Speaker
Mr
LAIFU LUO
(University of Science and Technology of China)
Description
Super Tau-Charm Facility (STCF) is one of the important options for accelerator-based particle physics after Beijing Electron-Positron Collider (BEPC-II) in China, which aims at ultra-precise measurement and new physics search in tau-charm energy region with about 100 times higher luminosity. The electromagnetic calorimeter (ECAL), as an important part of the spectrometer, needs to meet demand of high-efficiency and high-resolution gamma detection, electron and hadron discrimination, etc. Pure CsI (pCsI) is selected as the scintillation crystal for STCF ECAL owing to its fast response (30 ns decay time), high mass density and adequate radiation hardness. Due to the relative low light yield of pCsI crystal and the strong magnetic environment in the experiment, avalanche photodiode (APD) is proposed to convert the scintillation light into current signal with an internal gain of about 50 at the recommended operating voltage.
Charge sensitive amplifier (CSA) which has the feature of low noise and high charge measure performance has been used to read the APD signal. Considering the large capacitance of APDs (tens to hundreds of picofarads) which may result in poor noise performance, a low noise field effective transistor (FET) input stage is adopted. Then, a CR-(RC)^2 shaping circuit is employed to improve the signal-to-noise ratio (SNR), and the shaped analog signal is digitalized by an ADC chip. Digital data are pre-processed by FPGA and transmitted to the upper computer via Ethernet. The circuit block diagram and PCB (Front End Board and Back End Board) pictures are shown in figure-1.
Under high event rate and large dynamic range conditions, it is difficult for CSA to achieve extremely low noise, especially with an input capacitance of hundreds of picofarads. In this paper, a noise equivalent circuit containing eleven noise sources is established. Based on this, the optimization for large input capacitance is carried out, applying different types and numbers of JFETs. The result shows that the 3-JFET (2SK715) input stage CSA has the best noise performance at large input capacitance. Optimization of shaping time is also conducted to meet a balance between shot noise and thermal noise, thereby minimizing system noise. The result indicates that the system noise can reach minimum level at a shaping time of about 100 ns. Under above conditions, an equivalent noise charge (ENC) of 675 electrons of electronic noise and 1025 electrons with S8664-0505 APD is realized. Furthermore, optimization of the APD operating voltage to achieve best SNR is also under development.
Summary
A CSA-based electronics system is designed to readout the signal of pCsI crystal, which is the basic option of STCF ECAL. Noise optimization of the readout scheme for large input capacitance is carried out. System with 3-JFET (2SK715) input stage CSA and 100 ns shaping time has the best noise performance after testing. A noise level of 675 electrons for electronics alone and 1025 electrons with S8664-0505 APD connected is realized. Additionally, optimization of the APD operating voltage to achieve best SNR is also under development.
Primary author
Mr
LAIFU LUO
(University of Science and Technology of China)
