Module(DCM),which is in charge of collecting data and hit signals from front-end parts,uploading data to the computer and sending trigger and time information to the front-end parts.The basic performance test of the electronics system has been carried out and introduced in the paper.According to the test results,the electronics noise is less than 0.35fC,the dynamic range is 3.2pC,the Integral Non-Linearity(INL)is not higher than 0.2%,the error-free threshold can be set as low as 1.8fC with an inconsistent of less than 0.2fC between different channels.Besides,the Bit Error Rate(BER)is less than 1×10-12.The performance is in full compliance with the requirements of Si-W ECAL.Then a small prototype system with 4 sensitive layers is built to study the key performance.The system has a total of 256 detectors,each of which has a noise lower than 0.5fC and the noise level meets the demand.A joint test with a radioactive source of 241Am is carried out and the energy resolution measured by them system is 14.3%@59keV.This test proves that the system has a sufficient resolution for small signals.The system is also used to test the cosmic ray spectrum,the measured spectrum's Minimum Ionizing Particle(MIP)and Most Probable Value(MPV)are in accordance with the result of Geant4 simulation.The Signal-to-Noise Ratio(SNR)of each channel's MIP is higher than 10,which proves that the system detect the MIP signal effectively.Besides,the incident trajectory of cosmic ray is reconstructed by the hitting positions of different sensitive layers.At last,the requirements of the CEPC imaging ECAL,the optimized parameters of the Si-W ECAL,the structure of the prototype of Si-W ECAL,the design and performance of the readout system are summarized in the paper.In addition,the paper proposed some shortage of the current phase and some guidances for the next step.